FitForge

BMR Calculator

Your basal metabolic rate is the floor of your calorie needs: what you would burn doing nothing at all. See it from three formulas at once.

Units
Sex
yrs
lb
Height
ft
in
%

BMR (Mifflin-St Jeor)

1,768kcal / day

Calories burned at complete rest

Harris-Benedict

1,844kcal

Revised 1984

Katch-McArdle

–kcal

Needs body fat %

ActivityDaily calories
Sedentary ×1.22,120
Lightly active ×1.3752,430
Moderately active ×1.552,740
Very active ×1.7253,050
Extremely active ×1.93,360

The three BMR formulas

FormulaMenWomen
Mifflin-St Jeor (1990)10W + 6.25H − 5A + 510W + 6.25H − 5A − 161
Harris-Benedict (rev. 1984)13.397W + 4.799H − 5.677A + 88.3629.247W + 3.098H − 4.330A + 447.593
Katch-McArdle370 + 21.6 × lean body mass (kg)

W = weight in kg, H = height in cm, A = age in years.

Mifflin-St Jeor is the default across FitForge because it was derived from a modern population and validated best in independent reviews. Harris-Benedict, first published in 1919, tends to run 5% or so higher. Katch-McArdle ignores sex and age entirely and uses lean mass, which is why a muscular lifter often gets a higher, more realistic number from it.

What moves your BMR

  • Lean mass: the biggest lever you control. Each kilogram of muscle adds roughly 13 kcal a day at rest; organs cost far more per kilogram, which is why the effect of muscle is real but modest.
  • Body size: bigger bodies burn more, which is why BMR falls as you lose weight.
  • Age and sex: built into the formulas, largely because they track differences in lean mass.
  • Long, hard diets: metabolism adapts downward a little beyond what weight loss alone predicts. Diet breaks and higher protein help limit it.

Questions lifters ask

What is BMR?

Basal metabolic rate is the energy your body uses at complete rest, lying still in a warm room after an overnight fast, just to keep your heart, brain, lungs and other organs working. For most adults it is 60–70% of daily calorie burn.

Which BMR formula is the most accurate?

For the general adult population, Mifflin-St Jeor. A 2005 systematic review for the American Dietetic Association found it predicted measured resting metabolism within 10% for more people than Harris-Benedict. If you know your body fat and are lean or muscular, Katch-McArdle is often closer.

Should I eat my BMR to lose weight?

No. BMR is what you burn lying in bed all day. Your real daily burn (TDEE) is higher, and eating at BMR usually means a larger deficit than you need. Plan a cut from your TDEE instead.

Why does BMR drop with age?

Mostly because people lose muscle and gain fat as they age, and muscle costs more to maintain. Lifting weights and eating enough protein slows that decline.

What is the difference between BMR and RMR?

Resting metabolic rate is measured under looser conditions and runs slightly higher than true BMR. Calculators and most people use the terms interchangeably.

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