BMR Calculator

Two formulas, and when each one wins

Most people should use Mifflin-St Jeor. Published in the American Journal of Clinical Nutrition in 1990, it needs just four inputs — sex, age, height, and weight:

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

If you know your body fat percentage, Katch-McArdle has an edge. It takes a different route: BMR = 370 + 21.6 × lean body mass(kg). Because muscle is metabolically active tissue, calculating directly from lean mass tends to land closer to the truth for people whose muscle level is far from average — serious lifters and athletes in particular. It doesn't ask for sex or age at all; those differences are already baked into body composition.

How far apart do the two land? At 170 lbs and 20% body fat, Katch-McArdle returns 1,702 where Mifflin returns 1,737 — about a 2% gap. The further your body fat sits from the population average, the wider that gap grows. Select Katch-McArdle in the calculator and the result card shows the Mifflin number right alongside for comparison.

What actually determines your BMR

1. Body size. Weight and height carry the most weight in the equation — a bigger body simply has more tissue to keep running.
2. Muscle mass. At rest, a kilogram of muscle burns about 13 calories a day versus about 4.5 for a kilogram of fat. Real, but modest: ten extra pounds of pure muscle raises BMR by only ~60 calories a day. The "muscle is a calorie furnace" story is heavily oversold.
3. Age. BMR drifts down over the years, mostly through muscle loss and lower everyday activity rather than age itself. Recalculating every five years is a good habit.
4. Sex. At the same height and weight, men typically run a higher BMR — largely because they carry more lean mass, which is exactly why Katch-McArdle can skip the sex input.
5. Hormones and health. Thyroid function is the biggest single lever (an overactive thyroid pushes BMR up, an underactive one pulls it down). Long stretches of very low-calorie dieting also push metabolism into adaptation.

BMR vs. RMR: what's the difference?

RMR (resting metabolic rate) is measured under looser conditions: BMR assumes a strict morning, fasted, fully-rested state, while RMR is simply "sitting quietly." RMR typically comes in 3–10% higher than BMR. In everyday usage — including most online calculators — the terms are interchangeable; strictly speaking, equations like Mifflin-St Jeor estimate something in the RMR neighborhood. For meal planning, the distinction doesn't matter.

Can you eat below your BMR?

Not as a habit. Your BMR is the budget for the basics — heartbeat, breathing, body temperature. Eat below it for long and your body responds by breaking down muscle, dialing metabolism down, and draining your energy: precisely the opposite of what you want. Weight loss works better as a moderate deficit below your TDEE (BMR plus activity burn), not as a crash under your BMR. For that full daily-burn number, use the TDEE calculator.

How to use this calculator

1. Pick a Formula: Mifflin-St Jeor by default, or Katch-McArdle if you know your body fat percentage.
2. Toggle Imperial / Metric, then enter sex, age, height, and weight — results update as you type.
3. In Katch-McArdle mode, add your Body fat percentage.
4. Read the result: the big number is your daily BMR, with the hourly equivalent and the formula used below it. Katch-McArdle mode also shows your lean body mass and the Mifflin estimate for comparison.

Frequently asked questions

Straight answers to the questions people ask most about basal metabolic rate.

What is a good BMR?

There's no "good" or "bad" BMR — it's just a number set by your size, age, sex, and muscle mass. Adult women commonly land around 1,200–1,600 calories a day, men around 1,500–2,000. If your reading seems far off the estimate and you also feel constantly tired, cold, or notice unexplained weight changes, a thyroid check is worth discussing with a doctor.

How do I calculate my BMR?

Use the Mifflin-St Jeor equation: for men, 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5; for women, 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161. If you know your body fat percentage, Katch-McArdle is an alternative: 370 + 21.6 × lean body mass(kg). Or enter your numbers above — the calculator runs both and shows them side by side.

Does muscle really burn more calories than fat?

Yes, but the margin is oversold. At rest, muscle burns roughly 13 calories per kilogram per day versus about 4.5 for fat — ten pounds of new muscle works out to about 60 extra calories a day, less than half a can of soda. The metabolic case for strength training rests on the workout burn itself and on keeping muscle while you diet, not on turning your BMR into a furnace.

Why does BMR decrease with age?

Mostly because of muscle loss — adults lose roughly 3–5% of their muscle per decade after 30 — compounded by lower everyday activity. It isn't a mysterious aging switch: people who keep lifting weights see a much slower decline.

Which BMR formula is most accurate?

For the general adult population, Mifflin-St Jeor is the most widely validated equation in published research. For people with above- or below-average muscle mass and a reliable body fat measurement, Katch-McArdle often tracks closer. Both are population-level estimates with roughly ±10% individual error — only a metabolic chamber or indirect calorimetry gives a precise reading.

My BMR seems low. Should I worry?

Double-check your inputs first — a unit mix-up is the most common culprit. The equations also naturally predict lower values for people with less muscle; that's statistics, not disease. But a low reading paired with persistent fatigue, feeling cold all the time, or unexplained weight gain warrants a conversation with your doctor about thyroid function.

Sources: Mifflin-St Jeor equation, published in the American Journal of Clinical Nutrition (1990); Katch-McArdle formula, the standard lean-mass equation in exercise physiology; tissue metabolic rates from published body-composition research (skeletal muscle ≈ 13 kcal/kg/day, adipose tissue ≈ 4.5 kcal/kg/day). Example numbers were computed with the same engine that powers this calculator. Related tools: TDEE Calculator and the health calculators hub.

Disclaimer: This calculator uses widely accepted estimating equations from published research and is for general reference only — it is not medical or nutritional advice. If you're concerned about your metabolism, suspect a thyroid issue, or plan a major diet change, talk to a doctor or registered dietitian first.