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 off. 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 kettle, 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. Never set the adjustment against BMR.
• Metric or imperial: switch the tabs above the height and weight fields. The equation is defined in centimetres 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 centimetre 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 markers for a 500 calorie deficit and a 300 calorie surplus.
5. Weigh yourself the same way each time, ideally in the morning before eating. Day to day swings of a kilogram or two are water, and they move the estimate by about ten calories 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. Then set your goal against that total. A deficit of roughly 300 to 500 calories a day is the usual advice for losing fat at a pace you can keep up, and a surplus of 200 to 300 is enough to support muscle gain without much fat coming with it.

The activity factor is the weakest link, because people are poor judges of their own activity and the factors are coarse. Treat the first target as a hypothesis. Hold it for two or three weeks, track weight on the same scale at the same time of day, and adjust by 100 to 200 calories based on what the trend actually did rather than what the arithmetic promised. The TDEE calculator does the multiplication and shows the goal bands side by side.

Keeping the number and sharing it

• Select and copy: the result is ordinary text. Drag across it, then Ctrl+C on Windows, Cmd+C on a Mac, or tap and hold to select on a phone.
• In a tracking app: most of them ask for a maintenance or goal figure rather than a BMR. Put your TDEE-based target in that field and keep BMR in your own notes as the floor you do not want to sit under.
• For a dietitian, coach or doctor: bring the four inputs, the estimate and which equation produced it. Mifflin-St Jeor is the one most clinicians expect, so naming it saves a question.
• Sharing the page: the inputs are not stored in the address, so a link you send opens with the default values rather than yours. Screenshot the result if the exact figure matters.

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. Built on measurements of healthy adults and published in 1990, and generally the default in dietetic practice for people whose body composition is unremarkable.
• Harris-Benedict, revised. The original dates from around 1919 and the revision from 1984. It takes the same four inputs but usually reads higher than Mifflin-St Jeor for a modern adult, which is the main reason many calculators moved away from it. 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 needs a body fat measurement you trust, and in exchange it handles very lean and very heavy bodies better than height-and-weight equations do.
• Cunningham. Also lean-mass based, in the form of 500 plus 22 times lean mass in kilograms, and popular with athletes because it tends to read higher for muscular people.

Devices add another layer. A body composition scale estimates lean mass from a small electrical current, and that estimate shifts with hydration, time of day and how recently you trained, so the BMR it prints can move without your metabolism moving at all. Fitness watches blend a standard equation with heart rate and logged workouts without publishing the recipe, and food tracking apps rarely say which equation they picked. 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. If you are pregnant or breastfeeding, under 18, over 65, 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, the equation was not built for your situation and you should work from advice given by a doctor or a registered dietitian instead.

On eating below your BMR: it is a poor idea to sustain without supervision. Intakes that low make protein, fibre and micronutrient targets hard to hit, and people who try them commonly report low energy, disrupted sleep, feeling cold and losing strength in the gym. Medically supervised very low calorie diets do exist for specific clinical reasons, and the supervision is the point. Set any deficit against your TDEE.

A last word on precision. The result is shown as a whole number because a decimal would be absurd, but do not read it as exact. Treat it as a band of roughly plus or minus ten per cent, use it to set a starting point, and let two or three weeks of honest tracking tell you where your own line really sits.