The most accurate way to calculate BMR without laboratory equipment is to plug your sex, age, weight, and height into the Mifflin-St Jeor equation: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years), then add 5 for men or subtract 161 for women. Published in 1990 and validated against measured resting energy expenditure in healthy adults, this formula predicts calories burned at complete rest within roughly 10 percent for a larger share of people than older alternatives, which is why most current clinical and dietetic guidelines reach for it first. Basal Metabolic Rate itself is the number of calories your body burns at complete rest just to keep your heart beating, your brain active, your lungs exchanging air, your kidneys filtering, and your core temperature stable — before any movement, digestion, or exercise is layered on. That figure usually accounts for 60 to 70 percent of total daily energy use, making it the single largest slice of any calorie budget. Running a second respected equation alongside it — the 1984 revised Harris-Benedict formula — turns a single number into a defensible range, which is the honest answer to the question of how accurate a BMR estimate can get from four simple inputs.
The Honest Meaning of an "Accurate" BMR Estimate
BMR accuracy has two layers, and most readers only think about the first. The visible layer is whether the chosen equation matches what a metabolic cart would measure in a lab. The Mifflin-St Jeor formula was re-fit to a large sample and shown to predict measured resting metabolic rate within roughly ±10 percent for most healthy adults, while the original 1919 Harris-Benedict drifted further from real values in wider populations. That is why the revised 1984 Harris-Benedict (Roza and Shizgal) is the version still cited in clinical handbooks today, and why modern guidelines have shifted toward Mifflin-St Jeor as the default.
The invisible layer is individual variation. Body composition, thyroid activity, recent dieting, genetics, illness, and certain medications can push any one person's real resting metabolism 10 percent or more above or below the population average encoded in the formula. Because none of those variables appear in a height-and-weight equation, the honest output of any BMR calculator is a range, not a precise reading. A single number invites false confidence; two trusted equations disagreeing by a known amount invites better judgment.
The Two Equations Behind a Trustworthy BMR Reading
The Mifflin-St Jeor equation, published in 1990, looks like this for an adult man: 10 × weight + 6.25 × height − 5 × age + 5. For an adult woman of the same body size, the trailing term becomes −161 instead of +5, a swing of 166 calories that captures the average difference in lean body mass between sexes. Its strength is the deliberately simple structure: three continuous variables plus a single sex offset, calibrated against measured data.
The revised Harris-Benedict equation (Roza and Shizgal, 1984) uses larger constants and steeper coefficients. For men it is 88.362 + 13.397 × weight + 4.799 × height − 5.677 × age. For women it is 447.593 + 9.247 × weight + 3.098 × height − 4.330 × age. For a typical adult, the same inputs return a slightly higher number under revised Harris-Benedict than under Mifflin-St Jeor, which is a useful visual reminder that the two equations describe slightly different statistical populations.
A quick comparison of the two formulas:
| Property | Mifflin-St Jeor (1990) | Harris-Benedict, revised (1984) |
|---|---|---|
| Year published | 1990 | 1984 (re-fit by Roza and Shizgal) |
| Constant term (men) | +5 | +88.362 |
| Constant term (women) | −161 | +447.593 |
| Weight coefficient | 10 | 13.397 (men) / 9.247 (women) |
| Height coefficient | 6.25 | 4.799 (men) / 3.098 (women) |
| Age coefficient | −5 | −5.677 (men) / −4.330 (women) |
| Preferred by current guidelines | Yes (primary recommendation) | Common comparison reference |
The takeaway is that Mifflin-St Jeor is the more accurate choice for most healthy adults today, and the revised Harris-Benedict serves as a credible second opinion. The BMR Calculator runs both at once so the gap between them is visible immediately.
How to Calculate Your BMR Step by Step
- Pick the metric version. The tool accepts weight in kilograms and height in centimeters. If you only know pounds, convert to kilograms by dividing by 2.2046; for feet and inches, convert total inches to centimeters by multiplying by 2.54.
- Select your sex. The formula uses different constants for men and women, so this single click changes the result by roughly 100 to 170 calories at typical adult body sizes.
- Enter your age in years, weight in kilograms, and height in centimeters. These four inputs are all the equation needs, and rounding to whole kilograms and centimeters is fine.
- Read the Mifflin-St Jeor result as the primary figure. This is the estimate most current clinical guidelines prefer, and it is the headline number on the tool.
- Compare it with the revised Harris-Benedict estimate. A small gap — often 50 to 100 calories — is normal; a much larger gap is a sign to double-check your inputs.
- Treat the spread as your range. Your accurate BMR sits somewhere between the two values, and any individual factor can nudge your real metabolism higher or lower.
Everything runs locally in your browser. Nothing you type is uploaded or stored, so the inputs stay private from start to finish.
A Worked Example With the Mifflin-St Jeor Formula
Take a 30-year-old woman who is 165 cm tall and weighs 65 kg. Substituting into the female version of the Mifflin-St Jeor equation:
BMR = 10 × 65 + 6.25 × 165 − 5 × 30 − 161 BMR = 650 + 1031.25 − 150 − 161 BMR = 1370.25, which rounds to 1,370 calories per day.
Running the same inputs through the revised Harris-Benedict equation for women gives 447.593 + 9.247 × 65 + 3.098 × 165 − 4.330 × 30 = 1429.918, which rounds to 1,430 calories per day.
So her accurate BMR sits in a range of roughly 1,370 to 1,430 calories — a 60-calorie spread that reflects the two equations' slightly different assumptions about average lean mass and resting metabolic rate. For the exact numbers for your own body, run the same four inputs through the BMR Calculator.
Reading the Range Between the Two Equations
A 50- to 100-calorie gap between Mifflin-St Jeor and revised Harris-Benedict is typical for healthy adults and is not a sign that you did anything wrong. It simply reflects different calibration samples and different constant terms. A much larger gap — 200 calories or more — usually means one of your inputs is off, especially weight or height, since both are scaled by the largest coefficients in the formulas.
Use the higher figure as a cautious ceiling and the lower figure as a cautious floor when you plan calorie intake. Because individual metabolism can sit above or below the predicted range by 10 percent or more, treat the two values as a starting reference point and adjust over two to three weeks based on real-world results, not on a single equation's output alone.
Where Prediction Equations Lose Accuracy
Any formula that uses only sex, age, weight, and height assumes a "typical" body composition. Real bodies differ, and the most common reasons a BMR estimate misses a person's true resting calorie burn are:
- Lean body mass. Muscle burns more calories at rest than fat. Two people of identical height, weight, sex, and age can have noticeably different BMRs because of different muscle-to-fat ratios.
- Thyroid function. An under- or over-active thyroid shifts resting metabolism well outside the predicted range.
- Adaptation to dieting. Sustained caloric restriction suppresses resting metabolic rate, which is why an aggressive cut can stall before body weight predicts it should.
- Genetics and ethnicity. Population averages baked into the formulas do not capture every genetic background equally well.
- Illness, fever, and certain medications. Active infection, recovery from surgery, and some drugs raise or lower resting energy needs in ways no four-input equation can detect.
- Pregnancy, lactation, and elite athletic body composition. The formulas were not built around these states, so the outputs should be treated as rough references only.
For those cases, indirect calorimetry in a clinical setting is the only way to measure true resting energy expenditure. Outside a lab, pairing the Mifflin-St Jeor result with the revised Harris-Benedict comparison gives you a range built from two of the most widely used equations — a defensible starting point for any calorie plan, rather than a single number to over-trust.
Once you have a trustworthy BMR figure, the next step is layering activity on top of it. A full overview of basal metabolic rate on Wikipedia covers the physiology behind the number, while a daily calorie target that accounts for movement and goals is the job of a separate calorie calculator workflow. If you also want to know where your weight sits relative to your height, the BMI Calculator rounds out the picture.