What the Numbers Mean
Your BMR is the amount of energy your body burns if you stayed in bed all day. It covers the brain, heart, kidneys and all the tissues that run 24 hours a day. Your TDEE adds the energy cost of movement: walking, exercise, standing, fidgeting and the small amount your body spends digesting food. Eating around your TDEE keeps weight stable. Eating below it loses weight; eating above it gains weight. That is the entire math of body composition.
The Mifflin-St Jeor Equation
Published in 1990, the Mifflin-St Jeor equation replaced the older 1919 Harris-Benedict as the clinical default because it predicts resting metabolic rate more accurately for today's population.
For men: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age + 5
For women: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age - 161
TDEE is then BMR multiplied by an activity factor from 1.2 (sedentary) up to 1.9 (very active).
How to Use This Tool
- Pick metric or imperial units.
- Pick your biological sex, which the formula needs to apply the right constant.
- Enter your age, height and weight.
- Pick an activity level honestly. Most people overestimate. If in doubt, drop one level.
- Read the result card. BMR is the floor, TDEE is maintenance and the loss and gain numbers are typical calibrated deficits and surpluses.
Losing or Gaining Weight
A sustainable rate is roughly 0.5 to 1 percent of body weight per week. Faster than that risks muscle loss on a cut or fat gain on a bulk. The common rule is that 7700 kcal is one kilogram of fat, so a 500 kcal daily deficit is about half a kilogram per week on paper. Real world losses are often slower because the body adapts. Weighing in weekly and adjusting intake is more reliable than any formula.
Limits of the Formula
- BMR formulas are averages. Any individual can be 10 percent higher or lower.
- The activity multiplier is a guess. People who track with a smartwatch should trust actual data over this estimate.
- Body composition matters. Two people at the same weight can have different BMRs depending on muscle mass.
- The equation does not apply to children, pregnant or lactating women, or people with certain medical conditions.
Privacy
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Frequently Asked Questions
What is BMR?
Calories burned at full rest.
What is TDEE?
BMR times an activity factor: your estimated maintenance intake.
Which formula is used?
Mifflin-St Jeor, the modern clinical standard.
How much should I cut to lose weight?
Around 500 kcal per day loses about 0.5 kg per week.
How much to gain muscle?
A 250 to 500 kcal surplus with strength training.
Is the number exact?
No. Expect 10 percent variation for any individual.
Does it apply to kids or pregnancy?
No. Consult a doctor for those cases.