BMR Calculator

Where the equation came from

For most of the last century the default was Harris-Benedict, published in 1919 from measurements on a few hundred subjects who were, on average, leaner and more physically active than people are now. It works, but it tends to run a little high for modern bodies. In 1990 Mifflin, St Jeor, and their colleagues measured resting energy expenditure in roughly 500 healthy adults, deliberately including people across a wide weight range, and fit a simpler equation to that data. Later reviews of the prediction equations landed on Mifflin-St Jeor as the most reliable of the common options for healthy adults, which is why it is the number I lead with here and Harris-Benedict sits beside it for comparison.

A worked example

Take a 30 year old man at 180 pounds and 5 foot 10, which converts to 81.6 kg and 177.8 cm. Ten times the weight gives 816. Multiply height by 6.25 and you get about 1,111. Five times age is 150. So 816 plus 1,111, minus 150, plus the male offset of 5, lands at roughly 1,782 calories a day. The same numbers with the female offset of minus 161 give about 1,616. That gap is mostly a body composition difference baked into the equation, not a rule about individuals.

What the number does not cover

BMR is the cost of being alive and still: heart, lungs, brain, kidneys, liver, temperature, and repair. It does not include walking to the car, fidgeting, standing at work, training, or the energy spent digesting your food, which is a real chunk of your intake. That is why eating at your BMR is not a plan, it is a floor with everything you actually did all day missing from it.

Why it is an estimate

The equation only knows four things about you, and the biggest real driver of resting burn is lean mass, which it can only infer. Two people at the same height, weight, age, and sex can sit a couple of hundred calories apart. Age nudges it down, though the decline is gentler through your working years than the folklore suggests. Treat the output as a well-informed starting estimate, then let a few weeks of real scale data tell you where you actually sit.

What to do with your number

Multiply it by an activity factor to get the number you can plan against, which is what the TDEE calculator does for you. From there, set an intake with the calorie deficit tool and split it with macros. For the whole picture worked through in one place, see how many calories you burn a day. Recalculate after a weight change of ten pounds or so. If your weight is not moving the way the math says it should, adjust to what the scale is telling you, and take questions about your metabolism or any medication that might affect it to a doctor.