How it works
Mifflin–St Jeor: BMR = 10·kg + 6.25·cm − 5·age + s (s = +5 men, −161 women); TDEE = BMR × activity factor
Mifflin–St Jeor builds the estimate from three measurements and a sex constant: 10 times weight in kilograms, plus 6.25 times height in centimeters, minus 5 times age in years, then +5 for men or −161 for women. For a 30-year-old man at 80 kg and 180 cm that stacks up to 800 + 1125 − 150 + 5 = 1,780 kcal per day. The tool prints the revised Harris–Benedict number beside it — the two normally sit within a few percent of each other, and Mifflin–St Jeor takes the headline spot because it validates better on modern populations. TDEE then scales the BMR by your chosen activity level, from 1.2 for a sedentary week up to 1.9 for heavy training loads. Pound and inch inputs are converted to metric before any of this runs.
Sources
- Mifflin–St Jeor equation Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247.
- Harris–Benedict, revised 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.
- Activity multipliers (PAL) Total daily energy expenditure is estimated as BMR × a physical-activity level (PAL): ~1.2 sedentary to ~1.9 very active, a convention used widely in dietetics.
FAQ
What exactly does the BMR number represent?
The energy cost of simply existing — heartbeat, breathing, tissue repair — across 24 hours of full rest. It comes from the Mifflin–St Jeor coefficients (10 per kg of weight, 6.25 per cm of height, −5 per year of age, then +5 or −161 by sex), not from anything a watch records.
How do I pick the right activity level?
Match it to a typical week, not your biggest one. The five options run from sedentary (×1.2) through lightly, moderately, and fully active, up to very active (×1.9) for hard daily training or physical work. Runners logging four to six sessions a week usually land in the middle two choices.
Why show two formulas for the same thing?
As a sanity check. The 1990 Mifflin–St Jeor equation leads because it predicts resting energy best in modern validation work; the 1984 revised Harris–Benedict rides along as a second opinion. When the two agree within a few percent — which is typical — the ballpark is trustworthy.
Do I have to enter metric values?
No — weight accepts kilograms or pounds and height accepts centimeters or inches, and the equation always runs on the converted metric values. An entry of 176 lb and 71 in produces essentially the same BMR as 80 kg and 180 cm.
How far off can the estimate be for me personally?
Formulas like these fit population averages, and real metabolisms scatter around them — 10% either way is common, more if you carry unusual muscle mass or are very lean. Watch your weight trend over a few weeks against your intake; that feedback beats any equation.
Is eating under my BMR a fast way to lose weight?
It is a fast way to wreck training. BMR is what your body needs at rest, and runners add workout costs on top, so any deficit should come off the TDEE instead — and stay modest. Persistent intake below BMR risks hormonal and bone problems; involve a dietitian before cutting that deep.
Mifflin–St Jeor and Harris–Benedict are statistical fits, not metabolic measurements — your true resting burn can differ, sometimes by 10% or more. Nothing here supports aggressive calorie cutting, and none of it replaces individualized guidance from a doctor or registered dietitian.