BMR Calculator

What is BMR?

BMR stands for basal metabolic rate. It's the number of calories your body would burn if you stayed in bed all day and did absolutely nothing. No walking to the kettle, no scrolling on your phone while sitting up, just lying still, awake, in a comfortably warm room.

Even then, your body is busy. Your heart beats about a hundred thousand times a day. Your lungs keep moving air. Your liver, kidneys and brain are all working, and the brain alone uses roughly a fifth of your resting energy. Your cells are repairing themselves and your body is holding its temperature at around 37°C. All of that costs fuel, and the bill for staying alive is your BMR.

For most adults, BMR makes up somewhere between 60 and 75 percent of the calories they burn in a day. That's why it's the sensible starting point for any calorie plan. If you don't know this number, you're guessing at everything that comes after it.

BMR vs TDEE vs RMR: what's the difference?

These three terms get mixed up constantly, even on gym websites, so here's the short version.

BMR (basal metabolic rate)

The strict, lab-style number. It's measured after a night's sleep, after fasting for 12 hours or so, lying down and fully relaxed. It's the lowest amount of energy you need to keep going.

RMR (resting metabolic rate)

The practical cousin. It's measured under less strict conditions, so it comes out a little higher, usually by 10 percent or less. Most online calculators, ours included, are really estimating something between the two. People use "BMR" and "RMR" interchangeably, and for planning your meals the gap doesn't matter.

TDEE (total daily energy expenditure)

This is the one you actually eat for. TDEE is your BMR plus everything else: the calories you burn digesting food (about 10 percent of what you eat), your workouts, and all the small movement you don't think of as exercise, like cooking, pacing during phone calls or climbing stairs. Eat around your TDEE and your weight stays put. Eat below it and you lose weight. Eat above it and you gain.

How BMR is calculated

Nobody walks around with a lab breathing mask, so we estimate BMR with equations. Researchers measured the energy use of lots of people, then worked out a formula that predicts it from age, sex, height and weight. This calculator uses three of them.

We show Mifflin-St Jeor as the primary number because it has held up best in studies of ordinary, healthy adults. Harris-Benedict is older. The original dates back to 1919, and the version used here was updated in 1984 with better data. Katch-McArdle works differently, because it ignores age and sex and looks only at your lean mass. That makes it more useful for someone with a lot of muscle, and less useful if you're guessing your body fat.

Once you have BMR, you multiply it by an activity factor to get TDEE:

A worked example, step by step

Let's do one by hand so the numbers don't feel like magic. Meet Priya. She's 45, a mother of two, 165 cm tall and 68 kg. She works part time, walks the kids to school and goes to a yoga class twice a week. Not a gym machine, but not a sofa either. We'll call her lightly active.

Step 1: Mifflin-St Jeor

For women the formula is 10 × weight + 6.25 × height − 5 × age − 161.

• 10 × 68 = 680
• 6.25 × 165 = 1,031.25
• 5 × 45 = 225
• 680 + 1,031.25 − 225 − 161 = 1,325 kcal per day

Step 2: Compare with the other equations

Revised Harris-Benedict gives 447.593 + (9.247 × 68) + (3.098 × 165) − (4.330 × 45), which comes to about 1,393 kcal. If Priya knew her body fat was around 32 percent, her lean mass would be 68 × 0.68 = 46.2 kg, and Katch-McArdle would give 370 + (21.6 × 46.2) = about 1,369 kcal. Three methods, three answers within about 70 calories of each other. That's a good sign.

Step 3: Turn BMR into TDEE

Lightly active means a multiplier of 1.375, so 1,325 × 1.375 = about 1,822 kcal per day. That's roughly what Priya needs to hold her weight steady.

Step 4: Set a target

If she'd like to lose weight slowly, a 250 to 500 calorie daily gap works out to a target of roughly 1,320 to 1,570 kcal. Notice that her BMR is 1,325. Going much lower than that isn't sensible, and that's why our calculator won't show anything below 1,200 kcal for women.

For comparison, take Dan, a 30-year-old office worker, 178 cm and 82 kg. His Mifflin-St Jeor BMR is 10 × 82 + 6.25 × 178 − 150 + 5 = 1,788 kcal. With a desk job and a couple of casual gym visits, he's lightly active, so his TDEE comes to about 2,458 kcal.

What changes your BMR

Two people with the same height and weight can have noticeably different BMRs. Here's why.

Muscle mass

Muscle is more expensive to keep than fat. Rough estimates put muscle at about 13 calories per kilogram per day at rest, against about 4.5 for fat. Add a few kilos of muscle and your BMR rises, though by less than most gym ads suggest.

Age

For decades the rule was that metabolism slows steadily from your twenties. A large 2021 study that measured thousands of people suggests it's more nuanced. After adjusting for body size and organ mass, metabolism stayed fairly stable from the twenties through to about age 60, then drifted down by under one percent a year. A lot of the "slowing down" many of us feel in middle age comes from losing muscle and moving less. That part is partly in our hands.

Sex

Men usually have more lean mass and less body fat than women of the same weight, which is why the formulas add 5 for men and subtract 161 for women.

Body size

A bigger body takes more energy to run. That's the reason weight and height are in every formula, and it's also why BMR drops a little as you lose weight.

Genetics, hormones and health

Some people simply run hotter or cooler. An underactive thyroid can lower BMR, and an overactive one raises it. Pregnancy and breastfeeding increase energy needs. Fever, injury and some medications change them too. If you've got a medical condition, treat the calculator's answer as a rough sketch and check with your doctor.

Sleep, stress and crash dieting

Poor sleep doesn't slash your BMR much, but it does push up hunger and cravings, which can wipe out a calorie plan. Eating far too little for months can lower your metabolism a bit as your body adapts and you lose muscle. Temperature matters slightly as well, since your body works a bit harder in the cold.

Using your BMR for weight loss, maintenance or muscle gain

Your BMR is not your calorie goal. Your TDEE is the baseline, and your goal is built from there.

To maintain your weight

Eat around your TDEE. It won't be exact, so weigh yourself a few mornings a week and watch the average over two to three weeks. If it drifts up or down, nudge your intake by 100 to 150 calories.

To lose weight

Aim for a modest deficit, usually 250 to 500 calories below your TDEE. A 500 calorie gap works out to roughly 0.45 kg (1 lb) a week on paper. In real life the pace varies, because your body adapts and water weight bounces around. Slower is easier to stick to, and it protects your muscle. Don't eat below your BMR for long stretches, and never go beneath the safe minimums of 1,200 kcal for women and 1,500 kcal for men without medical supervision.

To build muscle

A small surplus of 250 to 500 calories above TDEE gives your body the material it needs, with less fat gain than a big "bulk". Pair it with strength training and plenty of protein, commonly around 1.6 to 2.2 grams per kilogram of body weight for people who lift regularly.

Don't forget food quality

Calories decide whether your weight goes up or down, but what you eat decides how you feel on the way. Protein keeps you full and protects muscle. Fibre-rich vegetables, fruit, beans and whole grains make a calorie budget feel generous. Our macro presets are only starting points. If you have diabetes, kidney disease or any other condition, ask a doctor or registered dietitian before changing how you eat.

Common myths and mistakes

"I have a slow metabolism, so I can't lose weight"

True differences between people exist, but they're smaller than most people assume. Two adults of the same size and age typically differ by a few hundred calories at most, not thousands. Underestimating how much we eat is far more common than a truly broken metabolism.

"Eating more often speeds up metabolism"

Meal timing has little effect on the total calories you burn. Three meals or six, the digestion cost depends on how much you eat overall. Pick whatever pattern you can stick with.

"Cutting calories as low as possible is fastest"

It's fastest at making you tired, hungry and fed up. Very large deficits cost muscle, and most people who crash diet regain the weight.

"My fitness tracker's calories are exact"

Wearables can be off by 20 percent or more, especially for calories burned in a workout. Treat them as hints.

"I exercise a lot, so I'm very active"

This is the biggest calculator mistake. If you train for an hour but sit for the other eleven waking hours, you're probably lightly or moderately active. When in doubt, pick the lower level.

Using the wrong units

Pounds typed into a kilogram box give silly numbers. Use the unit toggle and the calculator converts everything for you.

How accurate is a BMR calculator?

Honestly, it's a good estimate and nothing more. In a widely cited 2005 review, Mifflin-St Jeor predicted resting metabolism within 10 percent of the measured value for roughly eight in ten non-obese adults, and for around seven in ten adults with obesity. That means for a fair number of people the equation will be off by more than 10 percent. For someone with a BMR of 1,600, that's 160 calories either way.

These equations were built from averages. They can't see your muscle, your thyroid, your genes or your medication. The activity multipliers add another layer of guesswork, since "moderately active" means different things to different people.

So use the result as a starting point. Follow it for two to three weeks, watch what your weight actually does, and adjust. The scale and the mirror are your real data. The calculator only gets you to the right neighbourhood.

Frequently asked questions

Medical disclaimer

The information and calculations on this page are for general education only. They are estimates and are not medical advice, a diagnosis or a treatment plan. Please talk to a doctor or registered dietitian before you start a new diet or exercise programme, especially if you're pregnant or breastfeeding, under 18, have an eating disorder or a medical condition, or take regular medication. If you feel unwell, stop and seek professional care. Read our full Disclaimer.