How the Mifflin-St Jeor equation works
Basal metabolic rate (BMR) is the energy you burn at complete rest. Mifflin-St Jeor (1990) predicts it from weight, height, age and sex: BMR = 10 x weight in kg + 6.25 x height in cm - 5 x age, plus 5 for men or minus 161 for women.
Multiply BMR by an activity factor for total daily energy expenditure (TDEE): 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 active, 1.9 hard training most days. TDEE holds your weight steady.
Maintain, lose, or gain
Eat at TDEE to maintain. Eat about 20% below to lose weight while keeping lean mass. Eat about 15% above to build muscle or support a heavy block.
Large deficits cause low energy availability, which hurts recovery, hormones, bone health and immunity. Eat most around your hardest sessions.
Why this is a starting point, not a verdict
Identical stats can differ by a few hundred calories a day, and people overestimate their activity factor. Adjust from how your weight trends over two to three weeks.
Worked example
A 35-year-old man, 70 kg and 175 cm, training at a moderate level:
| BMR (Mifflin-St Jeor) | 1,624 kcal/day |
| TDEE (x 1.55 moderate) | 2,517 kcal/day |
| Lose weight (about 20% deficit) | 2,013 kcal/day |
| Gain / build (about 15% surplus) | 2,894 kcal/day |
He eats about 2,517 kcal to hold weight and adjusts after two to three weeks.
Frequently asked questions
What is the difference between BMR and TDEE?
BMR (basal metabolic rate) is the energy your body burns at complete rest. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor for movement and training. Eat to TDEE to maintain weight.
Is Mifflin-St Jeor the most accurate calorie equation?
Mifflin-St Jeor is the most accurate common prediction equation, beating Harris-Benedict in validation studies. It can still be off by a few hundred calories for an individual.
How big a calorie deficit should I use to lose weight?
A calorie deficit of about 20% below TDEE protects training quality and lean mass. Larger deficits speed early loss but raise the risk of low energy availability, poor recovery and muscle loss.
Why might the calculator be wrong for me?
The activity factor is the biggest source of error, and people routinely overestimate it. Genetics, muscle mass and non-exercise movement also shift expenditure. If your weight is not moving as predicted, adjust intake.
Should athletes eat more on hard training days?
Yes. The activity factor is a weekly average. Eat more carbohydrate and total calories around your longest, hardest sessions and slightly less on rest days.
Sources
- Mifflin, St Jeor et al. (1990). "A new predictive equation for resting energy expenditure in healthy individuals." Am J Clin Nutr 51(2):241-247. Link
- Academy of Nutrition and Dietetics. Evidence reviews finding Mifflin-St Jeor the most accurate common resting-energy prediction equation.
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