BMR and TDEE: Why Every Calculator Gives You a Different Number

Two numbers underpin every calorie recommendation you have ever seen. BMR is what your body burns at complete rest: the cost of keeping cells alive, blood moving and organs running. TDEE is BMR multiplied by an activity factor: everything you actually spend in a day.

Both are estimates. The second is an estimate built on the first.

The three equations

Mifflin-St Jeor (1990): the current default

with w in kg, h in cm, a in years. Derived from 498 subjects measured by indirect calorimetry, and the equation the Academy of Nutrition and Dietetics recommends for non-obese and obese adults alike.

Harris-Benedict (1919, revised 1984): the legacy option

The original dates from 1919 and reflects a population that was leaner and more physically active than today’s. The 1984 Roza-Shizgal revision improved it, but it still tends to overestimate by around 5% relative to measured values.

Katch-McArdle: the best one, if you can use it

This one ignores sex entirely, and rightly so: the sex difference in the other equations is largely a proxy for average differences in body composition. If you know your lean mass, you do not need the proxy. The catch is that it requires a body fat measurement, and a bad body fat estimate makes it worse than Mifflin-St Jeor rather than better.

The same person, three equations

A 35-year-old woman, 165 cm, 68 kg, 28% body fat:

A spread of 62 kcal at rest, about 4.5%. Modest. Then the activity multiplier arrives and amplifies everything.

The activity multiplier is the real problem

The full range spans 963 kcal: 58% of the sedentary figure. And these bands are self-selected from one-line descriptions. Is a nurse on a 12-hour shift "moderately" or "very" active? Does a 40-minute gym session three times a week plus a desk job put you at 1.375 or 1.55? Most people cannot answer that confidently, and the two answers differ by 240 kcal a day.

There is also a well-documented tendency to overestimate. The multiplier is supposed to account for all movement, including the substantial contribution of fidgeting, standing and walking about (non-exercise activity thermogenesis, or NEAT), which varies enormously between individuals and drops sharply when people are in a calorie deficit.

The honest error bar

Validation studies of predictive equations against indirect calorimetry typically find them within 10% of measured RMR for around 70-80% of subjects. Which means:

• Roughly one person in four or five falls outside ±10% even at the BMR stage.
• Add multiplier uncertainty and the realistic band on TDEE is ±15-20%.
• For a nominal 2,000 kcal TDEE, the true figure plausibly sits between 1,600 and 2,400.

Individual metabolic variation is real and not fully explained by the inputs these equations take. Thyroid function, medication, sleep, prior dieting history and genetics all move the number, and none of them appear in the formula.

How to use the number properly

The calculated TDEE is a starting hypothesis, not a measurement. The correct procedure is to test it:

1. Calculate a TDEE using Mifflin-St Jeor and the lowest activity band you can justify.
2. Eat at that level for two to three weeks, tracking intake as accurately as you reasonably can and weighing yourself under consistent conditions.
3. Read the trend, not the readings. Use a weekly average; daily weight swings of 1-2 kg from water, sodium and glycogen are normal and mean nothing.
4. Adjust from what actually happened. If weight is stable, that is your real maintenance, better than any equation. If it moved, adjust by 100-200 kcal and repeat.

After one such cycle you have an empirically derived maintenance figure for your own body. That is worth more than any amount of arguing about which equation to use.

On deficit size

A pound of body fat stores roughly 3,500 kcal, which is where the familiar "500 kcal deficit for 1 lb per week" comes from. It works reasonably at first and then decays, because TDEE falls as you get smaller, both mechanically and through reduced NEAT. Expect the same deficit to yield less over time, and recalculate every 4-5 kg.

Very aggressive deficits tend to backfire: more lean mass lost, worse adherence, sharper drops in spontaneous activity. A deficit of 15-25% below TDEE is the range most practitioners work in.

The BMR calculator and calorie calculator will do the arithmetic. Treat what comes out as the first line of an experiment rather than the last word on your metabolism.