BMR Calculator Math: How to Calculate Yours by Hand

Basal metabolic rate is the calories your body burns at complete rest — heart pumping, lungs breathing, brain thinking, kidneys filtering, cells dividing. For most adults, BMR accounts for 60 to 75 percent of total daily energy use. Get this number wrong by 200 calories a day and a year of "I'm doing everything right and not losing weight" makes sense.

You can calculate BMR by hand in under a minute with one formula. The most accurate is Mifflin-St Jeor (1990), within plus or minus 10 percent of measured values for most healthy adults — as good as you get without a metabolic chamber.

This article covers Mifflin-St Jeor and the older Harris-Benedict formula, worked examples for men and women in their 30s, 40s, and 50s, why BMR matters for weight loss, the difference between BMR and TDEE, and when to use the calorie target tool instead of doing the math yourself.

Mifflin-St Jeor vs Harris-Benedict

Two formulas dominate. They were developed 70 years apart on different populations. The newer one wins by a wide margin.

Mifflin-St Jeor (1990): developed by M.D. Mifflin and S.T. St Jeor on a wider weight range than earlier studies. Validated across many studies as the most accurate predictive equation for modern adults.

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

Harris-Benedict (1919, revised 1984): the original equation. Still in older textbooks and calculators.

Men: BMR = 88.362 + (13.397 × W) + (4.799 × H) − (5.677 × A)
Women: BMR = 447.593 + (9.247 × W) + (3.098 × H) − (4.330 × A)

For most healthy adults, Harris-Benedict overestimates BMR by 100 to 200 calories per day versus indirect calorimetry. Mifflin-St Jeor lands within plus or minus 10 percent for ~82 percent of subjects. The difference: a deficit set on Harris-Benedict might be eating 100 below maintenance when you thought 300 — the gap between losing 0.6 lb a week and barely moving.

Katch-McArdle uses lean body mass and is more accurate for very lean or very heavy people, but requires a body fat percentage measurement most people do not have reliably. Mifflin-St Jeor is the right default. If body composition matters for your case, plug stats into a body composition macro tool.

Worked Examples

These examples use Mifflin-St Jeor since it is the modern standard.

30-year-old man, 80 kg (176 lb), 178 cm (5'10"):

10 × 80 = 800
6.25 × 178 = 1112.5
5 × 30 = 150
Add the male constant: + 5
BMR = 800 + 1112.5 − 150 + 5 = 1767.5
Round to 1768 calories per day

This person burns 1768 calories just to stay alive. Add daily activity on top.

40-year-old woman, 70 kg (154 lb), 165 cm (5'5"):

700 + 1031.25 − 200 − 161 = 1370 calories per day

About 400 calories lower than the 30-year-old man, from lower weight, shorter height, older age, and the female constant. Sex differences in BMR are mostly lean body mass differences; the formula bakes in a fixed offset.

50-year-old man, 90 kg (198 lb), 180 cm (5'11"):

900 + 1125 − 250 + 5 = 1780 calories per day

Higher than the 30-year-old man's number despite being 20 years older, because he weighs 10 kg more. Weight is the dominant variable — every additional kilogram adds 10 calories.

50-year-old woman, 65 kg (143 lb), 162 cm (5'4"):

650 + 1012.5 − 250 − 161 = 1252 calories per day

This is one of the most common BMRs in the adult female population, which is why the standard "1200 calorie diet" is too aggressive a deficit to sustain. Subtracting more than a couple hundred calories from a 1250-cal BMR creates physiological stress that backfires.

If running by hand feels error-prone, a calorie needs calculator takes the same inputs instantly.

Why BMR Matters for Weight Loss

BMR is the foundation of every weight loss math problem. Get it wrong and the whole calculation downstream is off.

To lose weight, you eat fewer calories than you burn. To know how many you burn, you need BMR plus daily activity. Assume BMR is 1900 when it is 1700 and your "400-calorie deficit" plan is actually 200. After two months you have lost 4 lb instead of 8, you blame yourself, you give up.

BMR also goes down as you lose weight. A 200-lb person dropping to 180 lb sees BMR fall ~90 to 110 cal/day from the weight change alone, more if muscle was lost. The deficit that worked at 200 does not work at 180. People who do not recalculate every 10 to 15 lb hit a plateau that has nothing to do with willpower.

Crash diets make this worse. Eating far below maintenance for extended periods causes metabolic adaptation — BMR drops more than weight change predicts. Studies on extreme weight-loss show contestants have measured 300 to 500 calorie drops below predicted, persisting years after regain. Aim for moderate deficits (300 to 500 cal below maintenance) and recalculate every 10 lb. Plug your weight into a protein needs estimator to keep muscle through the cut.

BMR vs TDEE

BMR is calories burned at complete rest. TDEE — total daily energy expenditure — is calories burned across the whole day including movement. TDEE is what you actually need to know for weight management.

The standard way to estimate TDEE is to multiply BMR by an activity factor:

Sedentary (desk job, no exercise): BMR × 1.2
Lightly active (light exercise 1 to 3 days/week): BMR × 1.375
Moderately active (moderate exercise 3 to 5 days/week): BMR × 1.55
Very active (hard exercise 6 to 7 days/week): BMR × 1.725
Extra active (hard daily exercise plus physical job): BMR × 1.9

Take the 30-year-old man from the example above. BMR = 1768. If he sits at a desk all day and exercises 3 times a week:

TDEE = 1768 × 1.55 ≈ 2740 calories per day

That is what he needs to maintain his current weight. Eating 2240 (a 500-calorie deficit) would produce roughly 1 lb of fat loss per week. Eating 3240 (a 500-calorie surplus) would add roughly 1 lb per week, with composition depending on training and protein intake.

Activity multipliers are rough. Two people with the same exercise schedule can have wildly different total movement — the non-exercise component (NEAT) varies by 600 to 800 calories per day between similar people. If TDEE seems off after 2 to 3 weeks of tracking, adjust the activity factor by one notch.

Workflow: calculate BMR with Mifflin-St Jeor, multiply by activity factor for TDEE, set eating target 300 to 500 below TDEE for a cut or above for a lean bulk, reassess after 4 weeks. Cross-check in a calorie balance calculator the first time. If you train hard, know your protein floor with a daily protein target tool.

Free Tool

If you want to skip the arithmetic, the BMR and calorie target calculator on ScoutMyTool runs Mifflin-St Jeor, applies your activity factor, and returns BMR, TDEE, and recommended targets for cutting, maintaining, or bulking. Same inputs you would use by hand. No account, no email.

It also handles unit conversion (pounds to kg, feet/inches to cm) automatically — one of the most common sources of hand-calculation errors. People put 5'10" in as 70 instead of 178 cm and end up with a BMR that is wildly low. The calculator skips that trap.

Frequently Asked Questions

Q: How accurate is the Mifflin-St Jeor equation? A: For most healthy adults, it predicts measured BMR within plus or minus 10 percent. About 82 percent of subjects in validation studies fall in that range. It is less accurate at extremes of body composition — very lean athletes (underestimates) and people with significant obesity (can overestimate). For everyone in between, it is the most reliable predictive equation with simple inputs.

Q: Why does BMR decrease with age? A: Lean body mass tends to decrease as muscle is lost without targeted training, and metabolic activity per unit of lean tissue slows slightly. Mifflin-St Jeor captures this with the 5-cal-per-year subtraction. Regular resistance training largely offsets the lean-mass loss — a 60-year-old who has lifted consistently can have a BMR similar to a sedentary 35-year-old of the same weight.

Q: Should I subtract calories from BMR or TDEE for weight loss? A: TDEE. BMR is just the resting baseline; you also burn calories through movement and exercise. Subtracting from BMR leads to dangerously low intake. A safe deficit is 300 to 500 below TDEE, producing 0.6 to 1 lb of fat loss per week. Below 1200 cal/day for women or 1500 for men is not recommended without medical supervision.

Q: Does muscle really burn more calories than fat at rest? A: Yes, but less than popular numbers suggest. A pound of muscle burns ~6 cal/day at rest. A pound of fat burns ~2. So adding 5 pounds of muscle adds ~30 cal/day to BMR — meaningful but not "50 cal per pound." The bigger benefit of muscle is during and after exercise.

Q: How often should I recalculate my BMR? A: Every 10 to 15 lb of weight change either direction, or every 6 months if weight is stable but training changed. The same person at 200 lb and 180 lb has a BMR ~100 cal lower at the lighter weight. People who stop recalculating after losing hit a plateau that disappears the day they update.

Bringing It Together

BMR is the foundation of every honest weight management calculation. Mifflin-St Jeor gives you a number within 10 percent using only age, sex, height, and weight. Multiply by an honest activity factor for TDEE, set your eating target 300 to 500 above or below, and recalculate every 10 to 15 lb of progress. Skip Harris-Benedict, which overestimates and silently undermines weight loss math. Run it by hand once so you understand it, then let a calculator handle the arithmetic.