What you burn at rest: breathing, circulation, keeping warm. Estimated from weight, height, age and sex, and roughly three quarters of a typical day.
A multiplier from 1.2 (desk) to 1.9 (hard daily training) for everything on top of resting. You judge it yourself, which makes it the softest number in the sum.
BMR × activity factor: what you actually burn in a day, and the number every weight goal is measured from.
Mifflin–St Jeor lands within 10% of measured BMR for about 82% of non-obese adults and 70% of obese adults (Frankenfield 2005), so plan with a range.
kcal a day, for an 80 kg, 180 cm, 35-year-old man. The activity level you pick matters about four times more than the formula.
A lighter body needs less. Recalculate every 4–6 weeks, or every 5 kg of change, not once and never again.
Eat at the target, track your weight, and move the target by about 100 kcal a day if real results disagree with the prediction.
Women carry proportionally more body fat and less lean mass at the same weight and height, so they burn less at rest. The two constants are simply what the original regression found fit the measured data.
Because people count how hard a workout felt, not how many hours a week they are on their feet. Three hard sessions and a desk job the other four days is “lightly active”. Overestimating activity is the commonest reason an estimate runs high.
Within about 10% for most people: meaningful, not exact. Eat at the target for 2–3 weeks, watch the scale, and adjust by roughly 100 kcal a day if the result does not match.
Only if your activity level did not already include that exercise. A level picked because you train regularly already prices the training in; use “build it from your week” if you want exercise counted separately.
Everyday life is a multiple of your BMR (1.2 to 1.5), and each kind of exercise adds minutes × (MET − 1) × body weight, averaged over the week. Subtracting one MET leaves out the resting energy the multiplier already holds.
The 3,500 kcal rule assumes a fixed deficit forever, but a lighter body needs fewer calories, so the deficit quietly shrinks. The realistic line recalculates BMR at every step; published critiques find real results come in below the naive rule, more so the longer it runs.
The same intake with your BMR taken as 10% lower and 10% higher, because that is how far Mifflin–St Jeor is out for about one non-obese adult in five (three in ten if obese). A wide band means the plan is softer than it looks; let the scale say which edge you are on.
Protein first, from your grams-per-kilogram choice (typically 1.6–2.2 for active adults). The remaining calories are split 50/50 between carbohydrate (4 kcal/g) and fat (9 kcal/g).
TDEE is your BMR times an activity multiplier: 1.2 for sedentary (desk job, little exercise), 1.375 for lightly active (light exercise 1–3 days a week), 1.55 for moderately active (moderate exercise 3–5 days), 1.725 for very active (hard exercise 6–7 days) and 1.9 for extra active (a physical job plus daily training). A sedentary BMR of 1,600 kcal becomes a TDEE of 1,920 kcal.
On this scale “moderately active” means moderate exercise about 3–5 days a week, on top of an otherwise ordinary day. If you exercise one to three days a week choose lightly active (1.375); if you train hard six or seven days choose very active (1.725). People tend to overestimate, so when in doubt pick the lower level and adjust after a few weeks of real data.
Mifflin–St Jeor by default, multiplied by the activity factor to give TDEE. You can compare it with Harris–Benedict, and if you know your body fat there is a Katch–McArdle option that works from lean mass instead.
This page has changed in 194 archived releases, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.