Calculating your basal metabolic rate in 3 steps takes only your weight, height, age, sex, and the Mifflin-St Jeor formula. The result shows how many calories your body burns at rest to keep its vital functions running, and it becomes the foundation for any plan to lose weight, maintain, or build muscle.
Between 60% and 75% of the calories you burn in 24 hours come from your basal metabolic rate, according to the Cleveland Clinic. Anyone who ignores this number and cuts calories by guesswork gets stuck between two bad scenarios: a deficit too small to lose weight, or an aggressive deficit that stalls the metabolism within three weeks.
This guide shows the validated formula, examples with real numbers, the comparison between Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle, and the four mistakes that usually invalidate the calculation.
BMR in one sentence: what you're calculating
Basal metabolic rate is the minimum amount of energy your body needs at rest to keep the heart, lungs, liver, kidneys, and brain working. It's the fixed cost of existing, without including digestion or any voluntary movement. The unit is kilocalories per day.
This number accounts for up to 70% of daily energy expenditure in sedentary people. In athletes, the share drops to 50% to 60%, because physical activity adds a lot to the total. The complete basal metabolic rate guide gives more context on why this fixed cost is so high.
Calculating BMR doesn't replace an indirect calorimetry test. It delivers an estimate with a 5% to 10% margin of error, enough for any practical planning without paying for an expensive lab test.
Step 1: gather the 4 data points that change the result
The result depends on the quality of your input data. A 2 kg error in weight shifts the final calculation by 50 to 70 kcal a day. One centimeter more or less changes it by 6 kcal. Over a week, that becomes 350 to 500 kcal of accumulated imprecision. Collecting the data correctly removes that noise.
- Current weight. Weigh yourself fasted, after using the bathroom, in light clothing. Use a digital scale with one decimal place and always keep the same device.
- Height. In centimeters, no shoes, against a straight wall. Use a rigid tape or stadiometer, don't eyeball it.
- Age. In completed years. Update it on your birthday, without rounding up.
- Biological sex. The formula has different factors for men and women because average body composition differs in lean mass and fat tissue.
Athletes who know their body fat percentage can use an extra formula (Katch-McArdle), which also asks for lean mass in kg. For anyone without a recent body composition scan, Mifflin-St Jeor delivers an equivalent result without the extra data point.
ContaCal
Count calories and macros with just 1 photo
Snap your meal and the AI instantly calculates calories, protein, carbs and fat.
Step 2: apply the Mifflin-St Jeor formula (with worked examples)
The equation published in 1990 by Mifflin and St Jeor became the reference standard of the Academy of Nutrition and Dietetics thanks to its margin of error under 10% in healthy and overweight adults. The formula is straightforward.
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Example 1. A 32-year-old woman, 65 kg (143 lb), 165 cm (5'5"): (10 × 65) + (6.25 × 165) − (5 × 32) − 161 = 650 + 1,031 − 160 − 161 = 1,360 kcal. That's her basal burn over 24 hours.
Example 2. A 40-year-old man, 85 kg (187 lb), 180 cm (5'11"): (10 × 85) + (6.25 × 180) − (5 × 40) + 5 = 850 + 1,125 − 200 + 5 = 1,780 kcal. In both cases, the number is only the starting point. You still need to multiply by your physical activity level. To run the math without doing it by hand, the basal metabolic rate calculator applies the same formula and returns the result in 30 seconds.
Step 3: multiply by your real activity factor
BMR alone accounts only for resting burn. To reach your total daily calorie burn, multiply the result from step 2 by the factor that represents your real week, not your ideal week.
| Activity level | Practical description | Factor |
|---|---|---|
| Sedentary | Desk job, no training | 1.2 |
| Lightly active | 1 to 2 light workouts a week | 1.375 |
| Moderately active | 3 to 5 workouts a week | 1.55 |
| Very active | 6 to 7 intense workouts | 1.725 |
| Extremely active | Athlete with 2 daily sessions | 1.9 |
Back to the example of the sedentary 65 kg woman: 1,360 × 1.2 = 1,632 kcal a day to maintain weight. To lose fat, she subtracts 300 to 500 kcal and lands between 1,130 and 1,330 kcal. Heads up: the 1,130 floor is below her BMR and shouldn't be sustained for more than two weeks.
When in doubt about your level, pick the lower factor. Most people overestimate their own activity by one or two points on the scale. Conservative on the input, aggressive on the result: that's the rule that protects against a false deficit.
Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle: which to choose
There are more than 200 formulas to estimate BMR in the scientific literature. Only three have relevant clinical use today. The table below sums up the differences and shows which scenario each one fits best.
| Formula | When to use | Margin of error | Uses lean mass? |
|---|---|---|---|
| Mifflin-St Jeor (1990) | Healthy and overweight adults | 5% to 10% | No, but it's the most accurate without that data |
| Harris-Benedict (1919, revised) | Historical and comparative | 10% to 15% | No, and it overestimates today's sedentary people |
| Katch-McArdle | Athletes with measured body fat % | 3% to 7% | Yes, uses lean mass directly |
For most people, Mifflin-St Jeor is the right choice. Athletes with a recent body composition scan gain a bit of precision with Katch-McArdle. Harris-Benedict is only worth it for comparing against old chart calculations, not for current planning.
4 mistakes that destroy any BMR calculation
- Overestimating physical activity. A 40-minute workout doesn't cancel out 12 hours of sitting. Use the lower factor when in doubt.
- Not updating after a weight change. Every 5 kg lost drops BMR by 70 to 100 kcal. Redo the math every 4 to 6 weeks.
- Cutting calories below BMR. The body slows down 10% to 15% within three weeks. What was a deficit becomes maintenance and the scale stalls.
- Trusting visual measurements. Weighing in heavy clothing or estimating height distorts the calculation by up to 8%. Use a digital scale and a rigid tape.
Fixing two of these mistakes is usually enough to unstick a stalled diet. The combination of the correct formula, periodic updates, and consistent tracking delivers results in 4 to 8 weeks.
How ContaCal recalculates your BMR every week
Calculating your BMR only once in your life is a recipe for a plateau. The body changes. Weight fluctuates. Lean mass responds to training. Stress and sleep alter the burn. That's why ContaCal's built-in calculator recalculates BMR every week using the data you log across the 7 days.
The flow is simple. The app cross-references your updated weight with your meal-photo history and the burn measured by your smartwatch. If you lost 2 kg, the target drops. If you gained lean mass, the app raises the recommended protein and keeps the deficit. That continuous tracking is what avoids the frozen calculation that stalls the scale.
People who keep tracking for a few weeks in a row tend to get a finer adjustment than manual calculations done once in a spreadsheet. The difference isn't in the algorithm, it's in the consistent data collection the app sustains on autopilot.