BMR calculator
This BMR calculator estimates your basal metabolic rate — the number of calories your body burns at complete rest — using the Mifflin-St Jeor formula, the modern clinical standard. Enter your sex, age, height, and weight to see your resting calorie burn, a side-by-side Harris-Benedict comparison, and a quick breakdown of what your daily calorie needs could look like at different activity levels.
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What BMR is and why it's the foundation of calorie planning
Basal metabolic rate (BMR) is an estimate of how many calories your body burns in a day purely to stay alive — at complete rest, with no movement, digestion, or exercise factored in. It covers the energy cost of breathing, circulating blood, regulating body temperature, maintaining cell function, and keeping your organs running. For most people, BMR accounts for the largest single share of total daily energy expenditure, which is exactly why every serious calorie-planning method starts here: you cannot work out how much to eat for weight loss, maintenance, or gain until you know your resting baseline first.
The Mifflin-St Jeor formula (and Harris-Benedict, for comparison)
This calculator's primary figure uses the Mifflin-St Jeor equation, published in 1990 and now widely regarded as the modern clinical standard because it was derived from a more contemporary, representative population than its predecessors:
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
Alongside it, the calculator also computes the older Harris-Benedict equation (revised in 1984) purely as a point of comparison — it was the standard for decades and you'll still see it referenced in older sources and some clinical tools:
Men: BMR = 13.397 × weight (kg) + 4.799 × height (cm) − 5.677 × age (years) + 88.362
Women: BMR = 9.247 × weight (kg) + 3.098 × height (cm) − 4.330 × age (years) + 447.593
Both equations take the same four inputs — sex, age, height, and weight — and combine them with constants derived from real measured data, but each was fitted to a different population at a different point in time. Mifflin-St Jeor is generally considered more accurate for today's populations and is the equation most dietitians and clinical guidelines now lean on, which is why it's shown as this page's primary result. If you enter imperial units, the calculator first converts pounds to kilograms (kg = lb × 0.453592) and feet/inches to centimetres (cm = (feet × 12 + inches) × 2.54) before running either formula, so the maths underneath stays identical either way.
Worked example — a 30-year-old woman, 165 cm, 65 kg
Plug a 30-year-old woman who is 165 cm tall and weighs 65 kg into the Mifflin-St Jeor formula for women:
BMR = 10 × 65 + 6.25 × 165 − 5 × 30 − 161
BMR = 650 + 1,031.25 − 150 − 161 = 1,370.25 ≈ 1,370 kcal/day
Running the same numbers through Harris-Benedict gives BMR = 9.247 × 65 + 3.098 × 165 − 4.330 × 30 + 447.593 = 601.055 + 511.17 − 129.9 + 447.593 ≈ 1,429.9, or roughly 1,430 kcal/day — about 60 kcal/day higher than the Mifflin-St Jeor figure. That gap is typical: the two formulas usually land within a few dozen to a couple hundred calories of each other, which is part of why "accurate to the calorie" was never really the promise — both are estimates, and Mifflin-St Jeor is simply the better-calibrated one for most people today.
From that 1,370 kcal/day resting baseline, here's roughly what her daily calorie needs could look like once activity is factored in (BMR × activity multiplier):
| Activity level | Multiplier | Est. daily calories |
|---|---|---|
| Sedentary | × 1.2 | ≈ 1,644 kcal/day |
| Lightly active | × 1.375 | ≈ 1,884 kcal/day |
| Moderately active | × 1.55 | ≈ 2,124 kcal/day |
| Very active | × 1.725 | ≈ 2,364 kcal/day |
| Extremely active | × 1.9 | ≈ 2,603 kcal/day |
Try those same numbers — or your own — in the calculator above to see your personal Mifflin-St Jeor figure, the Harris-Benedict comparison, and your own activity-level breakdown rendered instantly.
BMR vs TDEE — and what affects your number
BMR and TDEE are easy to mix up, but they answer different questions. BMR is what your body burns doing absolutely nothing — lying still, awake, in a fasted, temperature-controlled state. TDEE (total daily energy expenditure) starts from that BMR and adds everything else: digesting food, walking around, working, exercising, fidgeting — all of it. TDEE is always higher than BMR for anyone who isn't bedridden, and it's TDEE — not BMR — that you actually compare your food intake against when planning to lose, maintain, or gain weight.
A handful of factors push your real BMR up or down relative to any population-average formula:
- Age — BMR tends to gradually decline as you get older, partly because muscle mass tends to decrease with age.
- Sex — the formulas use separate constants for men and women, largely reflecting average differences in body composition.
- Body composition and muscle mass — muscle tissue burns more calories at rest than fat tissue, so two people of identical height, weight, age, and sex can still have meaningfully different real BMRs.
- Genetics and hormones — thyroid function and other hormonal factors can shift metabolic rate independently of size, sex, or age, in ways no formula can see from the outside.
If you'd rather skip straight to a single, activity- and goal-adjusted daily calorie number — built on top of the same BMR maths — our Calorie Calculator takes your activity level and a weight goal and returns a ready-to-use daily target.
Limitations and disclaimer
These formulas are population-based estimates, not measurements. Mifflin-St Jeor and Harris-Benedict were both built by fitting equations to data from groups of real people and finding the constants that minimised average error across that group — they were never designed to nail any one individual's metabolism exactly. The gap between an estimate and your true resting metabolic rate (only directly measurable through clinical methods such as indirect calorimetry) tends to be widest for people with very high muscle mass, certain medical or hormonal conditions, or body weights at the extremes of the spectrum the formulas were derived from.
This is not medical advice. This calculator provides an estimate only and should not be used as a substitute for professional medical or nutritional guidance. Before making significant changes to your diet — particularly if you have a health condition, are pregnant, are underweight, or are considering a substantial calorie deficit or surplus — please consult a doctor or registered dietitian who can account for your individual health history.
Frequently asked questions
- What is BMR and why does it matter?
- Basal metabolic rate (BMR) is an estimate of how many calories your body burns at complete rest just to keep itself alive — breathing, circulating blood, maintaining body temperature, and running basic cell repair. It is the foundation of any calorie-planning exercise, because every other estimate of your daily energy needs starts by adding activity on top of this resting baseline.
- What formula does this BMR calculator use?
- The primary result uses the Mifflin-St Jeor equation, the modern clinical standard: for men, BMR = 10 × weight in kg + 6.25 × height in cm − 5 × age in years + 5; for women, BMR = 10 × weight in kg + 6.25 × height in cm − 5 × age in years − 161. The calculator also shows the older Harris-Benedict (revised 1984) result alongside it for comparison.
- How is Mifflin-St Jeor different from Harris-Benedict, and which is more accurate?
- Both formulas estimate resting calorie burn from weight, height, age, and sex, but they were built from different population data decades apart — Harris-Benedict dates to the early 1900s (revised in 1984) and Mifflin-St Jeor to 1990. Mifflin-St Jeor is generally considered more accurate for modern populations and is the equation most commonly recommended by dietitians today, which is why this calculator treats it as the primary figure and shows Harris-Benedict only as a secondary comparison.
- What is the difference between BMR and TDEE?
- BMR is the number of calories your body burns at complete rest, with zero activity. TDEE (total daily energy expenditure) takes that BMR and multiplies it by an activity factor to account for movement, exercise, and daily life — so TDEE is always higher than BMR for anyone who is not bedridden. The activity-level breakdown on this page shows roughly what your TDEE could look like at each common activity level.
- What factors affect my BMR?
- The biggest drivers are body size (taller and heavier bodies generally burn more at rest), age (BMR tends to decline gradually as you get older), sex (which is why the formula uses separate constants for men and women, largely reflecting differences in typical body composition), and muscle mass (muscle tissue burns more calories at rest than fat tissue does). Genetics, hormone levels, and certain medical conditions can shift your true BMR away from any population-based estimate as well.
- How do I use my BMR for weight management?
- BMR alone will not tell you what to eat — you need your full TDEE first, which adds your activity level on top. As a general guide, eating consistently below your TDEE tends to support weight loss, eating around it tends to support maintenance, and eating above it tends to support weight gain. Because BMR is the floor that TDEE is built on, knowing it helps you sanity-check any calorie target — diets that push you far below your BMR for long periods are generally discouraged without medical supervision.
- Why might my actual BMR differ from this estimate?
- Both Mifflin-St Jeor and Harris-Benedict are population-based formulas built from averages — they cannot see your individual muscle mass, body-fat percentage, hormone levels, genetics, or any medical conditions that influence metabolism. People with very high muscle mass, those at the extremes of the weight spectrum, and people with certain thyroid or metabolic conditions are the most likely to see a meaningful gap between the estimate and their measured resting metabolic rate, which can only be confirmed with clinical testing such as indirect calorimetry.
- How does this BMR calculator differ from your Calorie Calculator?
- This page focuses specifically on BMR itself — the Mifflin-St Jeor figure as the primary, modern-standard result, the Harris-Benedict figure as a side-by-side comparison, and a quick breakdown of what your calorie needs could look like at each activity level. Our Calorie Calculator goes a step further: it asks for your activity level and a weight goal up front and returns a single activity-adjusted TDEE plus a goal-adjusted daily calorie target, which is the better tool once you are ready to plan an actual eating plan.
- Is this BMR calculator accurate enough to plan my diet around?
- Treat the result as a well-informed starting estimate, not a precise measurement. It is built from formulas that work well on average across large groups of people but can be off by a meaningful margin for any one individual — especially someone with very high muscle mass, an underlying health condition, or a body weight far from the average the formulas were derived from. Use it as a sensible starting point, watch how your body actually responds over a few weeks, and adjust from there.
- Is this medical advice?
- No. This BMR calculator provides an estimate only and is not medical advice or a diagnosis. Resting metabolic rate varies between individuals for reasons a simple formula cannot capture, and significant changes to your diet — especially if you have a health condition — should be made in consultation with a doctor or registered dietitian.