What is BMR and how is it different from RMR?

Basal metabolic rate is the energy your body spends sustaining life at complete rest, measured after an overnight fast in a thermoneutral environment. Resting metabolic rate is the practical version of the same measurement and runs roughly 10 percent higher because it tolerates ordinary daytime conditions. The literature usually treats them as interchangeable in the context of calorie planning.

Which BMR formula is most accurate?

Mifflin-St Jeor (1990). A systematic review by Frankenfield in 2005 found it within ±10 percent of measured RMR more often than any other equation it compared, so it stays the headline number here even when you enter body fat. Katch-McArdle uses lean mass instead of weight. In people who train regularly and had lean mass measured in a lab, it came closer than Mifflin, which tends to underestimate them. With body fat estimated from a tape, calipers or a photo, treat Katch-McArdle as a comparison.

Why does my BMR matter for fat loss?

BMR is roughly 60 to 70 percent of total daily energy expenditure for a sedentary adult, and in real bodies it tracks lean mass closely. A cut that drops lean mass lowers resting burn until the tissue is rebuilt. That is one reason aggressive deficits stall; a smaller body and adaptive drops in energy use add to it.

Does BMR decrease with age?

When body composition is held constant, metabolic rate is largely stable from age 20 to 60. Pontzer and colleagues (Science 2021, n=6,421 across 29 countries) showed that the apparent pre-60 BMR decline is almost entirely explained by age-related lean-mass loss, not by metabolism itself, with the steepest true metabolic decline occurring only after 60. Resistance training and adequate protein protect most of the curve by keeping lean mass high.

Should I eat at my BMR?

No. Eating at BMR is the calorie level your body burns sleeping. Even a sedentary day adds physical activity, NEAT and the thermic effect of food, lifting maintenance to roughly 1.2 to 1.4× BMR. Plug BMR into the TDEE calculator with your real activity level and treat that number as the starting line.

How often should I recompute my BMR?

Every time weight changes by 3 to 5 kg, or every 3 months during an active phase. The formula uses weight, so it cannot see fat swapped for muscle at a stable weight. Body fat adds a lean-mass comparison, and your weight trend is the better check.

BMR Calculator: Basal Metabolic Rate and Calories

Three ways to estimate BMR

Mifflin and Harris-Benedict use height, weight, age and sex. Katch-McArdle uses lean mass instead of body weight, so it is only as accurate as the body-fat figure behind it.

Mifflin-St Jeor

Men: BMR = 10 × weight (kg) + 6.25 × height (cm) - 5 × age (years) + 5

Women: BMR = 10 × weight (kg) + 6.25 × height (cm) - 5 × age (years) - 161

800 + 1,125 − 150 + 5 = 1,780 kcal (M, 30y, 180cm)

Source: Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr, 1990.

Katch-McArdle370 + 21.6 · LBM370 + 21.6 · 65.6 kg = 1,787 kcal

Source: Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. Am J Clin Nutr, 1991;54(6):963-969.

Harris-Benedict (1984 revision, men)88.36 + 13.4·kg + 4.8·cm − 5.68·age88.4 + 1,072 + 864 − 170 = 1,854 kcal (M)

Source: Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr, 1984;40(1):168-182.

Example numbers come from an 80 kg, 1.80 m, 30-year-old male at 18 percent body fat. The three formulas land within about 75 kcal of each other here, but the real uncertainty is larger: the best-validated equation predicts measured resting rate within 10 percent for most adults, about 180 kcal in this example.

BMR is the energy cost of being alive

Basal metabolic rate is the number of calories your body spends sustaining life at complete rest. Heart, brain, liver, kidneys, lungs, cell turnover. About 60 to 70 percent of total daily energy expenditure for a sedentary adult, scaling almost linearly with lean mass.

The calculator above asks for weight, height, age, sex and body fat. Your BMR comes from Mifflin-St Jeor, the equation best validated in adults. Body fat adds a Katch-McArdle comparison built on your estimated lean mass, and fills in the lean-mass figures in the result panel.

The result panel shows the BMR number itself plus what it means in context: kcal per hour, lean mass in kg, kcal per kilogram of lean mass, food equivalents, and how the number drifts with age while everything else is held still.

Where BMR sits inside total daily energy expenditure

Typical shares for a sedentary-to-moderately-active adult. Athletes push the activity slice into the 35 to 50 percent range.

BMR

≈ 65%

Resting burn. Scales with lean mass. The largest, most stable slice.

NEAT

≈ 15%

Steps, fidgeting, posture, chores. Largest hidden variable between people.

TEF

≈ 10%

Digesting food. Protein costs 20 to 30 percent of its calories.

Workouts

≈ 5%

Direct training cost plus the small post-exercise tail.

Shares aligned with McMurray et al., Med Sci Sports Exerc 2014, and Westerterp, Nutr Metab 2004. Athlete activity range from Westerterp Annu Rev Nutr 2017 doubly-labelled water meta-analysis.

The 200 kcal that every cut quietly loses

A kilogram of skeletal muscle burns roughly 13 kcal a day at rest. Lose two kilograms of muscle during a cut and resting burn drops about 26 kcal, or roughly 35 to 45 kcal of daily maintenance once activity is multiplied in, until you rebuild it.

Most calorie calculators stop at the BMR estimate and never close the loop. Eat at the deficit, lose weight, and a chunk of that weight is the engine that was setting BMR. The next plateau hits sooner. The next deficit has to go deeper to keep moving.

The fix is structural. Resistance training plus enough protein preserves most of the engine through a moderate deficit. For most people who train, 1.6 to 2.2 g of protein per kilogram of body weight is a well-supported range (Morton 2018, in people who were not in a calorie deficit). For lean, trained people dieting for a contest, Helms 2014 recommends 2.3 to 3.1 g per kilogram of lean mass. Helms 2014 and Garthe 2011 both show that the slower the cut, the more of the loss is fat and the less of it is BMR.

BMR is an estimate of resting energy needs, not a measurement of body composition. An unchanged result does not establish that muscle was retained. Review weight, measurements and training performance together.