Skip to content
ToolBoxGenie

BMR Calculator

Calculators · Added 15 August 2026

Work out your basal metabolic rate — the energy your body uses at complete rest — using all three of the standard equations at once. They disagree with each other by a couple of hundred calories, and showing that spread is more honest than presenting a single figure as if it were measured.

Units
Sex
kg
cm
yrs
%

Only needed for the Katch-McArdle equation, which works from lean mass

How to use the bmr calculator

  1. 1Choose metric or imperial units.
  2. 2Enter your sex, age, height and weight.
  3. 3Optionally add a body fat percentage, which unlocks the Katch-McArdle equation.
  4. 4Read the BMR from each equation and the gap between them.
  5. 5To find what you actually need in a day, multiply by an activity factor using the TDEE calculator.

Examples

A 30-year-old man

Input
75 kg, 178 cm, 30 years
Result
Mifflin-St Jeor 1,717 kcal · Harris-Benedict 1,779 kcal

The two equations differ by about 60 kcal here.

A 30-year-old woman

Input
62 kg, 165 cm, 30 years
Result
Mifflin-St Jeor 1,340 kcal

The equations use different constants by sex, largely reflecting typical differences in lean mass.

With body fat known

Input
75 kg, 178 cm, 30 years, 18% body fat
Result
Katch-McArdle 1,698 kcal

Katch-McArdle ignores height and age entirely, working only from lean body mass.

About the bmr calculator

Where these equations came from

The original work is Harris and Benedict's, published in 1919 at the Carnegie Institution. They measured the resting metabolism of a few hundred subjects and fitted equations relating it to height, weight, age and sex. Those equations dominated for most of a century and were revised by Roza and Shizgal in 1984 with better data.

Mifflin and St Jeor published their equation in 1990, working from a more representative modern sample. Subsequent validation studies comparing predictions against indirect calorimetry — the actual measurement, done by analysing inhaled and exhaled gases — generally found it the most accurate of the general-purpose equations, and it has become the standard recommendation.

Katch-McArdle takes a different approach entirely, predicting from lean body mass rather than total weight. This is theoretically better because fat tissue is metabolically much less active than muscle, so two people of identical weight and very different composition have genuinely different resting rates. Its weakness is practical: it needs a body fat percentage, and consumer methods of measuring that are themselves imprecise.

What drives metabolic rate, and what does not

The largest determinant is simply body size, and specifically lean mass. More tissue requires more energy to maintain, and muscle requires considerably more than fat. This is why larger people have higher BMRs and why the equations are dominated by the weight term.

Age matters, though less dramatically than folk wisdom suggests. Recent large-scale work using doubly labelled water found that metabolic rate per unit of lean mass is remarkably stable from about twenty to sixty, and that much of the apparent age-related slowdown is really loss of muscle mass rather than a change in metabolism itself. That reframes the practical advice considerably: maintaining muscle addresses more of the problem than eating less does.

Things commonly credited with large effects mostly have small ones. Meal frequency does not meaningfully change total energy expenditure. Specific foods and drinks produce measurable but tiny and short-lived increases. Thyroid disorders genuinely do shift metabolic rate substantially, which is a medical matter rather than a dietary one, and is one reason a person whose numbers persistently make no sense should be talking to a doctor rather than to a calculator.

Reading an estimate honestly

This page deliberately shows several numbers rather than one, because a single figure invites more confidence than the method supports. Two equations built by careful researchers on real data disagree by several percent for the same person, and both may be several percent from that person's measured value.

The practical implication is not that the estimate is useless — it is a much better starting point than a guess — but that it should be treated as a hypothesis to test. Eat at a calculated level for several weeks, track weight honestly against the trend rather than day to day, and adjust based on what happens. The feedback from your own body outperforms any equation.

It is also worth remembering what these numbers were built for. Population regressions predict group means, and clinical use of them — in hospital nutrition planning, for instance — comes with professional judgement attached. Nothing here diagnoses anything, and any decision about eating that is driven by a health condition belongs with a qualified professional rather than with an online calculator.

Frequently asked questions

What is BMR, exactly?
The energy your body uses at complete rest to keep itself running — breathing, circulation, cell repair, brain function and maintaining temperature. It is measured under strict conditions: fully rested, fasted, at a comfortable temperature, having not exercised. For most people it accounts for roughly 60 to 70 percent of total daily energy use, which is why it is the starting point for any calorie calculation.
Which equation should I use?
Mifflin-St Jeor is the usual default. Published in 1990 and validated against indirect calorimetry, it is generally the most accurate of the general-purpose equations for a modern adult population. Harris-Benedict, even in its 1984 revision, tends to read a little high. Katch-McArdle is the most accurate of the three if you genuinely know your body fat percentage, because lean mass drives metabolic rate more than total weight does — but a guessed body fat figure makes it worse than the others, not better.
Why do the equations disagree?
Because they are regressions fitted to different populations at different times. Each was derived by measuring a group of people and finding the formula that best predicted the group. They agree closely on average and diverge on individuals, particularly at the extremes of body composition. The gap between them is a reasonable indication of how precise any single one can be for a person it was not fitted to.
How accurate is a calculated BMR for me personally?
It is an estimate with real uncertainty. Studies of these equations typically find they predict measured BMR within about 10% for most people, which on a 1,700 kcal BMR is a range of roughly 170 kcal either way. Individual variation comes from body composition, thyroid function, genetics, medication and adaptation to previous dieting — none of which the inputs capture. Use the number as a starting point and adjust against what actually happens.
Should I eat my BMR?
No — BMR is what your body uses at complete rest, not what you need in a day. Anyone who gets out of bed uses more. Eating at BMR level would be a substantial deficit for most people. The figure you want for daily intake is TDEE, which is BMR multiplied by an activity factor. This calculator is general information rather than dietary advice; for guidance on your own intake, speak to a doctor or registered dietitian.

Related tools