TDEE Calculator — Mifflin-St Jeor with Activity Multipliers

The Mifflin-St Jeor formula is your baseline

This calculator uses the Mifflin-St Jeor equation, the current standard for estimating resting metabolic rate and generally more accurate than the older Harris-Benedict equation for modern populations. The formula is:

• Men: BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age + 5
• Women: BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age − 161

TDEE — total daily energy expenditure — is that BMR multiplied by an activity factor, from 1.2 for sedentary up to 1.9 for very active. Worked through for a 30-year-old woman of 65 kg and 165 cm at moderate activity: BMR = 650 + 1031.25 − 150 − 161 = 1370, and TDEE = 1370 × 1.55 = 2124 calories per day.

The activity multiplier is where most of the error lives

BMR is reasonably consistent across a population. The activity multiplier is not, and this is the part worth understanding before you trust the output.

Two people who both honestly choose "moderate" can differ by several hundred calories a day. The bracket is defined by exercise frequency — 3 to 5 days a week — and that description covers an enormous range of actual lives. A desk worker who trains hard three times a week and a nurse working twelve-hour shifts can select the same bucket and burn very different amounts.

The gap is largely non-exercise activity: the walking, standing, fidgeting and general moving you do outside deliberate workouts. It varies enormously between individuals and frequently accounts for more of the daily total than formal exercise does — and an exercise-frequency bucket captures none of it. This is why picking one level higher than your real life is the most common way people end up with a target that quietly does not work.

Treat the number as a hypothesis, not a measurement

The output is a starting point, and the method matters more than the number. Eat at the calculated intake consistently for two to three weeks, then track your weekly average weight rather than daily readings — day-to-day water shifts are large enough to hide the actual trend entirely — and adjust from what you observe.

The readings tell you what the equation could not. If you are losing faster than intended, your real deficit is larger than predicted. If weight holds steady where you expected loss, your true expenditure is lower than the estimate. Reality overrides the formula every time; the formula exists only to give you a sensible place to begin.

Individual variation is significant

These are population averages, and individuals genuinely deviate from them because of differences in body composition, hormones and thyroid function. Prediction equations land within roughly 10% for most healthy adults, and further out for some.

The practical implication is to stop reading the output as a precise figure. If the calculator returns 2100, the honest interpretation is "somewhere in the region of 1900 to 2300, and the trial will tell me where." Carrying that number to the calorie is false precision — the input you gave it, your own bodyweight, was probably not accurate to the gram either.

Safety and scope

This is a general fitness estimate, not medical or dietary advice. The Mifflin-St Jeor equation is not appropriate for children or teenagers, whose energy needs during growth follow different patterns entirely.

Anyone who is pregnant or breastfeeding, or managing a condition that affects metabolism or nutrition — thyroid disease, diabetes, PCOS, a history of disordered eating, or medications that alter appetite or metabolism — should work from clinical guidance rather than a generic calculator. For splitting a calorie target into protein, carbohydrate and fat, see the macro calculator.