BMR Calculator

What This Calculator Really Does

Your basal metabolic rate (BMR) is the energy your body spends keeping you alive at complete rest — breathing, circulation, cell repair, brain function — before any activity is counted. This calculator estimates it from the two formulas clinicians and dietitians actually cite: Mifflin-St Jeor (the current default in nutrition practice) and the older Harris-Benedict equation.

BMR is the foundation number behind calorie targets: add activity on top of it and you get TDEE (total daily energy expenditure), which is what weight plans are really built on. An honest BMR estimate therefore comes with an honest error bar — the formulas were derived from population studies, and individuals can sit well above or below the line.

Everything runs locally in your browser; nothing about your body is transmitted or stored.

The Formulas, Explained Plainly

Mifflin-St Jeor: BMR = 10×weight(kg) + 6.25×height(cm) − 5×age + 5 (men) / −161 (women)

Revised Harris-Benedict: men: 88.362 + 13.397×W + 4.799×H − 5.677×A; women: 447.593 + 9.247×W + 3.098×H − 4.330×A

• Weight (W) — kilograms; the dominant term in both equations.
• Height (H) — centimetres, capturing body size beyond weight alone.
• Age (A) — years; BMR falls slowly with age in these models.

The two formulas can differ by about 5% for the same person. That spread is not an error — it is an honest picture of how precise any population-derived equation can be for one individual.

How to Use It — In Order

1. Choose units — metric (kg/cm) or imperial, consistently.
2. Enter sex, age, height, weight — the four inputs both formulas use.
3. Read your BMR — the rest-day energy floor in kcal/day.
4. Multiply by activity — 1.2 sedentary up to 1.9 very active gives TDEE, the number that actually governs weight change.
5. Treat it as a starting point — track real-world weight trends for two to three weeks and adjust.

Worked Example, Verified by Hand

Man, 35 years, 175 cm, 75 kg — Mifflin-St Jeor:

1. 10 × 75 = 750 (weight term).
2. 6.25 × 175 = 1,093.75 (height term).
3. 5 × 35 = 175 (age subtraction).
4. 750 + 1,093.75 − 175 + 5 = 1,673.75 ≈ 1,674 kcal/day.

Sedentary TDEE: 1,674 × 1.2 ≈ 2,009 kcal/day. Moderately active (×1.55): ≈ 2,595 kcal/day. Those two numbers bracket where his maintenance calories actually sit.

Common Mistakes to Avoid

• Eating at BMR and wondering why you are exhausted — BMR excludes daily activity; plan around TDEE, not BMR.
• Using morning weight after a salty meal — weigh fasted, several days, and average.
• Assuming muscle vs fat makes no difference — lean mass raises real energy use above what the formula predicts.
• Applying athletic-body numbers to the equations — they were derived from general populations.
• Changing targets daily — give any plan two to three weeks before adjusting.

Limitations

• Population equations carry individual error commonly cited around ±10% or more.
• Very muscular, older, or clinical populations fall outside the derivation data.
• Illness, medication, sleep and stress shift real expenditure in ways no static formula captures.
• This is a fitness planning tool, not a medical assessment.

Expected Accuracy

Mifflin-St Jeor is the formula dietetics practice treats as the best general predictor, but any BMR equation is an estimate of a quantity best measured with calorimetry. Expect the true value for an individual to differ from the estimate — use trends, not the single number, to steer.

Privacy — Your Data Never Leaves This Device

Height, weight and age are computed locally and never transmitted. No account, no tracking, works offline after first load.

Related Tools & Guides

Natural next steps from BMR:

• All Health Calculators
• Calorie Calculator — TDEE with activity
• BMI Calculator
• Body Fat Calculator
• Macronutrient Calculator

Sources & Standards

Both formulas are published, peer-reviewed equations used across nutrition practice.

• NIH — energy expenditure research
• CDC — healthy weight resources
• Academy of Nutrition and Dietetics — evidence-based practice guidelines

Bottom Line

BMR is the floor, TDEE is the plan. Use this calculator to get an honest starting estimate, then let two to three weeks of real-world weight data — not the formula — make the final call.

Frequently Asked Questions