BMR calculator: calories your body burns at rest

Written byDr. Pooja Sahu

Last Updated Oct 29, 2026

Your basal metabolic rate (BMR) is the energy your body uses at complete physical, mental and digestive rest. Resting energy use makes up about 60 to 70% of all the energy you use in a day, which makes it the largest part of your daily energy budget.

The calculator above estimates your BMR from your sex, age, weight and height with the Mifflin-St Jeor equation. It also shows the 1984 Harris-Benedict equation for comparison and multiplies your BMR by an activity factor to estimate your total daily energy expenditure (TDEE). Choose US units to enter pounds, feet and inches, and the calculator converts them for you. Every result is an estimate drawn from population data, so treat it as a starting point that your own weight trend will refine.

What is the difference between BMR, RMR and TDEE?

Three measurements sit behind your results, and each covers a different slice of the day.

Source: Dietary Reference Intakes for Energy, National Academies (2023)

Most calculators skip one detail. Mifflin-St Jeor was built from resting measurements, which run about 10% above strict basal ones, so the number this tool labels “BMR” sits closer to your RMR.

That gap rarely changes a meal plan. It matters if you compare your result with a lab report from indirect calorimetry, the breathing test that calculates energy use from the oxygen you take in and the carbon dioxide you breathe out.

The two formulas behind your result

Mifflin-St Jeor supplies the main number. Its authors derived it from resting measurements of 498 healthy adults aged 19 to 78, 247 women and 251 men, of whom 234 had obesity. Each person was measured by indirect calorimetry.

Harris-Benedict is much older. The original equations date from 1918 and 1919, and this calculator uses the coefficients from a 1984 revision of them.

Source: Mifflin-St Jeor equations from the 1990 study in healthy adults; Harris-Benedict coefficients from the 1984 revision.

When the 1990 team tested the original Harris-Benedict equations against their own measurements, those equations overestimated resting burn by 5%. In the worked example further down, the revised version reads 63 kcal higher than Mifflin-St Jeor.

How close is the estimate to a lab measurement?

Within about 10% for most people. A systematic review for the American Dietetic Association compared the common prediction equations and found that Mifflin-St Jeor came within 10% of measured resting metabolic rate in more people, with and without obesity, than any other equation, and had the narrowest error range. Older adults and US ethnic minorities were underrepresented in the studies used to build and test these equations, which leaves more uncertainty in their estimates.

For an estimate of 1,400 kcal/day, a 10% margin covers 1,260 to 1,540 kcal/day. Some people fall outside even that.

The formula sees only weight, height, age and sex. In the 1990 study, fat-free mass (muscle, organs, bone and body water) explained more of the difference between people than body weight did, with an R² of 0.64 against 0.56. Two people with identical inputs but different amounts of muscle receive the same estimate, even though their measured resting burn can differ.

Six results from one set of inputs

A 40-year-old woman who weighs 70 kg (154 lb), stands 165 cm (5 ft 5 in) and is lightly active gets the results below. The values are invented for illustration.

Source: calculated with the two equations shown earlier.

The two formulas disagree by 63 kcal. The moderate weight-loss target matters more: at 1,384 kcal, it sits only 14 kcal above her resting estimate. When TDEE is modest, a 500-calorie cut can bring intake close to BMR.

Most of the error comes from the activity setting

Across the five activity levels, the same woman’s daily total runs from 1,644 kcal (sedentary, × 1.2) to 2,603 kcal (extra active, × 1.9). That 959-calorie spread is about 15 times the gap between the two formulas.

The multiplier is a ratio the national energy guidelines also use. They divide total daily burn by basal burn and call the result physical activity level (PAL), and their 2023 report groups adults into four bands drawn from doubly labeled water studies of people living their normal lives.

Source: Dietary Reference Intakes for Energy, chapter 7

The typical inactive adult in that data sits near 1.4, above this tool’s sedentary factor of 1.2. If you work at a desk but walk, cook and do chores, the lightly active setting often fits better. Save sedentary for days spent mostly sitting or lying down.

One caution applies. The guideline ratio is built on basal burn, while this calculator multiplies a slightly higher resting estimate, so use the table to choose a setting and let your weight trend do the fine-tuning.

If your weight holds steady for two to four weeks at a given daily intake, that intake is your maintenance level.

Things that move your BMR up or down

Each input shifts the estimate by a fixed amount.

Source: calculated from the Mifflin-St Jeor coefficients shown earlier.

Lean tissue changes your true resting burn without appearing in the formula. Fat-free mass was the strongest single predictor in the original Mifflin-St Jeor data, so two people of the same weight can differ if one carries more muscle. The body fat calculator gives a rough split between fat and lean mass.

Thyroid hormone sets the pace of metabolism throughout the body, and an underactive thyroid slows it. When tiredness, feeling cold or unexplained weight gain come along with it, a TSH blood test is the usual first check, and the page on what a high TSH result means covers the follow-up tests.

Age matters later than most people expect. A Science analysis of energy use in people aged 8 days to 95 years found that, once adjusted for fat-free mass, total and basal energy use stayed steady from about age 20 to 60, even during pregnancy, and declined after that.

Long, large calorie deficits lower energy use as well. Underfeeding brings compensating metabolic changes and lean-tissue loss, and both reduce daily energy expenditure. Because the formula drops 50 kcal for every 5 kg lost, a recalculation after each 4 to 5 kg change keeps your numbers current.

Set your calorie target from your TDEE

BMR leaves out every step you take and the energy spent digesting food, so meal planning starts from TDEE. The calorie deficit calculator turns your TDEE into a daily target for weight loss.

With that target in hand, the macro calculator splits it into protein, carbohydrate and fat, and the protein intake calculator estimates how much protein to eat while you are in a deficit.

Plan your target with a doctor or registered dietitian if any of these apply:

• You are under 19, since the equation was built on adults aged 19 to 78.
• You are pregnant or breastfeeding.
• Your BMI is under 18.5, or an eating disorder is part of your history.
• Your weight has changed without explanation, or a condition or medicine affects your weight or thyroid.

Note: If calorie numbers push you toward eating less than feels safe, or you have a history of an eating disorder, the National Alliance for Eating Disorders helpline (866-662-1235, weekdays 9 a.m. to 7 p.m. ET) offers free support and treatment referrals from licensed therapists.

For everyone else, recalculate after a lasting change in weight or activity, and compare the new TDEE with what your scale has shown over the past few weeks.

Questions people ask about BMR

Is it bad to eat fewer calories than your BMR?

Sustained intake below your BMR means a large deficit once your activity is counted. The National Academies report notes that ongoing underfeeding brings compensating metabolic changes and loss of lean tissue, both of which lower energy expenditure.

Is BMR the same as the calories you burn while sleeping?

No. Lab measurements of BMR are taken while you are awake and lying still, usually right after waking from an overnight fast, so the number is a close stand-in for overnight energy use rather than a measurement of sleep itself.

How this was made

About this content

How this article was put together: researched from recognised health sources, drafted with the help of AI tools, and edited by hand, with sources linked throughout.

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Dr. Pooja Sahu

Researched and written from recognised health sources

Dr. Pooja Sahu, MBBS, is a medical reviewer for MyMedicineAdvisor. She is a clinical physician whose experience includes serving as a Junior Resident in Nephrology at Dr. Ram…

Content on My Medicine Advisor is researched from recognised sources, drafted with AI assistance, and edited by hand, with sources linked so you can check them. Articles medically reviewed by Dr. Pooja Sahu, MBBS, show her name and the review date. This is general health education, not medical advice.