BMR, or Basal Metabolic Rate, is the number of calories your body burns at complete rest just to keep your most basic systems running — breathing, circulating blood, regulating temperature, and powering the brain and organs. The most reliable way to calculate BMR for a healthy adult is the Mifflin-St Jeor equation, which uses your sex, age, weight in kilograms, and height in centimeters to return a single daily calorie figure that represents your resting energy expenditure. A practical approach is to run your numbers through a BMR calculator that shows the Mifflin-St Jeor result as the primary value and the revised Harris-Benedict equation right next to it, so you can see how two trusted formulas compare for the same body.
Calculating BMR by hand is straightforward once you have the inputs, but the equations are easy to mis-key, and small slip-ups with units (pounds instead of kilograms, or inches instead of centimeters) throw the result off by hundreds of calories. That is the practical case for using a small online tool: you enter the four variables, the tool applies the formulas, and you get a side-by-side reading you can actually trust. The rest of this guide walks through what BMR really means, which formulas are worth using, how to read a BMR result, and how the BMR Calculator on this site gets the job done in a few clicks.
What BMR Actually Measures
BMR is the energy cost of staying alive when you are awake, lying still, in a warm room, and have not eaten for several hours. It covers the work your body does automatically: keeping your heart beating, your lungs expanding, your liver and kidneys filtering, and your cells turning over. It does not include walking, typing, digesting a meal, or training at the gym — those costs are layered on top of BMR to build your total daily energy expenditure.
Because the conditions under which BMR is measured are strict, the formulas that estimate it are designed to be used in the same way: with your body stats, not with your activity level. Once you have a BMR, you multiply it by an activity factor to estimate daily calorie needs, or hand it to a calorie calculator that does the same thing automatically. The BMR step is the foundation; everything calorie-related that follows sits on top of it.
The Two Formulas Worth Using
Modern BMR estimation leans on two equations, both of which have been tested in clinical settings.
The Mifflin-St Jeor equation, published in 1990, is now the equation most dietitians and researchers reach for first. It is the form recommended by the American Dietetic Association for healthy adults and tends to estimate resting energy expenditure within about 10 percent for most people. For men, the formula is:
BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
For women, the constant on the end is different:
BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
The revised Harris-Benedict equation, updated by Roza and Shizgal in 1984, is an older but still useful cross-check. It uses larger constants and tends to run a little higher than Mifflin-St Jeor for most adults. For men: BMR = 88.362 + 13.397 × weight (kg) + 4.799 × height (cm) − 5.677 × age. For women: BMR = 447.593 + 9.247 × weight (kg) + 3.098 × height (cm) − 4.330 × age.
Neither equation is "the truth" — both are population averages applied to one person. That is exactly why seeing them side by side is useful: when the two agree, you can be reasonably confident in the number, and when they differ by more than usual, it is a hint to double-check your inputs (especially the units).
Side-by-Side Comparison of the Two Formulas
The table below summarizes the practical differences between the two equations, so you can pick the right one for the situation in front of you. Use it as a quick reference card, not as a final verdict — the body stats you plug in matter more than the formula you pick.
| Feature | Mifflin-St Jeor | Revised Harris-Benedict |
|---|---|---|
| Year published | 1990 | 1984 (Roza & Shizgal update of the 1919 original) |
| Inputs required | Sex, age, weight (kg), height (cm) | Sex, age, weight (kg), height (cm) |
| Common use today | Default choice for healthy adults in clinical and fitness settings | Cross-check and historical comparison |
| Typical output for the same body | Slightly lower resting calorie figure | Slightly higher resting calorie figure |
| Best for | Modern nutrition planning, weight management, general estimates | Sanity-checking Mifflin-St Jeor, older studies, athletic populations |
| Limitations | Population average, not tuned to elite athletes or clinical conditions | Same average-based limits; tends to run higher than Mifflin-St Jeor |
How to Calculate BMR With the BMR Calculator
- Open the BMR Calculator on this site and pick your sex from the dropdown — male or female. This selects which version of each equation the tool will run.
- Enter your age in whole years. The formulas treat age as a linear term, so a 32-year-old and a 33-year-old should expect roughly a 5-calorie difference from this variable alone.
- Enter your weight in kilograms. If you only know your weight in pounds, divide by 2.2046 first; using pounds by mistake inflates the result dramatically.
- Enter your height in centimeters. If you have inches, multiply by 2.54 to convert before typing the number in.
- Read your BMR from the Mifflin-St Jeor equation in the primary result panel. This is the figure most nutrition references will quote when they talk about your BMR.
- Look at the revised Harris-Benedict value displayed below it. Treat the two numbers as a range: if they sit within a few percent of each other, your input is almost certainly clean and your BMR is somewhere between the two.
- Take that BMR figure into a Calorie Calculator (or multiply it by an activity factor yourself) to estimate total daily calorie needs for maintenance, weight loss, or weight gain.
Reading the Side-by-Side Result
The reason the BMR Calculator shows both equations together is that no single formula is perfect for every body. A tall, lean 28-year-old man and a short, muscular 55-year-old woman will not respond to the same average the same way, and the two equations have slightly different sensitivities to weight, height, and age.
As a rule of thumb, when Mifflin-St Jeor and revised Harris-Benedict land within roughly 50–80 calories of each other, you can treat the average as a solid working BMR. When the gap is wider, re-check that you entered kilograms and centimeters, not pounds and inches — a units slip is the most common reason the two numbers disagree by a lot. A second, smaller source of spread is the formula design itself: Harris-Benedict tends to weight muscle mass slightly differently, so athletic users often see Harris-Benedict a touch higher.
You can also sanity-check the result against BMI or ideal weight figures for your height. BMI and BMR are not the same thing, but both depend on the same body stats, so wildly off-base inputs will produce an obviously wrong pair of outputs.
A Quick Worked Example
Suppose you are a 30-year-old woman who weighs 65 kg and is 168 cm tall. Plugging into the Mifflin-St Jeor equation for women:
BMR = (10 × 65) + (6.25 × 168) − (5 × 30) − 161
BMR = 650 + 1,050 − 150 − 161
BMR = 1,389 calories per day
For the revised Harris-Benedict equation with the same numbers:
BMR = 447.593 + (9.247 × 65) + (3.098 × 168) − (4.330 × 30)
BMR = 447.593 + 601.055 + 520.464 − 129.900
BMR ≈ 1,439 calories per day
Both formulas land in the 1,400 range, and the 50-calorie gap is exactly the kind of small spread you expect from two well-validated equations applied to the same body. If you see a worked example like this in the BMR Calculator and your own inputs, the arithmetic is easy to redo by hand, which is a useful way to confirm the tool is behaving the way the formulas say it should.
Common BMR Mistakes and How to Avoid Them
The single most expensive error is mixing up units. A 70 kg adult and a 70 lb child are wildly different bodies, and the formula does not know which one you meant — it just multiplies. Always check the unit label on the input before you read the result. A second common error is using the wrong equation for the wrong sex: the constant at the end is different, so swapping the male and female forms by accident shifts the result by well over 100 calories.
A third issue is treating BMR as if it were your daily calorie budget. BMR is the floor: it is what you burn doing nothing. As soon as you sit up, walk across a room, or eat a sandwich, your actual expenditure climbs. Use a Calorie Calculator or multiply your BMR by a known activity factor to translate it into something you can plan a diet around.
Finally, BMR does not adapt to short-term changes such as a single workout or a single meal. The equations describe a stable, fasted, resting state. If your weight is fluctuating, your BMR estimate is only as current as the weight you typed in.
Where to Go After You Have Your BMR
A BMR figure is the starting point for most nutrition planning, but it is rarely the final answer. The most common next steps are turning BMR into a daily calorie target using an activity multiplier, splitting those calories into carbohydrate, protein, and fat grams, or comparing your BMR-derived ideal weight to a BMI reading for a fuller picture of where you sit on the body-composition spectrum. If your goal is training, you can also pair your BMR with a target heart rate to plan cardio zones that match your current fitness level.
For a quick sanity check with no math at all, a free browser calculator can also run the Mifflin-St Jeor arithmetic if you would rather see the formula produce the number in front of you than trust any single result. Whichever path you take, the principle is the same: BMR is a baseline, the two equations in the BMR Calculator are both credible ways to estimate it, and a small spread between them is normal and expected.
If you're weighing options, How to Calculate Body Fat Percentage at Home with a Tape Measure covers this in detail.