How the Mifflin-St Jeor equation works
Your basal metabolic rate (BMR) is the energy your body burns at complete rest just to stay alive. The Mifflin-St Jeor equation, published in 1990, predicts it from weight, height, age, and sex: BMR = 10 x weight in kg + 6.25 x height in cm - 5 x age, then add 5 for men or subtract 161 for women. It is the most accurate of the common prediction equations for most people.
BMR alone is not what you eat. To get total daily energy expenditure (TDEE), you multiply BMR by an activity factor that accounts for movement and training: 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 active, and 1.9 for athletes training hard most days. TDEE is the number of calories that holds your weight steady.
Maintain, lose, or gain
To maintain weight, eat at your TDEE. To lose weight at a sustainable rate, take roughly a 20% deficit below TDEE, which protects training quality and lean mass better than a crash diet. To build muscle or support a heavy training block, a modest 15% surplus gives your body the energy to adapt without excess fat gain.
For endurance athletes, large deficits are risky. Under-fuelling chronically drives low energy availability, which hurts recovery, hormones, bone health, and immunity. If you are training seriously, err toward the smaller deficit and prioritise eating enough around your hardest sessions.
Why this is a starting point, not a verdict
Prediction equations carry real error: two people with identical stats can differ by a few hundred calories a day in true expenditure. Use this number as your starting estimate, then adjust based on how your weight, performance, and recovery actually trend over two to three weeks.
The activity multiplier is the biggest source of uncertainty. People routinely overestimate their activity. If your weight is not moving the way the maths predicts, recheck which factor honestly matches your week, not your best week.
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 around 2,517 kcal to hold weight, then nudges up or down based on two to three weeks of real data.
Frequently asked questions
What is the difference between BMR and TDEE?
BMR, your basal metabolic rate, is the energy your body burns at complete rest. TDEE, your total daily energy expenditure, is BMR multiplied by an activity factor to include movement and training. TDEE is the number that actually maintains your weight, so it is the one you eat to.
Is Mifflin-St Jeor the most accurate calorie equation?
For the general population it is the most accurate of the common prediction equations, beating the older Harris-Benedict formula in validation studies. It still carries error of a few hundred calories for any individual, so treat its output as a starting estimate and refine it from how your weight actually trends.
How big a calorie deficit should I use to lose weight?
A deficit of about 20% below your TDEE is a sustainable target that protects training quality and lean mass. Larger deficits speed early loss but raise the risk of low energy availability, poor recovery, and muscle loss. For endurance athletes, the smaller, slower deficit almost always wins long term.
Why might the calculator be wrong for me?
The biggest source of error is the activity factor, which people routinely overestimate. Genetics, muscle mass, and non-exercise movement also shift true expenditure. If your weight is not changing the way the maths predicts after two to three weeks, adjust your intake based on that real-world feedback, not the formula.
Should athletes eat more on hard training days?
Yes. The activity factor gives a weekly average, but your real needs swing with your schedule. Eating more carbohydrate and total calories around your longest and hardest sessions, and slightly less on rest days, supports performance and recovery far better than the same flat intake every day.
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.