Almost every calorie calculator starts by estimating your basal metabolic rate (BMR) — the energy your body burns at complete rest. But there is more than one way to estimate it, and the two formulas you will meet most often are the Harris-Benedict equation and the Mifflin-St Jeor equation. They look similar, they use the same inputs, yet they can disagree by a couple of hundred calories. This guide explains where each comes from, how they differ, and which one you should trust.
What both formulas are trying to do
Both equations predict resting energy expenditure from four simple inputs: weight, height, age, and sex. Neither measures your metabolism directly — that requires lab equipment called indirect calorimetry. Instead, they are statistical models built by measuring real people and finding the equation that best fits the data. That is the key idea: a BMR formula is a population average applied to you, not a personal readout.
You can run either calculation on this site with the BMR calculator and the Harris-Benedict calculator.
The Harris-Benedict equation (1919, revised 1984)
The Harris-Benedict equation is the original. It was first published in 1919 and revised by Roza and Shizgal in 1984. For decades it was the default in nutrition textbooks and software.
The revised version estimates BMR like this:
- Men: 88.36 + (13.40 × weight in kg) + (4.80 × height in cm) − (5.68 × age)
- Women: 447.6 + (9.25 × weight in kg) + (3.10 × height in cm) − (4.33 × age)
The structure is intuitive: more weight and height raise the number, more age lowers it. The Harris-Benedict equation works well, but it was derived from a relatively small sample of lean, early-20th-century adults. Because average body composition has changed since then, it tends to overestimate BMR for many modern people, particularly those carrying more body fat.
The Mifflin-St Jeor equation (1990)
The Mifflin-St Jeor equation was published in 1990 in the American Journal of Clinical Nutrition. It was built on a larger, more contemporary sample and quickly became the new standard.
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
The Academy of Nutrition and Dietetics reviewed the major predictive equations and concluded that Mifflin-St Jeor is the most reliable for estimating resting energy in healthy, non-critically-ill adults. That is why it is the default formula behind our TDEE calculator.
Head-to-head: how they differ
For a typical adult the two formulas usually land within 5 percent of each other, but the gap is not random:
- Harris-Benedict tends to read higher. Its older dataset skews the estimate upward for the average modern body, sometimes by 100 to 200 calories.
- Mifflin-St Jeor tends to be more conservative and more accurate across a wider range of body types, which matters most if your goal is fat loss.
- Both lose accuracy at the extremes — very lean athletes, people with obesity, and the elderly are all harder to model, because neither equation accounts for body composition.
A worked example makes it concrete. Take a 35-year-old woman, 70 kg, 165 cm. Mifflin-St Jeor gives roughly 1,396 calories. Harris-Benedict gives roughly 1,449 calories. That ~53-calorie difference is small here, but it grows with body size, and it compounds once you multiply by an activity factor to get total daily energy.
So which one should you use?
For most people, the answer is Mifflin-St Jeor, because it is the better-validated formula for today's population and it is less likely to overestimate. That makes it the safer foundation for a calorie target.
There are still good reasons to know Harris-Benedict:
- It remains common in older apps, textbooks, and clinical references, so you may need to reconcile a number a coach or tool gives you.
- Comparing both can be useful as a range rather than a single point — if both formulas agree, you can be more confident in your estimate.
If you know your body fat percentage, a third option — the Katch-McArdle formula — can be more accurate still, because it is based on lean body mass rather than total weight. It is the best choice for lean, muscular people whom weight-based formulas tend to underestimate.
From BMR to calories you actually use
Whichever formula you pick, BMR is only the starting point. To get the number you plan meals around, you multiply BMR by an activity factor to estimate total daily energy expenditure (TDEE). If that distinction is fuzzy, our guide on TDEE vs BMR breaks it down, and the calorie deficit calculator turns your TDEE into a safe weight-loss target.
The most important habit is to treat any estimate as a starting point: follow it for two to four weeks, watch your weight trend, and adjust based on real results rather than the formula alone.
Frequently asked questions
Is Mifflin-St Jeor more accurate than Harris-Benedict?
For healthy modern adults, yes. The Academy of Nutrition and Dietetics identifies Mifflin-St Jeor as the most accurate of the common predictive equations, while the older Harris-Benedict equation tends to overestimate resting energy for the average person today.
Why do the two formulas give me different numbers?
They were built from different populations using different statistics. Harris-Benedict's data came from lean early-1900s adults, so it usually reads higher than the more contemporary Mifflin-St Jeor equation. A difference of 100 to 200 calories is normal.
Which formula should I use to lose weight?
Use Mifflin-St Jeor as your baseline because it is less likely to overestimate, then build a moderate deficit from your TDEE. If you have a reliable body fat measurement, the Katch-McArdle formula based on lean body mass can be even more precise.
Are BMR formulas accurate at all?
They are good estimates, typically within about 10 percent for healthy adults, but they are not measurements. Factors like muscle mass, genetics, and hormones affect real metabolism in ways no four-input formula can capture.
This article is for general educational purposes only and is not medical advice. Consult a qualified healthcare professional before making significant changes to your diet or exercise routine.