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?

Katch-McArdle is the most accurate when lean body mass is known, because it uses lean mass directly. When you only have height, weight, age and sex, Mifflin-St Jeor (1990) is the modern default. A systematic review by Frankenfield in 2005 found it within ±10 percent of measured RMR more often than any other equation.

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 the share that scales with lean mass. A cut that drops lean mass lowers BMR permanently, which is the mechanism behind the plateau most aggressive deficits hit after the first month.

Does BMR decrease with age?

Adult BMR drops by approximately 1 to 2 percent per decade after age 20, with the largest acceleration after 60. Pontzer and colleagues (Science 2021, n=6,421 across 29 countries) showed that the steepest decline is age-related lean-mass loss, not metabolism itself. Resistance training and adequate protein protect most of the curve.

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. BMR is roughly linear in weight, so the formula tracks the change closely until you start swapping fat for muscle. Once that happens, body fat percentage becomes the input that matters most.

BMR Calculator: Basal Metabolic Rate from Body Fat

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 so it can pick the most accurate equation for you. With body fat measured, Katch-McArdle becomes available and replaces the population approximation with a number built on your actual lean mass.

The result panel shows the BMR number itself plus what it means in context: kcal per hour, share of a sedentary day, 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.

Three ways to estimate BMR

Mifflin and Harris-Benedict use height, weight, age and sex. Katch-McArdle uses lean mass directly and wins whenever body fat is known.

Katch-McArdle370 + 21.6 · LBM370 + 21.6 · 65.6 kg = 1,786 kcal

Mifflin-St Jeor10·kg + 6.25·cm − 5·age + s80 + 1,125 − 150 + 5 = 1,805 kcal (M, 30y, 180cm)

Harris-Benedict88.36 + 13.4·kg + 4.8·cm − 5.68·age88.4 + 1,072 + 864 − 170 = 1,854 kcal (M)

Example numbers come from an 80 kg, 1.80 m, 30-year-old male at 18 percent body fat. The 70 kcal spread across the three formulas is the practical error bar around any BMR estimate.

The 200 kcal that every cut quietly loses

Every kilogram of lean mass burns about 22 kcal a day. Drop two kilograms of muscle during a cut and BMR sinks ~44 kcal. Multiply by activity and a typical day quietly loses 60 to 100 kcal of maintenance, permanently.

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 1.6 to 2.2 g of protein per kilogram of lean mass preserves most of the engine through a moderate deficit. 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 not the goal. It's the receipt. Watching it stay flat across a cut tells you the lean mass underneath stayed flat. That's the difference between a year of progress and a year of redoing the same first month.