calchy.
Home/💪 Health

BMR Calculator

Basal Metabolic Rate — the calories you burn at rest.

Your BMR is the minimum number of calories your body needs to keep the engine running: heart pumping, lungs breathing, cells repairing, temperature regulated. It accounts for 60–70% of your total daily energy expenditure. Everything else — moving, digesting food, exercising — sits on top of this baseline.

BMR
1,699kcal/day
Per hour
70.8 kcal
Per week
11,891 kcal

BMR is what you'd burn in bed for 24 hours. To get total daily calories (TDEE), multiply by an activity factor — or use our TDEE calculator.

BMR vs TDEE — what's the difference?

BMR is the floor. TDEE (Total Daily Energy Expenditure) is the real number: how many calories you burn on a typical day including movement, digestion, and exercise. TDEE = BMR × activity multiplier.

For a sedentary office worker (mostly sitting, light walking), the multiplier is about 1.4–1.5. For someone training 4–5 days a week, it's closer to 1.6–1.75. For athletes in heavy training, 1.9–2.1 or higher.

What affects your BMR?

The Mifflin-St Jeor formula captures the four biggest variables. Here's why each matters.

  • Weight: heavier bodies have more metabolically active tissue (even fat tissue burns a small amount). More mass = higher BMR.
  • Height: taller people have more surface area, which loses more heat and requires more energy to maintain temperature.
  • Age: after ~25, lean muscle mass gradually decreases. Less muscle = lower BMR. This is the main driver of 'slowing metabolism with age' — the metabolism itself changes little; the body composition does.
  • Sex: men carry more muscle mass on average, hence the +5 adjustment (vs −161 for women) in the Mifflin-St Jeor formula.

Why muscle mass is the most controllable variable

Of the four inputs, sex, height, and age are fixed. Weight is modifiable, but weight alone is a blunt lever — losing 5 kg of fat doesn't raise BMR much. Adding 5 kg of muscle does, significantly.

A kilogram of muscle burns approximately 13 kcal per day at rest. A kilogram of fat burns about 4–5 kcal. This gap compounds over years. Someone who strength trains from 30 to 50 and maintains lean mass may have a BMR 100–200 kcal/day higher than a same-weight, same-age peer who didn't — without any change to the scale weight.

Using BMR to set a calorie target

The standard approach: calculate TDEE, then apply a deficit (cut) or surplus (bulk) from there.

  • Cutting (fat loss): TDEE minus 300–500 kcal/day. Produces ~0.3–0.5 kg/week of fat loss. Aggressive deficits (−750–1000 kcal) lose weight faster but risk muscle loss.
  • Maintenance: eat at TDEE. Body weight stays stable over time.
  • Bulking (muscle gain): TDEE plus 200–300 kcal/day. Slow, 'clean' bulks minimize fat accumulation. Larger surpluses produce faster scale weight increases but more fat gain.
  • Never eat below BMR for extended periods — this is below what's needed just to stay alive at rest, and will cause lean tissue loss, hormonal disruption, and metabolic adaptation.

How to use your BMR in practice

Calculate the number, then put it to work.

  1. 1
    Get your stats
    Weigh yourself in the morning. Measure height without shoes. Use the correct biological sex — the formula uses population data tied to typical hormonal profiles.
  2. 2
    Calculate your BMR
    Enter the values above. Your BMR appears in kcal/day — this is your resting calorie floor.
  3. 3
    Multiply by your activity level
    Sedentary (desk job, minimal exercise): × 1.4. Lightly active (3–4 walks/week): × 1.5. Moderately active (4–5 workouts/week): × 1.6. Very active (daily training): × 1.75. Use the TDEE calculator for a more structured activity breakdown.
  4. 4
    Set your calorie target
    From your TDEE: subtract 300–500 kcal to cut, add 200–300 to build, hold flat to maintain. Track for 2–3 weeks and adjust if the scale doesn't respond as expected.
  5. 5
    Re-calculate every 5–10 kg of weight change
    BMR changes with body weight. Losing 10 kg means your TDEE is lower — calorie targets that worked earlier may need adjustment.

FAQ

What's the Mifflin-St Jeor formula?
Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5. Women: same calculation, minus 161. It's the most accurate widely-used BMR formula for healthy adults and was validated against indirect calorimetry measurements.
Is BMR the same as RMR?
Close, but not identical. BMR (Basal Metabolic Rate) is measured in a strictly controlled environment: lying still, completely fasted, after a full night's sleep. RMR (Resting Metabolic Rate) is measured in more relaxed conditions and tends to run 10–15% higher. Most calorie calculators actually use RMR, but call it BMR for simplicity.
Why does my BMR drop with age?
Two main reasons: loss of lean muscle mass (which burns more calories at rest than fat), and reduced cellular metabolic activity. From age 30–70, BMR typically drops 2–3% per decade. Resistance training slows this by preserving lean mass.
How accurate is Mifflin-St Jeor vs other formulas?
Mifflin-St Jeor (1990) outperforms the older Harris-Benedict formula (1919) for most adults. The Katch-McArdle formula is more accurate for lean athletes — it uses body fat percentage rather than weight alone, which avoids overestimating BMR in muscular people.
Is my BMR the same as my daily calorie target?
No — BMR is what you'd burn if you stayed completely still for 24 hours. Your actual daily target (TDEE) is BMR multiplied by an activity factor. For most people with desk jobs and occasional exercise, that's BMR × 1.5–1.6.

Related health calcs