MariposasBMR Calculator
By Zachary, builder of Mariposas · Reviewed against published exercise-science sources (references)
· Last updated June 2026
Nutrition & Calories Your BMR (basal metabolic rate) is the energy your body uses just to keep you alive at rest. We use the Mifflin-St Jeor equation, the most accurate for the general population, and show Harris-Benedict alongside.
⚕️ This calculator is a general-information estimate from standard formulas, not professional or medical advice. Individual results vary; don’t use it to make medical or dietary decisions.
How it works
The Mifflin-St Jeor equation calculates BMR from four variables: body weight in kilograms, height in centimeters, age in years, and biological sex. For men the formula is (10 × weight) + (6.25 × height) - (5 × age) + 5, and for women it replaces that final constant with -161. A 1990 study by Mifflin and colleagues found this equation outperformed older alternatives on measured resting metabolic rate across a diverse adult sample, which is why most registered dietitians and research bodies now treat it as the standard starting point. The Harris-Benedict equation, published in 1919 and revised in 1984, uses slightly different coefficients and consistently runs a bit higher than Mifflin-St Jeor, particularly for women. Showing both lets you see the range rather than treating one number as gospel, because real-world metabolic rates vary by several percent even among people with identical inputs.
When to use it
BMR is the foundation for any calorie target, so this calculator matters most when someone is trying to figure out how much to eat to lose fat, gain muscle, or simply maintain weight without guessing. It also helps athletes and active people understand the floor: no matter how much they train, their body is already burning a meaningful number of calories just to breathe, pump blood, and run basic cellular functions. From BMR you multiply by an activity factor to get total daily energy expenditure, which is the number actually used to set a food target.
Worked example
A 32-year-old woman who weighs 72 kg and stands 165 cm tall would calculate her BMR using Mifflin-St Jeor as (10 × 72) + (6.25 × 165) - (5 × 32) - 161, which works out to 720 + 1031.25 - 160 - 161 = 1430 calories per day. That 1430 figure represents her estimated calorie burn at complete rest, meaning if she stayed in bed all day she would need roughly that many calories just to break even. In reality she moves around, exercises, and digests food, so her actual daily need is higher. Multiplying 1430 by a moderate activity factor of 1.55 puts her total daily expenditure around 2217 calories, giving her a concrete number to build a diet around rather than a vague sense that she should "eat less."
Tips for an accurate result
- Use your weight on a typical, non-bloated morning after using the bathroom. A single heavy meal or extra water can shift the number by a kilogram or two, which meaningfully changes the output.
- Enter your height carefully. A 2 cm error in height changes the Mifflin result by about 12 calories, which sounds small but compounds when you multiply by an activity factor.
- Treat the result as a starting point, not a hard truth. BMR equations are population averages with a margin of error around plus or minus 10 percent. Track your actual intake and weight for two to three weeks and adjust from there.
- If you have a large amount of muscle mass relative to your weight, your true BMR may run higher than either equation predicts, since muscle tissue burns more energy at rest than fat tissue does. The formulas use total weight, not lean mass.
- Re-run the calculator if your weight changes by more than 5 to 7 pounds in either direction. Because weight is multiplied by 10 in the Mifflin formula, a 5 kg change shifts your BMR result by 50 calories before the activity multiplier is applied.
Formula & sources: methodology · references.
Now go hit the number Mariposas turns every workout, run and class into progress · collect a cute pet 🐾FAQ
- What is the difference between BMR and TDEE?
- BMR is how many calories your body burns doing absolutely nothing, at complete rest in a thermoneutral environment after an overnight fast. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor that accounts for movement, exercise, and the energy cost of digesting food. BMR is a component of TDEE, not the number you actually eat to.
- Why do the Mifflin-St Jeor and Harris-Benedict results differ?
- The two equations were derived from different populations using different regression methods. Harris-Benedict used a relatively small sample from the early 20th century and tends to overestimate BMR by about 5 percent compared to more recent direct measurements. Mifflin-St Jeor was validated against a larger, more contemporary group and shows better accuracy on average, which is why it is generally preferred today.
- Does muscle mass affect BMR?
- Yes, significantly. Skeletal muscle has a higher resting metabolic rate than adipose (fat) tissue, somewhere in the range of 13 calories per kilogram of muscle per day versus roughly 4.5 for fat, based on organ-level metabolic rate research. Both equations use total body weight, so two people with the same weight, height, age, and sex will get the same BMR estimate even if one carries 10 more kilograms of lean mass. That person likely has a meaningfully higher true BMR.
- Can I use this calculator if I am pregnant or postpartum?
- Pregnancy and the postpartum period substantially alter metabolic needs in ways that a standard BMR formula cannot capture. This calculator reflects equations developed for non-pregnant adults. Anyone in those circumstances would get more useful guidance from a healthcare provider or registered dietitian who can account for the specific changes involved.
- How often does my BMR change?
- BMR shifts gradually as body weight, age, and body composition change. Aging alone tends to lower BMR slightly each decade, partly due to changes in lean mass. Significant weight loss also lowers BMR both because you weigh less and because the body can adapt metabolically over time. For most people, recalculating every few months or whenever weight shifts noticeably keeps the number reasonably current.