01What BMR means
Your Basal Metabolic Rate (BMR) is the number of calories your body burns each day just to keep its core systems running — breathing, circulating blood, repairing cells — even if you stayed in bed all day. It is the largest single part of how much energy you use, and it is the starting point for almost every calorie or weight plan.
BMR is best understood as a baseline: it tells you the minimum energy your body needs at complete rest. The Overview tab covers what the calculator does, how activity turns BMR into your total daily needs, and practical guidance; the Formula tab shows the maths and a worked example.
02What this calculator does
This BMR calculator takes four inputs — your age, sex, height, and weight, in metric or imperial units — and estimates your Basal Metabolic Rate using the Mifflin-St Jeor equation, the formula most clinicians and modern calculators rely on.
It then multiplies your BMR by a set of activity factors to show your Total Daily Energy Expenditure (TDEE) — the calories you actually burn in a day — across five activity levels. The calculator rejects impossible entries (a blank field, zero, a negative number, or an out-of-range age) so the result stays meaningful.
03Activity levels & daily calories
BMR only counts the calories you burn at rest. To find your Total Daily Energy Expenditure (TDEE) — the number to use when planning how much to eat — multiply your BMR by an activity factor that matches your week:
TDEE, not BMR, is the figure to build a calorie target around. For weight loss, a common starting point is a deficit of about 300–500 calories a day below your TDEE; for muscle gain, a small surplus above it.
A BMR calculator gives a solid estimate, but it is a population-based formula, not a personal measurement. A few things shift the real number:
- Muscle vs fat. Muscle burns more energy at rest than fat, so two people of the same age, height, and weight can have different BMRs depending on body composition.
- Age. BMR tends to fall as you get older, partly because muscle mass declines — so it is worth recalculating every few years.
- Hormones and health. Thyroid levels, genetics, illness, and some medications all nudge BMR up or down.
- Pregnancy and children. The standard adult equation is not designed for pregnancy or for under-18s, whose needs differ.
- Out-of-range inputs are rejected. Age must be 15–80 and height and weight within sensible limits; the calculator flags anything outside them instead of returning a misleading figure.
- Pick Metric (cm, kg) or Imperial (ft/in, lb), then choose your sex.
- Enter your age, height, and weight.
- Tap Calculate to see your BMR and a table of daily calories (TDEE) for five activity levels.
Tips for using your result
- Use your TDEE, not your BMR alone, as the baseline for any calorie target — BMR is just the resting portion.
- For fat loss, a deficit of roughly 300–500 calories below TDEE is a common, sustainable starting point.
- Recalculate every few weeks if your weight changes by more than a couple of kilograms, since BMR moves with it.
01The BMR formula
This calculator uses the Mifflin-St Jeor equation — the formula most widely used in clinical practice and considered more accurate than the older Harris-Benedict method. It uses the same inputs for everyone, with a small adjustment for sex:
Where:
- weight = your body weight in kilograms (the calculator converts pounds for you)
- height = your standing height in centimetres
- age = your age in years
- +5 / −161 = the sex constant: +5 for men, −161 for women
The only difference between the two is the constant at the end (+5 for men, −161 for women), which reflects average differences in body composition. The result is your BMR in calories per day.
02Worked example
Take a 28-year-old man who weighs 80 kg and stands 178 cm tall. Substitute into the men’s formula one step at a time:
At a sedentary activity level (×1.2) that is about 2,133 calories a day; at a moderate level (×1.55), roughly 2,755. Those TDEE figures — not the bare BMR — are what you would build a calorie target around.
BMR Calculator
| Activity level | × | Cal/day |
|---|---|---|
| Sedentary little/no exercise | 1.2 | — |
| Light 1–3 days/week | 1.375 | — |
| Moderate 3–5 days/week | 1.55 | — |
| Very active 6–7 days/week | 1.725 | — |
| Extra active hard job / 2× training | 1.9 | — |