BMR Calculator
Find your basal metabolic rate — the calories your body burns at complete rest — using the Mifflin-St Jeor equation, then see your daily needs at different activity levels.
What basal metabolic rate actually tells you
Your basal metabolic rate (BMR) is the number of calories your body would burn in a full day if you did nothing but lie still — no eating, no moving, just keeping you alive. That energy runs your heart and lungs, maintains body temperature, keeps your brain and kidneys working, and repairs cells around the clock. For most people BMR is the single biggest slice of daily calorie use, often 60–70% of the total, which is why it’s the natural starting point for any calorie plan.
What BMR does not tell you is how many calories you actually need to eat. It deliberately excludes movement, exercise and the energy spent digesting food. It’s also a population-average estimate from an equation, not a measured value — two people with identical stats can differ by a couple of hundred calories. Treat the number as a well-grounded ballpark rather than a precise daily quota.
How to use this calculator
Enter your age, sex, height and weight and the calculator returns your estimated BMR in calories per day. Sex and age both matter because they’re proxies for body composition: men on average carry more lean tissue, and metabolism tends to slow gradually with age. To turn BMR into the calories you actually need — your total daily energy expenditure, or TDEE — multiply it by an activity factor, then adjust up or down for your goal. Our calorie calculator does that step and suggests sensible targets.
How it’s calculated
This tool uses the Mifflin-St Jeor equation, generally regarded as one of the most accurate for healthy adults. In words: multiply your weight in kilograms by 10, add 6.25 times your height in centimetres, then subtract 5 times your age in years. Finally add 5 if you’re male, or subtract 161 if you’re female. The older Harris-Benedict equation works the same way with different coefficients and tends to read slightly higher; Mifflin-St Jeor was published in 1990 to correct that drift and is now the common default.
To get TDEE, BMR is multiplied by an activity factor — roughly 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active and 1.9 for extremely active.
A worked example
Take a 35-year-old woman who is 165 cm tall and weighs 65 kg. The maths runs: (10 × 65) + (6.25 × 165) − (5 × 35) − 161 = 650 + 1031.25 − 175 − 161 = about 1,345 calories per day at rest. If she’s moderately active, her TDEE would be roughly 1,345 × 1.55 ≈ 2,085 calories a day — the amount that would keep her weight steady.
Activity factors and what they multiply BMR by
| Activity level | Typical week | Multiply BMR by |
|---|---|---|
| Sedentary | Little or no exercise, desk job | 1.2 |
| Lightly active | Light exercise 1–3 days | 1.375 |
| Moderately active | Moderate exercise 3–5 days | 1.55 |
| Very active | Hard exercise 6–7 days | 1.725 |
| Extra active | Physical job or twice-daily training | 1.9 |
What drives your BMR — and its limitations
The main driver is your amount of lean (fat-free) mass: muscle is metabolically active and burns more at rest than fat, so larger, more muscular people have higher BMRs. Body size, sex and age all feed into the estimate for the same reason. Because the equation only sees height, weight, age and sex, it can’t account for an unusually muscular or lean build, thyroid conditions, certain medications, or the temporary metabolic dip that comes with aggressive dieting. It also isn’t validated for children, during pregnancy, or for people with serious illness. Use it as a planning estimate, track your real-world results over a few weeks, and check in with a doctor or registered dietitian before making big dietary changes.
Frequently asked questions
What is BMR?
Basal metabolic rate is the number of calories your body uses at rest to keep basic functions going — breathing, circulation and cell repair. It's the largest part of most people's daily calorie burn.
What formula does this use?
The Mifflin-St Jeor equation, widely regarded as one of the most accurate for healthy adults: 10 × kg + 6.25 × cm − 5 × age, then +5 for men or −161 for women.
What's the difference between BMR and TDEE?
BMR is calories burned at rest. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor to include movement and exercise — that's the figure to base eating targets on. Eat below your TDEE to lose weight and above it to gain.
Is BMR the same as RMR?
They’re close but not identical. BMR is measured under strict conditions — fully rested, fasted and in a controlled environment — while resting metabolic rate (RMR) is measured under slightly less strict conditions and usually comes out a little higher. In everyday use the terms are often used interchangeably, and equation-based estimates like this one are a practical stand-in for both.
Why does my BMR seem to drop as I get older?
Metabolism tends to slow with age, largely because people gradually lose muscle and gain fat, and muscle burns more energy at rest. The equation reflects this by subtracting calories for each year of age. Staying active and keeping muscle through resistance training is one of the most effective ways to support your metabolic rate over time.
How can I increase my BMR?
The most reliable lever is building and maintaining lean muscle through regular strength training, since muscle raises your resting burn. Staying well-fed enough to avoid prolonged aggressive dieting also helps, because very low calorie intake can suppress metabolic rate. Beware of supplements or “metabolism boosters” promising dramatic results — the effects are small and short-lived.
Is this number a diagnosis or medical advice?
No. This is a general screening estimate from a standard formula, not a measurement or a medical assessment. Individual metabolism varies with genetics, hormones, medications and health conditions. If you have concerns about your weight, energy or a possible thyroid or metabolic issue, speak with a healthcare professional.
Sources & references
- Mifflin MD, et al. — "A new predictive equation for resting energy expenditure in healthy individuals," Am J Clin Nutr (1990) — the original study behind the Mifflin-St Jeor equation this calculator uses.
- NIH (NIDDK) — Body Weight Planner — a research-based tool for estimating the calories needed to reach or maintain a goal weight, built on the same energy-balance principles as BMR/TDEE.
Last updated: July 2026 · How we calculate
BriskToolbox provides estimates for general information only and is not medical advice. Talk to a healthcare professional about your individual health.