What BMR actually measures
Your basal metabolic rate (BMR) is the number of calories your body burns in 24 hours at complete rest — lying still, awake, in a comfortable temperature, having fasted. It is the energy cost of simply being alive: your heart beating, your lungs breathing, your brain thinking, your liver and kidneys filtering, and every cell repairing itself. For most sedentary adults, BMR accounts for roughly 60 to 75 percent of all the calories they burn in a day, which is why it is the foundation every other calorie number is built on. This page calculates BMR on its own, then shows what happens when ordinary daily activity is layered on top of it.
The Mifflin-St Jeor formula
Published in 1990, the Mifflin-St Jeor equation is the formula most dietitians reach for first, because validation studies have repeatedly found it the most accurate of the common prediction equations for healthy adults — typically within about 10 percent of a laboratory measurement. The structure is easy to read: every kilogram of body weight adds about 10 calories, every centimetre of height adds 6.25, every year of age subtracts 5 (metabolism slows as lean tissue is lost with age), and the final constant adjusts for the average difference in body composition between men and women of the same size.
BMR is not TDEE, and neither is a calorie target
Three numbers are often confused, so be clear about which one you are looking at. BMR is rest only — nobody actually lives at rest, so nobody maintains their weight eating exactly their BMR. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor, and it estimates the calories that maintain your current weight in your real life; the five activity figures on this page are TDEE estimates. A calorie target then adjusts TDEE up or down for gaining or losing. If TDEE is what you need, go straight to the TDEE Calculator; if you want maintain, lose and gain targets worked out in one place, use the Calorie Calculator. This BMR page exists for the layer underneath both — useful when a TDEE figure looks suspicious and you want to check the foundation it was built from.
Choosing an activity multiplier honestly
The activity multipliers — 1.2, 1.375, 1.55, 1.725 and 1.9 — are the roughest part of any BMR calculation, because they compress your whole lifestyle into one number. The most common mistake is overestimating: training hard for an hour does not make the other 23 hours active. A desk worker who trains three times a week is usually light to moderate, not active. People on their feet all day — nurses, builders, warehouse and retail staff — often land a band higher than gym-goers who sit the rest of the day. When in doubt, pick the lower band: if your weight holds steady on the calories it predicts, the estimate was right; if it drifts, adjust by 100 to 200 calories and watch for another two weeks rather than recalculating from scratch.
Worked example
A 30-year-old man, 80 kg and 180 cm: BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 800 + 1,125 − 150 + 5 = 1,780 kcal per day at complete rest. Multiplied out, that is about 2,136 kcal sedentary, 2,448 kcal lightly active, 2,759 kcal moderately active, 3,071 kcal active and 3,382 kcal very active.
How to use this calculator
- Select male or female — the Mifflin-St Jeor equation uses a different constant for each.
- Enter your age in years, your weight in kilograms and your height in centimetres.
- Read your BMR in the headline panel: the calories you burn at complete rest.
- Find your honest activity level in the breakdown table to see your maintenance calories (your TDEE estimate) at that level.
- For goal-based targets, continue to the TDEE Calculator or the Calorie Calculator linked in the text.
Why two people of the same weight can have different BMRs
Weight is the biggest single driver of BMR, but it is not the only one, and the formula's other terms hint at why. Taller people tend to have more lean tissue and more skin surface, both of which cost energy. Muscle is metabolically expensive tissue while fat is comparatively cheap to maintain, so two people at the same weight and height — one muscular, one not — genuinely burn different amounts at rest, and no equation using only weight, height, age and sex can see that difference. Age matters for the same reason: lean mass typically declines from middle age onward, which is the real story behind a "slowing metabolism." Hormones, illness, fever, pregnancy, breastfeeding and some medications also shift true BMR, sometimes substantially.
Treat the result as a well-founded estimate, not a measurement. If you track your food and weight carefully for three or four weeks, your own data will refine the number better than any formula can — adjust in small steps of 100 to 200 calories based on the trend in your weight, and use this page as the sensible starting point it is designed to be.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.