BMR and TDEE: the Calorie Numbers That Matter

By Longevity Lab · Published: August 23, 2026

Based on peer-reviewed research — full source list at the end of this article. This is educational information, not medical advice.

Basal metabolic rate is what your body spends staying alive at complete rest — breathing, circulating, maintaining temperature and replacing tissue — and it accounts for roughly 60–70% of daily energy use in a typical adult. Total daily energy expenditure (TDEE), total daily energy expenditure, is that figure plus everything else you do. TDEE is the number that determines whether weight moves, and every method of estimating it from a formula carries about ±10% of error for an individual.

The four components of daily energy use

The surprise in this table for most people is the last two rows. Non-exercise activity — walking, standing, fidgeting, the ordinary movement of a day — varies far more between individuals than structured exercise does, and it is not something you can decide to increase by an act of will in the way a gym session is.

It also falls when you eat less. That adaptation is part of why a calculated deficit under-delivers.

Estimating BMR: the Mifflin-St Jeor equation

The best-validated prediction equation for healthy adults is Mifflin-St Jeor, published in 1990:

• Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
• Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

For a 40-year-old woman of 68 kg and 165 cm: (10 × 68) + (6.25 × 165) − (5 × 40) − 161 = 680 + 1031 − 200 − 161 = 1,350 kcal/day.

A systematic comparison by the American Dietetic Association found Mifflin-St Jeor predicted resting metabolic rate within 10% of measured values more often than Harris-Benedict or the other common equations — which also means it is outside 10% for a substantial minority. On 1,350 kcal, ten percent is 135 kcal a day.

From BMR to TDEE

Multiply BMR by an activity factor: 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for heavy, 1.9 for very heavy. Our 1,350 kcal example at 1.375 gives a TDEE of about 1,856 kcal.

These multipliers are coarse and the boundaries between them are not well defined, so the honest reading of “1,856” is “somewhere between about 1,650 and 2,050”. Treat it as a starting point to be corrected by observation, never as a specification.

The correction is straightforward: eat at the estimate for two to three weeks, weigh yourself consistently, and adjust by 100–200 kcal based on what actually happened. Two weeks of your own data beats any equation, because it measures you rather than a population average.

Why body composition drives the number

BMR tracks fat-free mass much more closely than it tracks total weight. Muscle, liver, kidney and brain tissue are metabolically expensive; stored fat is comparatively cheap to maintain.

That is why two adults at 80 kg and the same height can differ by 250–300 kcal a day in resting expenditure — one carries more Fat-free mass (FFM) than the other. It is also why weight loss lowers BMR: you lose some lean tissue along with the fat, and the smaller body costs less to run.

Equations that use lean mass instead of total weight — the Katch-McArdle formula is the common one — are more accurate when you have a reliable body composition measurement, and worse when you do not, because a bad body-fat estimate propagates straight into the result. Body composition assessment gives the fat and lean split from tape measurements, which is enough to try it.

What these numbers are good for

Setting a starting point, and nothing more precise than that. A deficit of 300–500 kcal a day below TDEE is the usual recommendation for fat loss, and it corresponds to roughly 0.3–0.5 kg a week at first, slowing as you get lighter.

They are also useful for the opposite reason — recognizing how small the numbers are. A 45-minute run costs perhaps 400–450 kcal in a 70 kg adult, which is about one large muffin. Energy balance is won on the intake side for most people, and the expenditure side is worth training for every reason except arithmetic.

If you want to see how your metabolic profile compares with your age rather than just its calorie total, Metabolic age from your metabolism — no lab tests converts the same inputs into an estimated metabolic age, which is a more interpretable output than a kilocalorie figure with a 10% error bar. Metabolism as a whole is more than its resting rate.

Frequently asked questions

Sources

Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.