BMR Calculator

What basal metabolic rate is, and what it is not

Basal metabolic rate is the energy your body spends keeping you alive while you do nothing: breathing, circulating blood, holding your core temperature, and running a brain, liver and pair of kidneys that never clock out. In a laboratory it is measured after an overnight fast and a night of sleep, lying still and awake in a room that is neither warm nor cold. For most people it is the largest single slice of the day’s energy use.

The naive move is to treat that figure as a daily calorie target. It is not one. BMR describes a body at complete rest, and you spent today walking to the kitchen, sitting upright, digesting food and possibly training. Eat to your BMR and you have quietly set a deficit of several hundred calories without deciding to. The number is a floor to build on, not a budget.

This page uses the Mifflin-St Jeor equation, published in 1990, which predicts resting energy from four inputs:

Men:   BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) − 161

Four inputs cannot see how much of your weight is muscle, how your thyroid is behaving, what medication you take or what you inherited. That is why this is an estimate, and why two people with identical height, weight, age and sex can genuinely burn a couple of hundred calories a day apart. Use it as the starting assumption you then correct with a few weeks of real-world results.

Which number you actually need

• BMR: the resting floor. Useful as the input to everything else, and as a sanity check that a diet plan is not absurdly low.
• RMR: resting metabolic rate, the same idea measured under looser conditions than a true basal test. It reads slightly higher than BMR. Apps, clinics and calculators use the two words almost interchangeably, so do not chase the difference.
• TDEE: BMR multiplied by an activity factor. This is the number that matters for eating, because it includes walking, work, fidgeting and training.
• A calorie target: TDEE with a deliberate adjustment. Below it to lose fat, above it to gain.
• Metric or imperial: switch the tabs above the height and weight fields. The equation is defined in centimeters and kilograms, so imperial entries are converted before the sum is done. Both routes give the same answer.

How to work out your BMR

1. Choose your sex. The equation applies a flat correction of plus 5 for men and minus 161 for women, standing in for the average difference in muscle mass.
2. Enter your age in whole years. Each year subtracts 5 calories from the estimate.
3. Pick metric or imperial, then enter height and weight. In imperial the height is split into feet and inches, and the converted centimeter and kilogram figures are shown underneath so you can check them.
4. Read the result in kcal per day. The bar chart under it shows what the same body would burn in total at five activity levels, with a red marker 500 kcal below and a green marker 300 kcal above the lightly active total. The red marker is left off when 500 below would fall under 1,200 kcal a day for women or 1,500 for men, the same floor the TDEE calculator applies to its fat loss target.
5. Weigh yourself the same way each time, ideally in the morning before eating. Day to day swings of two to four pounds are mostly water, and they move the estimate by only four or five calories per pound (ten per kilogram).

How BMR turns into a daily calorie target

BMR is the first term in a chain. Multiply it by an activity factor and you get TDEE: about 1.2 for a desk job with little exercise, 1.375 for light activity, 1.55 for moderate, 1.725 for hard training most days. A goal is then set against that total. The chart on this page marks 500 kcal below and 300 kcal above the lightly active figure. Harvard Health Publishing describes eating 500 to 1,000 calories under maintenance as the way to lose 1 to 2 pounds a week, and the CDC describes that same 1 to 2 pounds a week as the gradual, steady pace that people are more likely to keep off.

The activity factor is the weakest link, because people are poor judges of their own activity and the factors are coarse, so the first target is an estimate to check against a few weeks of weigh-ins. The TDEE calculator does the multiplication and walks through that correction.

Why other calculators, apps and scales disagree

Nobody measures your metabolism with a calculator, so every result you see is an equation picking a line through other people’s data. Four are in common use.

• Mifflin-St Jeor, used here. Derived in 1990 from indirect calorimetry on 498 healthy adults aged 19 to 78, about half of them obese. A 2005 systematic review found it predicted resting metabolic rate to within 10 percent of the measured value in more people than the other equations it compared.
• Harris-Benedict. The original dates from 1918 and 1919 and was recalculated in 1984; both versions take the same four inputs. The Mifflin-St Jeor paper found that the original overestimated measured resting energy by 5 percent in its own sample, so if another site gives you a noticeably bigger number, this is often why.
• Katch-McArdle. Ignores sex and height and works from lean body mass alone, in the published form of 370 plus 21.6 times lean mass in kilograms. It is only as good as the body fat measurement behind it.
• Cunningham. Also lean-mass based, from a 1980 reanalysis of the Harris-Benedict data: 500 plus 22 times lean mass in kilograms. For the same lean mass it always reads at least 130 kcal above Katch-McArdle.

Devices add another layer. A body composition scale estimates lean mass from a small electrical current, and that estimate shifts with hydration, so the BMR it prints can move without your metabolism moving at all. A watch or a food tracking app may be running any of the equations above. Choose one source and watch its trend. Comparing a watch, a scale and three websites tells you about the equations, not about you.

Limits, and a note about medical advice

This calculator is a general information tool, not medical advice, and it cannot diagnose anything. The equation was derived from healthy adults aged 19 to 78. If you are pregnant or breastfeeding, under 18, an athlete with unusual body composition, recovering from illness or surgery, taking medication that affects metabolism, or living with a thyroid condition or an eating disorder, it was not built for your situation and you should work from advice given by a doctor or a registered dietitian instead. The 2005 review cited below also found older adults underrepresented in the studies that tested it.