Mifflin-St Jeor vs Harris-Benedict: which equation to use

When you look up how to calculate basal metabolic rate, you will find two equations that everyone seems to have heard of. One dates from 1918. The other replaced it in 1990. This guide covers what actually changed, how much the choice affects your number, and why this site uses the newer one.

What the two equations have in common

Both take the same basic inputs — weight, height, age and sex — and both return an estimate of the calories your body would burn at rest. Neither measures anything. Both are statistical models fitted to groups of people, which means the result is a best guess for an average person, and a worse guess the further you are from average.

Why 1990 replaced 1918

The Harris-Benedict equation was derived in 1918 from a small group of men, and it systematically overestimates resting burn in most people today. Mifflin and St Jeor re-derived the relationship in 1990 using a larger and more representative sample, and the newer equation is the standard recommendation since.

This site uses Mifflin-St Jeor throughout, which is why the TDEE calculator, the calorie calculator, and every target derived from them all rest on the same number.

Does the choice actually matter?

Less than most comparisons imply. The two equations usually land within a few percent of each other for a healthy adult of average build, and both are then multiplied by the same activity factor — which, as the TDEE guide explains, is the weaker part of the estimate anyway.

Where the equations are weakest

Both are fitted to average adults, and they degrade as you move away from that. The error is larger for very muscular people, because muscle is metabolically active and the equations do not know how much of a given weight is muscle. It is also larger for very light people, and it is generally less accurate in people over 60, which is why the calculators here cap age at 80 and why anyone outside the range the equation was fitted to should treat the output as rough.

Neither equation accounts for thyroid function, illness, medications, or pregnancy. Those are all things that move resting burn substantially, and all of them are things a published formula cannot see. If any of them apply to you, a number from this page is not a useful input to a plan.

What this means in practice

Use the TDEE calculator for what it is good for: a starting estimate, within about 10%, that you then correct with real data. Spend your effort on the activity multiplier rather than the equation, and validate the result against your actual weight trend over a few weeks. The formula is the easy part. The feedback loop is what produces a number that is actually true for you.

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