CALCULATOR
BMR Calculator
Your basal metabolic rate from three equations side by side, plus what it becomes once activity is added.
BMR is the energy you burn at complete rest. General guidance, not medical advice.
What basal metabolic rate actually measures
Basal metabolic rate is the energy your body uses at complete rest simply to stay alive: circulation, breathing, brain activity, cell repair and maintaining body temperature. For most people it accounts for roughly 60 to 70 percent of total daily energy expenditure, which is why it is the foundation every calorie target is built on.
This calculator runs three published equations at once so you can see the spread between them rather than trusting a single figure. Mifflin-St Jeor is the modern default and generally the most accurate for the general population. Harris-Benedict is older and typically reads a little higher. Katch-McArdle uses lean body mass instead of total weight, making it the most accurate option for lean, trained people who know their body fat percentage.
One thing your BMR is not is a target. Eating at basal rate for extended periods is how people lose muscle, wreck their training quality and end up worse off than when they started.
The three BMR equations, written out
Every calorie target you will ever see begins with an estimate of basal metabolic rate. Three equations dominate the field, and they were derived from different populations at different times, which is why they disagree with each other by 100 kcal or more for the same person. This calculator shows all three rather than hiding the disagreement behind a single confident number.
Women: BMR = 10W + 6.25H - 5A - 161
Women: BMR = 9.247W + 3.098H - 4.330A + 447.593
where LBM = weight x (1 - body fat percentage / 100)
W is weight in kilograms, H is height in centimetres and A is age in years. Mifflin-St Jeor was validated against a broader modern population than Harris-Benedict and is the equation most dietetics bodies now recommend. Katch-McArdle only outperforms them when you have a reliable body fat measurement, because it works from lean tissue rather than total mass.
How the equations compare in practice
| Person | Mifflin-St Jeor | Harris-Benedict | Difference |
|---|---|---|---|
| Man, 30 yr, 80 kg, 175 cm | 1,749 kcal | 1,822 kcal | 73 kcal |
| Woman, 30 yr, 65 kg, 165 cm | 1,353 kcal | 1,406 kcal | 53 kcal |
| Man, 50 yr, 95 kg, 180 cm | 1,830 kcal | 1,928 kcal | 98 kcal |
| Woman, 25 yr, 55 kg, 160 cm | 1,220 kcal | 1,296 kcal | 76 kcal |
From BMR to the number you actually eat
BMR is not a target. It is the floor of your energy use, covering only what keeps you alive while lying still. Everything else you do that day adds to it: walking, digesting food, fidgeting, working and training. Multiply BMR by an activity factor to get total daily energy expenditure, which is the figure your calorie target is set against.
Activity multipliers and what BMR becomes
| Activity level | Multiplier | BMR 1,750 becomes |
|---|---|---|
| Sedentary, desk job | 1.2 | 2,100 kcal |
| Light, 1-3 sessions per week | 1.375 | 2,406 kcal |
| Moderate, 3-5 sessions | 1.55 | 2,713 kcal |
| Active, 6-7 sessions | 1.725 | 3,019 kcal |
| Very active, physical job | 1.9 | 3,325 kcal |
What makes up your daily energy use
| Component | Share of total | Can you change it? |
|---|---|---|
| Basal metabolic rate | 60-70% | Slowly, by gaining lean mass |
| Thermic effect of food | About 10% | Slightly, protein costs most to digest |
| Non-exercise activity | 15-30% | Yes, and it varies hugely between people |
| Deliberate exercise | 5-15% | Yes, but less than most people assume |
What actually changes your BMR
- Lean mass. The main lever you control. More muscle means a higher resting burn, though the effect is modest at roughly 13 kcal per kg of muscle per day.
- Body size. Larger bodies cost more to maintain, which is why BMR scales with both height and weight.
- Age. BMR declines gradually through adulthood, largely because lean mass declines. Resistance training slows that.
- Prolonged dieting. Extended deficits reduce BMR somewhat beyond what weight loss alone predicts, a phenomenon known as adaptive thermogenesis.
- Genetics and thyroid function. Real, individual, and outside your control. If your measured intake and weight change consistently contradict every equation, that is worth a conversation with a doctor.
The practical takeaway: use the estimate as a starting point, hold it for two to three weeks, and let your actual weight trend correct it. Your bathroom scale is a better calorimeter than any equation.
Frequently asked questions
What is a normal BMR?
Is BMR the same as TDEE?
Should I eat my BMR to lose weight?
Does BMR fall when you diet?
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General information, not medical advice. Consult a qualified professional before changing your diet, especially with a medical condition.