BMR formulas compared — which one should you use?
Three main equations are used to estimate BMR. Each was developed from different populations and uses different variables. Here is what the research says about each:
| Formula |
Year |
Inputs needed |
Best for |
Accuracy |
| Mifflin–St Jeor Recommended |
1990 |
Weight, height, age, sex |
General population, most people |
±10% in 82% of subjects |
| Katch–McArdle Best accuracy |
1996 |
Lean body mass only |
Athletes, very lean or muscular individuals |
Most accurate when body fat % is known |
| Harris–Benedict (revised) |
1919 / 1984 |
Weight, height, age, sex |
Historical reference, clinical settings |
Tends to overestimate by 5–15% |
The formulas in full
Mifflin–St Jeor:
Male: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Female: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Katch–McArdle (requires body fat %):
LBM = weight × (1 − body fat% / 100)
BMR = 370 + (21.6 × LBM in kg)
Harris–Benedict (Roza & Shizgal 1984 revision):
Male: BMR = 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
Female: BMR = 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)
Step-by-step BMR calculation example
Male, 32 years old, 80 kg, 178 cm — step by step
Step 1 — Weight term10 × 80 = 800
Step 2 — Height term6.25 × 178 = 1,112.5
Step 3 — Age term5 × 32 = 160
Step 4 — Sex constant (male)+5
BMR = 800 + 1,112.5 − 160 + 5= 1,757 kcal/day
TDEE (moderately active ×1.55)1,757 × 1.55 = 2,723 kcal/day
Female, 28 years old, 65 kg, 165 cm — step by step
Step 1 — Weight term10 × 65 = 650
Step 2 — Height term6.25 × 165 = 1,031.25
Step 3 — Age term5 × 28 = 140
Step 4 — Sex constant (female)−161
BMR = 650 + 1,031.25 − 140 − 161= 1,380 kcal/day
TDEE (lightly active ×1.375)1,380 × 1.375 = 1,898 kcal/day
How to use your BMR to reach your goal
BMR is a floor, not a target
Your BMR is the minimum calories your body needs to sustain basic organ function — breathing, circulation, cell repair. Do not try to eat at your BMR. Since you move, exercise, and digest food, your actual calorie need (TDEE) is always higher than BMR.
Use your TDEE as the baseline for setting calorie targets. Your BMR is most useful as a safety check: if your cut calories are close to or below your BMR, the deficit is too aggressive.
Fat loss: cut from TDEE, not BMR
A 500 kcal deficit below TDEE produces ~1 lb/week of fat loss. Eat above your BMR at all times to avoid muscle loss and hormonal disruption. A deficit of 20–25% of TDEE is the safe practical maximum.
Muscle gain: eat slightly above TDEE
Add 200–350 kcal above TDEE for a lean bulk. Larger surpluses produce faster fat gain without meaningfully faster muscle growth for natural athletes.
Why results change as you lose weight
BMR is directly tied to body weight. Every 10 lbs (4.5 kg) of weight lost reduces BMR by roughly 50–80 kcal/day. This is why weight loss stalls — if you don't recalculate, the deficit you started with slowly disappears. Recalculate every 4–8 weeks.
Why different BMR calculators give different numbers
Common reasons for BMR discrepancies
1Different formula used. Mifflin–St Jeor and Harris–Benedict can differ by 100–200 kcal for the same inputs. Neither is wrong — they reflect different research populations. Mifflin–St Jeor is recommended for most people.
2Unit conversion rounding. If one calculator uses lbs/inches natively and another converts internally, rounding at different steps produces slightly different results.
3Body fat % included or not. A calculator using Katch–McArdle with your body fat % will give a very different result from one using Mifflin–St Jeor with just your weight, especially if you are lean or muscular.
4RMR vs BMR confusion. Some tools display RMR (Resting Metabolic Rate), which is 10–20% higher than true BMR. They are often used interchangeably but measured differently.
Frequently asked questions about BMR
What is the difference between BMR and TDEE?
BMR is your calorie burn at complete rest. TDEE (Total Daily Energy Expenditure) is BMR multiplied by an activity factor that accounts for movement and exercise. For a moderately active person, TDEE is approximately 1.55× BMR. Always use TDEE as your calorie planning baseline.
Should I eat at my BMR or my TDEE?
Always eat at (or relative to) your TDEE — not your BMR. BMR is the absolute minimum your body needs at complete rest. Since you move, exercise, and digest food throughout the day, eating only at BMR would create a large, unhealthy deficit for most people and cause muscle loss, fatigue, and hormonal disruption.
Why are the Mifflin–St Jeor and Katch–McArdle results different?
Mifflin–St Jeor uses total body weight, while Katch–McArdle uses lean body mass. If you have average body composition, results will be close (within 50–100 kcal). If you are particularly muscular, Katch–McArdle will be higher — and more accurate. If you have high body fat, Katch–McArdle will be lower. The difference can exceed 200 kcal for people at body composition extremes.
Can BMR change over time?
Yes. BMR decreases with age (roughly 1–2% per decade after 30), with prolonged calorie restriction (metabolic adaptation), and with weight loss. It increases with gaining muscle mass. This is why recalculating every 4–8 weeks during a diet or training phase is important.
Is BMR the same as RMR?
Not exactly. RMR (Resting Metabolic Rate) is measured after a period of rest but not under the strict fasting and controlled temperature conditions required for true BMR measurement. RMR is typically 10–20% higher than BMR. Most online calculators technically estimate RMR but use the term BMR colloquially — this is standard practice and the difference is not clinically significant for calorie planning.
How accurate is a BMR calculator?
The Mifflin–St Jeor equation predicts BMR within 10% of measured values for approximately 82% of the general population. It is less accurate for very lean athletes (use Katch–McArdle) and people with certain medical conditions. For practical calorie planning, ±10% accuracy is sufficient — treat the result as a starting estimate and adjust based on 2–4 weeks of real weight data.