BMR calculator

What BMR actually measures

Basal metabolic rate is the energy your body uses at complete rest, in a neutral temperature, several hours after eating. It pays for the things that never stop: pumping blood, breathing, maintaining body temperature, running the kidneys and liver, and replacing cells. It is not the same as the calories you burn in a day — it is the floor beneath that number.

For most people BMR accounts for roughly 60 to 75% of daily energy use. That surprises people who assume exercise dominates. An hour of moderate cycling might add 500 kcal; simply being alive for 24 hours costs most adults somewhere between 1,300 and 1,900.

The equation used here

This calculator uses the Mifflin-St Jeor equation, published in 1990:

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

Mifflin-St Jeor replaced the older Harris-Benedict equation, which dates from 1919 and tends to overestimate by around 5% in modern populations. Where the two disagree, Mifflin-St Jeor is the better bet: it was validated against indirect calorimetry in a more representative sample, and it is the equation most dietetic guidance now defaults to.

It is still an estimate from four inputs. Two people of the same sex, age, height and weight can differ by 200 kcal a day or more, mostly because the equation cannot see body composition. Muscle is metabolically expensive; fat is comparatively cheap. A lean, muscular person sits above the prediction and a sedentary person with the same weight sits below it.

From BMR to the number you can actually use

BMR on its own is not a calorie target. Multiply it by an activity factor to get total daily energy expenditure — TDEE — which is roughly what you burn across a real day:

• 1.2 — desk job, no deliberate exercise
• 1.375 — light exercise one to three days a week
• 1.55 — moderate exercise three to five days
• 1.725 — hard exercise six or seven days
• 1.9 — twice-daily training, or a physically demanding job on top of training

Most people overestimate their activity factor. If you train hard four times a week but spend the rest of your time sitting, 1.55 is closer to honest than 1.725. The multiplier is meant to cover the whole week, not the days you feel most active.

Using it for weight change

A kilogram of body fat stores roughly 7,700 kcal, so a deficit of about 550 kcal a day predicts half a kilogram of loss per week. The calculator shows that figure, but treat it as a starting hypothesis rather than a promise. Metabolic rate adapts downward during a sustained deficit, activity outside training tends to fall without you noticing, and self-reported food intake is famously understated — by 20% or more in careful studies.

The practical approach is to take the estimate, eat at it consistently for two or three weeks, track weight as a rolling average rather than day to day, and adjust from what actually happens. Your own trend data beats any equation after a fortnight.

When the estimate does not apply

Mifflin-St Jeor was derived from healthy adults. It is not appropriate for children or adolescents, who need age-specific equations, and it is unreliable at the extremes of body composition — very high BMI, competitive bodybuilders, or people with significant muscle wasting. Pregnancy and breastfeeding add substantial requirements that no BMR equation accounts for. Thyroid disorders and some medications shift metabolic rate independently of body size.

Common questions