Your Basal Metabolic Rate (BMR) is the calorie cost of staying alive at complete rest — no walking, no thinking hard, no digesting. It is the foundation of every calorie-target calculation, and it takes about ninety seconds to work out by hand.
Quick Answer
The modern default formula is Mifflin-St Jeor (1990):
- Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
You can punch the numbers into our BMR Calculator and skip the arithmetic.
Step 1: Pick Units
Mifflin-St Jeor takes kilograms for weight and centimetres for height.
- 1 kg = 2.20462 lb
- 1 in = 2.54 cm
- Example: 5 ft 9 in = 69 in = 175.3 cm; 154 lb = 69.85 kg
Our calculator converts for you under the Imperial toggle.
Step 2: Plug In Your Numbers
For a 30-year-old man, 175 cm, 70 kg:
BMR = 10 × 70 + 6.25 × 175 − 5 × 30 + 5 BMR = 700 + 1,093.75 − 150 + 5 = 1,648.75 kcal/day
For a 30-year-old woman, 165 cm, 60 kg:
BMR = 10 × 60 + 6.25 × 165 − 5 × 30 − 161 BMR = 600 + 1,031.25 − 150 − 161 = 1,320.25 kcal/day
That's it — one equation, four inputs.
Step 3: Cross-Check With Harris-Benedict (Optional)
If you want a comparison number, the revised Harris-Benedict (Roza & Shizgal, 1984) is:
- Men: 88.362 + 13.397·W + 4.799·H − 5.677·A
- Women: 447.593 + 9.247·W + 3.098·H − 4.330·A
For the same 30-year-old man:
88.362 + 13.397 × 70 + 4.799 × 175 − 5.677 × 30 88.362 + 937.79 + 839.83 − 170.31 ≈ 1,696 kcal/day
A small disagreement is normal. Mifflin-St Jeor is generally considered more accurate for modern populations; Harris-Benedict slightly over-estimates for sedentary adults.
A Full Worked Example in Imperial Units
Many readers think in lb and inches. The math is the same, the conversions are the friction. A 35-year-old man, 5'10" (70 in), 180 lb:
- Convert weight: 180 lb ÷ 2.20462 = 81.65 kg
- Convert height: 70 in × 2.54 = 177.8 cm
- Apply Mifflin-St Jeor:
- BMR = 10 × 81.65 + 6.25 × 177.8 − 5 × 35 + 5
- BMR = 816.5 + 1,111.25 − 175 + 5 = 1,757.75 kcal/day
A 35-year-old woman, 5'5" (65 in), 140 lb:
- Convert: 63.5 kg, 165.1 cm
- BMR = 10 × 63.5 + 6.25 × 165.1 − 5 × 35 − 161
- BMR = 635 + 1,031.9 − 175 − 161 = 1,330.9 kcal/day
Round to the nearest 10 kcal in practice — the formula's true precision doesn't justify decimals.
Validating Your BMR With Real Data
The number from any formula is a starting estimate. Three weeks of disciplined tracking turns it into your actual BMR:
- Week 1: Track baseline. Log everything you eat (precise weights), step count, and morning weight. Eat to maintain — that is, at your predicted TDEE.
- Week 2: Hold and observe. If average weight is stable within 0.3 kg either way, your formula BMR was right.
- Week 3: Adjust if needed. If you gained, drop ~10% of calories; if you lost, add 10%. Re-test.
After three cycles like this — usually nine weeks — you know your true maintenance to within 50 kcal. From there, every cutting or bulking number lands accurately.
This is more valuable than any "more accurate" formula. The scale is your real BMR meter.
Step 4: Don't Confuse BMR With Daily Need
BMR is what you'd burn at complete rest. To get your daily calorie need you multiply BMR by an activity factor — that's TDEE, covered in our TDEE Calculator.
For a quick estimate:
| Lifestyle | Multiplier | |---|---| | Sedentary | 1.2 | | Lightly active | 1.375 | | Moderately active | 1.55 | | Very active | 1.725 | | Extra active | 1.9 |
The 30-year-old man at Moderately active: 1,649 × 1.55 ≈ 2,556 kcal/day TDEE.
When Mifflin-St Jeor Is Less Accurate
- Very lean athletes (BMI under 18.5) — the formula was derived from a broader population
- Very heavy adults (BMI over 35) — actual BMR may run lower than predicted
- Older adults (65+) — sarcopenia lowers BMR; the formula gets less reliable
- Children and teens — use pediatric equations and growth charts, not adult Mifflin-St Jeor
For these cases, treat the calculator output as a rough start, then validate against 2–3 weeks of actual food intake and weight change.
Why the Men and Women Formulas Differ
The two equations look almost identical except for the final constants: +5 for men, −161 for women. The difference (166 kcal/day) reflects the average difference in lean body mass between sexes at the same height and weight. Women carry, on average, more essential body fat and proportionally less skeletal muscle — and skeletal muscle is the main BMR contributor.
For a muscular woman or a lean man, the formula's sex assumption may over- or under-correct. That's another reason Katch-McArdle (which uses lean body mass directly) is more accurate for athletes — it removes the sex assumption entirely:
BMR = 370 + 21.6 × lean body mass (kg)
Estimate lean body mass via our Body Fat Calculator, then plug it in.
When to Suspect Your BMR Is Atypical
If your formula BMR predicts maintenance at, say, 2,500 kcal and you consistently gain weight at 2,000 — or lose weight at 3,000 — something other than the formula is at work:
- Thyroid issues (under-active: BMR runs 10–30% low; over-active: 10–40% high)
- Long deficit history producing adaptive thermogenesis
- Significant water-weight noise masking the true trend (look at 14-day averages, not daily numbers)
- Tracking error — under-counting calories is the most common cause of "low BMR" complaints
- Recent body composition change the formula doesn't yet reflect
In order of likelihood: tracking error > body composition shift > adaptation > medical. Investigate in that order.
Common Mistakes
- Using lb directly in the kg formula. A 154 lb input ≠ 154 kg. Convert first.
- Confusing BMR and TDEE. BMR is rest only; TDEE adds activity.
- Updating BMR only once a year. Drop ~5 kg of weight and your BMR has changed too. Recompute every meaningful weight shift.
- Trusting smart-scale BMR readings. Many estimate via bio-impedance — useful for trend, less for absolute numbers.
Frequently Asked Questions
Is Mifflin-St Jeor accurate for me?
For most healthy adults aged 19–65, within about ±10%. That's close enough to start a calorie plan; the 2–3 week trial closes the gap.
Do I need to refigure BMR daily?
No. Every 4–5 kg of weight change, or every few months, is plenty.
What about RMR — is it different?
Resting Metabolic Rate (RMR) is measured (typically by indirect calorimetry) at quiet rest. It's slightly higher than the theoretical BMR (which assumes you just woke up in a thermo-neutral room). The two are used interchangeably in everyday fitness math.
Why does my BMR seem too low?
Three honest checks: (1) recompute with the correct units, (2) account for activity properly (your "low" BMR may be fine, with TDEE looking small only because activity multiplier is sedentary), (3) consider that Mifflin-St Jeor is a population average — individuals vary ±10%.
Can I increase my BMR?
Slightly. Each kilogram of muscle adds about 13 kcal/day at rest. Bigger gains come from raising TDEE through activity, not BMR directly.
Bottom Line
Calculating BMR is one of the fastest, most useful things you can do for any nutrition plan:
- Convert weight to kg, height to cm.
- Plug into Mifflin-St Jeor — or use our BMR Calculator.
- Multiply by activity factor for TDEE → TDEE Calculator.
- Apply your goal with our Calorie Calculator.
Three calculators, three numbers. The same logic powers every reasonable diet plan in print.
Health and fitness calculations are estimates only and not medical advice. People with thyroid disorders, eating-disorder histories, or extreme body-fat profiles should validate BMR estimates with a healthcare provider.