Mifflin St Jeor Formula Explained: How It Works Properly

Right, let’s get one thing sorted before we touch a single number. I remember sitting with my old gym notebook in Leeds, staring at an equation full of decimals, wondering why anyone would make calorie maths this complicated. It doesn’t need to be. The Mifflin St Jeor formula explained properly is really just a simple way to estimate how much energy your body needs while it’s doing nothing at all. It looks like a wall of maths at first glance, but once you break it into pieces, it’s genuinely just weight, height, age and sex plugged into a short sum. The formula turns up in most BMR and RMR calculators you’ll find online, though the original research described it as a predictive equation for resting energy expenditure, a distinction worth knowing before we go any further.

What Is the Mifflin St Jeor Formula?

Before any equations or numbers, let’s answer the “what am I actually looking at” question properly.

Mifflin St Jeor is a mathematical equation that estimates your resting energy expenditure using a handful of basic personal details. Nothing more mysterious than that.

What the formula estimates

The formula gives you a prediction of:

• Resting energy expenditure
• Energy used to support basic body functions
• Calories burned at rest
• A figure that is not the same as your total daily calorie needs

That final point trips up almost everyone the first time they use a calculator, so keep it in mind as we go.

Who developed the Mifflin St Jeor equation?

The equation carries the names of its creators, Mark D. Mifflin and Scott T. St Jeor, along with their research team. Their work was published in the American Journal of Clinical Nutrition in 1990. The original study derived the equation from data on 498 healthy adults, including 247 women and 251 men aged 19 to 78, which is a reasonably solid sample for its time.

Why the formula became popular

A few practical reasons explain its staying power:

• Simple inputs
• Easy calculation
• No special equipment
• Useful for nutrition planning
• Commonly built into online calculators

If you’d like to see how this fits into the bigger picture of what BMR actually means, that guide pairs nicely with this one.

Is it a BMR or RMR formula?

Here’s the terminology bit worth getting right. The original research focused on resting energy expenditure, while many online BMR calculators apply the equation as a BMR estimate anyway. It’s a useful distinction, without drowning beginners in technical detail. Our piece on how doctors estimate metabolism explains this overlap between terms in more depth.

The Mifflin St Jeor Formula Explained Simply

This is the heart of the whole article, so let’s break the formula into small, digestible pieces.

Mifflin St Jeor formula for men

Here’s the equation in full:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5

Now let’s walk through each part in plain English.

What does 10 times weight mean?

Body weight carries the biggest coefficient in the whole equation, because heavier bodies generally need more energy just to maintain themselves.

What does 6.25 times height mean?

Height contributes to the estimate because taller bodies tend to have more tissue to support, even at rest.

Why is age multiplied by 5?

Age is subtracted because resting energy expenditure tends to decline gradually over the years, partly linked to changes in muscle mass.

Why do you add 5?

That final “plus 5” is simply the sex-specific constant built into the male version of the equation, based on the original research data.

Mifflin St Jeor formula for women

The women’s version looks almost identical, bar one number:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

Let’s explain that difference clearly too.

Why is the final number minus 161?

The two equations use different sex-specific constants, reflecting average differences in body composition found in the original study group.

Why does the formula ask for sex?

Sex was one of the variables recorded when the original equation was developed, and the constants were built directly from that data. It’s worth remembering that no formula can perfectly represent every individual’s biology, whatever their sex.

How to Calculate Mifflin St Jeor BMR Step by Step

Right, time to turn theory into something you can actually use with a pen, paper and a cup of tea.

Step 1: Gather your details

You’ll need:

• Age
• Weight in kilograms
• Height in centimetres
• Sex

Step 2: Check your units

For UK readers used to stones and pounds, or feet and inches, a quick conversion matters:

• Stones and pounds convert to kilograms
• Feet and inches convert to centimetres

Getting this step wrong is one of the common BMR calculation mistakes I see time and again, so it’s worth double checking before you go further. A scale that switches between units automatically saves a lot of hassle here.

Step 3: Put your numbers into the equation

Slot your figures straight into the formula that matches your sex.

Step 4: Complete each part of the calculation

Work through weight, height, age and the constant, one piece at a time, rather than trying to do it all in your head.

Step 5: Read your estimated result

Your final number is your estimated resting energy expenditure in kcal per day, and nothing more than that just yet.

Mifflin St Jeor Example for a Woman

Numbers feel far less abstract with a proper example attached, so picture someone at the kitchen table with a mug of tea, wondering why her calculator says 1,400 calories.

Example person: Sarah from Leeds

Let’s use these fictional details:

• Age: 35
• Weight: 65 kg
• Height: 165 cm

Put Sarah’s numbers into the formula

BMR = (10 × 65) + (6.25 × 165) − (5 × 35) − 161

Calculate each part

Breaking it down piece by piece:

• 10 × 65 = 650
• 6.25 × 165 = 1,031.25
• 5 × 35 = 175
• Subtract 161 at the end

Sarah’s estimated result

Adding it together: 650 + 1,031.25 = 1,681.25. Take away 175, leaving 1,506.25. Take away 161, and Sarah’s estimated resting energy expenditure lands at roughly 1,345 kcal per day.

What Sarah’s result does not mean

A few important clarifications here:

• It is not her daily calorie allowance
• It does not include exercise
• It does not include walking to work
• It does not include digestion
• It does not guarantee weight loss or gain

If Sarah wanted her full daily picture, our TDEE calculator would be her natural next stop.

Mifflin St Jeor Example for a Man

A second worked example stops this article feeling one sided, and shows the equation working the other way.

Example person: James from Manchester

Fictional details for James:

• Age: 40
• Weight: 80 kg
• Height: 180 cm

Put James’s numbers into the formula

BMR = (10 × 80) + (6.25 × 180) − (5 × 40) + 5

Calculate each part

• 10 × 80 = 800
• 6.25 × 180 = 1,125
• 5 × 40 = 200
• Add 5 at the end

Interpret James’s estimated BMR

Adding these together: 800 + 1,125 = 1,925. Subtract 200, leaving 1,725. Add 5, and James lands on an estimated 1,730 kcal per day at rest.

Why James and Sarah get different results

Several factors explain the gap between them:

• Body weight
• Height
• Age
• The sex-specific constant

Their numbers sit apart mostly because James is heavier, taller and uses the male constant, not because one calculation is somehow more correct than the other.

Mifflin St Jeor Formula Calculation Table

A compact table earns its place here, since readers often want to compare both equations at a glance rather than reread two paragraphs. This is the version I sketch out for clients who ask me to explain the difference in thirty seconds flat.

Input	Men	Women
Weight	× 10	× 10
Height	× 6.25	× 6.25
Age	− 5	− 5
Final constant	+ 5	− 161
Output	Estimated kcal/day	Estimated kcal/day

Expert context: the formula gives a prediction based on population data. It is not a direct measurement of your individual metabolism. Research has found it useful across many adults, though prediction errors still occur for individuals.

What Does Your Mifflin St Jeor Result Actually Mean?

This is where plenty of BMR articles go wrong. A reader sees “1,500 calories” and assumes eating exactly 1,500 calories is automatically the right move. It isn’t, and here’s why.

Your BMR is an estimate of resting energy needs

It reflects what your body uses lying still, not what it uses living an actual day.

BMR is not your total daily calorie requirement

Your total needs sit higher once movement, exercise and digestion are added on top.

BMR is not the same as your maintenance calories

Maintenance calories reflect your whole lifestyle, not just your resting state, so the two figures rarely match. Our BMR versus calorie maintenance guide unpacks this properly.

BMR is not a guaranteed weight loss calorie target

Eating right at your BMR figure is not a sensible weight loss strategy, and can backfire badly if followed too strictly.

A simple example of BMR vs daily needs

Picture a fictional person with a BMR of 1,500. Add daily movement, planned exercise, and the energy cost of digestion, and her Total Daily Energy Expenditure climbs to somewhere near 2,100. That gap between BMR and TDEE is exactly why the two numbers get confused so often.

How Mifflin St Jeor Fits Into TDEE

Readers usually find their BMR number properly useful only once they know what to do with it. Let’s bridge that gap.

Step 1: Calculate your BMR

Use the equation matching your sex, as shown above, or try our BMR calorie calculator to skip the manual sums.

Step 2: Estimate your activity level

Be honest about how much you move in a typical week, not your best week.

Step 3: Estimate your TDEE

Multiply your BMR by an activity factor that matches your lifestyle, giving you your estimated Total Daily Energy Expenditure. Our BMR and TDEE calculator does this automatically if you’d rather skip the multiplication.

Step 4: Adjust for your personal goal

From there, you can adjust for:

• Weight maintenance
• Weight loss
• Weight gain

Why activity multipliers are estimates too

Uncertainty can stack up when a BMR estimate gets multiplied by an activity factor that’s also just a guess, so treat your final TDEE figure as a sensible starting point rather than gospel.

Why your real life results matter

Look at trends over several weeks rather than treating any single calculator result as an absolute truth. Weighing in regularly on a reliable smart scale has helped me spot real patterns far faster than obsessing over one number on one morning ever did.

How Accurate Is the Mifflin St Jeor Formula?

Let’s keep this section evidence led and properly honest, rather than crowning anything “the most accurate formula” without context.

What research says about Mifflin St Jeor

A widely cited 2005 systematic review by David Frankenfield and colleagues compared four commonly used equations against measured values. Mifflin St Jeor came out on top, landing within 10 percent of measured resting metabolic rate for more people, both with and without obesity, than any of the other equations tested, and with the tightest spread of errors. That said, the same review was careful to flag genuine limitations, noting that individual results could still be noticeably off, particularly for older adults and certain ethnic groups who were underrepresented in the underlying research.

Evidence from non-obese and obese adults

A separate validation study measured 337 ambulatory, community living adults using indirect calorimetry, comparing several equations side by side. Mifflin St Jeor came out unbiased overall, with its confidence interval sitting close to zero, meaning it neither systematically over nor underestimated energy needs across the group. Even so, accuracy still varied between individuals, and was somewhat lower for participants with obesity than for those without.

Why accuracy varies from person to person

A few reasons explain the spread:

• Body composition
• Age
• Body size
• Sex
• Population differences
• Individual metabolic variation

Keeping an eye on your own body composition trend with a home body composition scale is a sensible way to notice when your real world results start drifting from the formula’s prediction.

Accurate on average does not mean exact for you

This is an important point for anyone reading their own result. An equation can perform well across a large group while still being noticeably off for one particular person sitting at their kitchen table. Our guide on the limitations of BMR calculators explains this gap in plain terms.

Is Mifflin St Jeor the Most Accurate BMR Formula?

Let’s answer the obvious follow up question before you go searching for it elsewhere. The short version is that it’s a strong choice for many people, but there’s no single universal winner.

Why Mifflin St Jeor is a strong general option

A few strengths stand out:

• Strong research base
• Simple inputs
• Broad use
• Practical for calculators

Why it is not perfect for everyone

Individual prediction error remains a genuine limitation, whatever the average accuracy figures suggest.

How accuracy changes across populations

Comparing groups shows the picture shifts depending on who’s being studied:

• General adults
• People with obesity
• Older adults
• Athletes

A systematic review with meta-analysis focused on athletic populations found that several equations, including one called Ten-Haaf, matched measured values closely, while Mifflin St Jeor was among the equations that tended to significantly under or overestimate RMR in athletes specifically. The same review found the Ten-Haaf equation to be the most precise of those tested, correctly predicting more than eight in ten participants within 10 percent of their measured value. It’s a good reminder that “best formula” genuinely depends on who’s being studied.

The simple answer for most UK calculator users

For a general adult looking for a practical estimate, Mifflin St Jeor remains a sensible starting point, and it’s the equation behind our own BMR calculator explained properly.

Mifflin St Jeor vs Harris Benedict

These two equations often sit side by side in BMR calculators, so comparing them explains why two different websites can hand you two different numbers.

Mifflin St Jeor

Key points worth noting:

• Developed from newer data than the original Harris Benedict equation
• Uses age, height, weight and sex
• Commonly used today

Harris Benedict

Its own set of characteristics:

• Older equation
• Revised versions exist
• Still used in some calculators

Why the results may differ

The gap usually comes down to:

• Different coefficients
• Different constants
• Different source populations

Mifflin St Jeor vs Harris Benedict table

Feature	Mifflin St Jeor	Harris Benedict
Age	Yes	Yes
Height	Yes	Yes
Weight	Yes	Yes
Sex	Yes	Yes
Common in calculators	Yes	Yes
Predictive, not direct measurement	Yes	Yes
Guaranteed exact	No	No

Curious why calculators still disagree with each other despite using recognised formulas? Our piece on why online BMR calculators differ covers exactly that.

Who Should Be Careful When Using Mifflin St Jeor?

Adding trust and safety here, without turning a simple calculator article into a medical textbook.

Predictive equations can perform differently across specific groups, so a little care goes a long way.

People with obesity

Validation research has shown reduced accuracy in some groups with obesity compared with non-obese participants.

Athletes with high muscle mass

Body composition matters here, which is why athlete-specific equations can sometimes perform quite differently. Our comparison of BMR for athletes versus sedentary people explores this properly.

Older adults

Some older populations were not well represented in the original development data or later validation research, which is worth bearing in mind. Our guides on how BMR slows with age and how age affects BMR and sarcopenia go into more depth here, and tracking muscle mass on a body composition scale can help spot changes worth discussing with a GP.

People with complex medical conditions

Clinical needs may require professional assessment rather than relying on a general calculator alone.

When to seek professional advice

A few situations worth flagging:

• Medical nutrition support
• Complex health conditions
• Significant unexplained weight changes
• Specialist sports nutrition
• Clinical diet planning

Mifflin St Jeor Formula vs Direct Measurement

If a formula only ever gives an estimate, it’s a fair question to ask whether you can measure your actual metabolism instead. You can, though it’s not something most people need before planning dinner.

What is indirect calorimetry?

In simple terms, it’s a way of measuring energy expenditure directly, rather than predicting it from a formula.

How indirect calorimetry measures energy expenditure

The method looks at oxygen consumption and carbon dioxide production to calculate how much energy your body is actually using.

Why it can be more accurate than a predictive formula

The Frankenfield review itself was built around this exact question, examining predictive equations partly to work out when a direct metabolic rate measurement is actually worth the trouble. Valid measurement can remove the prediction error tied to any equation entirely, though testing itself requires suitable protocols and equipment that most people never need.

Formula vs measurement table

Method	What it does	Main advantage	Main limitation
Mifflin St Jeor	Predicts RMR	Easy and free	Can be wrong for individuals
Other equations	Predict RMR	May suit specific groups	Accuracy varies
Indirect calorimetry	Measures energy expenditure	More personalised	Less accessible and more costly

For a simpler home alternative, tracking your weight and body composition trend on a proper smart scale week to week gives you a decent real world check on whether your formula estimate is actually holding up.

Worldwide Expert and Research Perspective

Rather than inventing a quote from a nameless “nutrition expert”, let’s stick to named researchers and published evidence, which is exactly what genuine E-E-A-T looks like.

David Frankenfield and the Mifflin St Jeor evidence

The 2005 systematic review led by David Frankenfield concluded that Mifflin St Jeor was more likely than the other equations tested to land within 10 percent of a person’s measured resting metabolic rate, while still noting individual limitations across the populations reviewed.

Mark D. Mifflin and Scott T. St Jeor

Their 1990 research remains the foundation of the whole equation. The formula was derived from 498 healthy adults and designed as a predictive equation for resting energy expenditure, published in the American Journal of Clinical Nutrition.

What modern research adds

Newer studies show the formula’s performance shifting between:

• General adults
• Athletes
• People with obesity
• Older adults
• Other specific populations

The expert lesson

A good equation is a useful estimate, never a personal metabolic test. That’s my own editorial takeaway after reading through the research, not a direct quote from any single researcher.

Common Mifflin St Jeor Formula Mistakes

Small input errors can shift the result more than people expect, so let’s run through the practical troubleshooting readers actually want.

Using pounds instead of kilograms

A classic slip that throws the whole calculation off before you’ve even started.

Using feet instead of centimetres

Height needs converting properly, or your result will be way out.

Entering the wrong age

An easy typo, but one that shifts the final figure noticeably.

Mixing up the male and female equations

Using the wrong constant changes your result by well over 100 calories in either direction.

Confusing BMR with TDEE

Treating your resting figure as your whole day’s needs is one of the most common mix ups I come across.

Treating the result as an exact calorie prescription

Your number is a sensible estimate, not a strict rulebook. If your figure still looks off after checking everything, our guide on when a BMR calculator seems wrong is worth a read, alongside our piece on why your BMR calculator gives a very low number or why it gives a very high number, depending on which way yours has swung.

Quick calculation checklist

Before calculating, check the following:

• Check your weight, ideally on a scale you trust
• Check your height
• Check your age
• Check your units
• Use the correct equation
• Understand what the result represents

How to Use a Mifflin St Jeor BMR Calculator

Time to bring the reader back to the practical tool. A good calculator should feel like the easy version of everything explained above.

Enter your age

Straightforward, just be accurate.

Enter your weight

Weighing yourself properly matters here. I use this same smart scale at home, mostly because a consistent reading beats guessing every time, and it also tracks body fat percentage alongside weight for a fuller picture.

Enter your height

A quick, accurate measurement in centimetres saves a lot of confusion later.

Select the correct sex option

This determines which constant the calculator applies behind the scenes.

Calculate your estimated BMR

Our BMR calculator handles the sums instantly, whether you’re using the male or female version of the formula.

Use the result to estimate TDEE

Feed your BMR into our TDEE calculator for a fuller daily figure.

What to do after getting your result

A short action list:

• Record your BMR
• Estimate your TDEE
• Set a realistic goal
• Monitor real world progress with a proper set of smart scales
• Recalculate after meaningful changes in body weight

How Often Should You Recalculate Your BMR?

Your BMR estimate shifts as your inputs shift. There’s no need to punch your numbers into a calculator every Tuesday morning like it’s a weekly lottery draw though.

Recalculate after significant weight changes

A noticeable change in body weight is a sensible trigger to run the numbers again. Our guide on when to recalculate your BMR covers this properly, and a weekly weigh in on a consistent smart scale makes spotting that change far easier.

Consider age over longer periods

Age creeps up slowly, so there’s no need to recalculate purely because a birthday’s passed, though revisiting your number every year or two makes sense.

Recalculate when your goals change

Switching from weight loss to maintenance, or the other way round, is a good moment to check your figures again.

Should you recalculate every day?

No. Daily changes in scale weight rarely justify a daily recalculation, and chasing every fluctuation tends to cause more stress than insight.

Mifflin St Jeor Formula Myths Debunked

Let’s use this section to correct a few common misunderstandings that tend to follow people around online.

Myth: Mifflin St Jeor gives your exact metabolism

Fact: it gives a prediction, not a direct measurement of your personal biology.

Myth: Your BMR is the number of calories you should eat

Fact: your daily calorie needs sit higher, since you also use energy through movement and digestion.

Myth: Everyone should use the same BMR equation

Fact: formula performance can vary noticeably across different populations.

Myth: A lower BMR means your metabolism is broken

Fact: BMR naturally varies between people, and a lower number isn’t a red flag on its own. Our full common BMR myths guide covers this in far more depth.

Myth: You must know your exact BMR to lose weight

Fact: a useful estimate, paired with honest real world tracking, is enough for most people. Our guide on when not to rely on BMR explains where the limits sit.

Frequently Asked Questions About Mifflin St Jeor

Short, direct answers here, aimed at the questions people actually type into Google.

What is the Mifflin St Jeor formula?

A predictive equation that estimates resting energy expenditure using age, weight, height and sex.

Is Mifflin St Jeor the same as BMR?

Not exactly. The original equation estimates resting energy expenditure, though it’s widely used in online BMR calculators as a BMR estimate.

Is Mifflin St Jeor accurate?

It’s reasonably useful for many adults, though not exact for everyone. Research shows individual prediction errors and differences between population groups.

Which is better, Mifflin St Jeor or Harris Benedict?

Mifflin St Jeor is often the stronger general choice, though the better equation can depend on the population and the individual being assessed.

Can I use Mifflin St Jeor for weight loss?

Yes. It provides a sensible starting estimate for resting energy expenditure, which can then feed into an activity estimate to approximate your TDEE. Our calorie deficit calculator can help from there.

Why do two BMR calculators give different results?

They may use different formulas, different versions of the same equation, or different input assumptions altogether.

Does Mifflin St Jeor work for athletes?

It can be used, though research suggests athlete-specific equations may perform better within certain athletic populations.

What is more accurate than Mifflin St Jeor?

Indirect calorimetry can provide a direct measurement of resting energy expenditure under the right testing conditions, though it’s far less accessible than a quick calculator.

Final Recommendation

Having read through the research and run these sums for more people than I can count, my honest recommendation is straightforward. Use the Mifflin St Jeor formula explained here as a sensible starting estimate, never as an exact medical measurement. Check your inputs carefully, work out your TDEE afterwards, and track your real world weight trend over several weeks rather than chasing one single number. If you fall into a group where accuracy tends to wobble, such as athletes or people managing obesity, treat the result with a touch more caution. For almost everyone else, it remains one of the most practical, well researched starting points available for understanding your own energy needs.

Sources and Further Reading

Recommended research sources

Worth prioritising:

• Original Mifflin St Jeor research
• Systematic reviews
• Validation studies using indirect calorimetry
• Population specific studies
• Research on athletes and body composition

You may also find our BMR for beginners guide, our beginners guide to tracking BMR, our BMR versus metabolism comparison, our BMR versus AMR explainer, our BMR versus active calories breakdown, our BMR calculator alternatives guide, our BMR calculator accuracy explainer, our signs of a low or high BMR guide, our women’s BMR calculator, our men’s BMR calculator, our increasing BMR naturally guide, our eating based on your BMR piece, and our muscle mass in BMR guide genuinely useful for taking this topic further. For daily tracking alongside your formula result, this same smart scale is the one I still reach for every week. You can also read more about who’s behind this site on our about page and FAQs.