Basal Metabolic Rate Calculator
Estimate the calories your body burns at rest using the Mifflin-St Jeor formula, and see your daily calorie needs at different activity levels.
| Activity level | Estimated daily calories |
|---|---|
| Sedentary | – |
| Light exercise | – |
| Moderate exercise | – |
| Active | – |
| Very active | – |
BMR Calculator: Mifflin-St Jeor Formula & Complete Guide
Written by Mathew | Financial Tools & Calculation Specialist · Last updated August 21, 2026
In two sentences: A BMR calculator estimates the minimum calories your body needs at complete rest using the Mifflin-St Jeor equation — 10×weight(kg) + 6.25×height(cm) − 5×age + 5 for men, or −161 for women — which research has shown to be more accurate than the older Harris-Benedict formula for most modern adults. This guide breaks down the formula, worked examples, and how BMR differs from the total daily calorie number most people actually need.
What Is a BMR Calculator?
A BMR calculator estimates your Basal Metabolic Rate — the number of calories your body burns at complete rest just to maintain essential functions like breathing, circulation, and cell production. This is not the number you eat to maintain your current weight; it’s the floor beneath that number, representing energy use before any movement, digestion, or exercise is factored in.
The BMR Formula: Mifflin-St Jeor Equation
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
The only difference between the two formulas is the final constant — +5 for men, −161 for women — reflecting average differences in body composition between sexes at the same height, weight, and age.
Worked Examples
Example 1: Calculating BMR for a woman
A 32-year-old woman, 165 cm tall, weighing 63 kg.
BMR = (10 × 63) + (6.25 × 165) − (5 × 32) − 161
= 630 + 1,031.25 − 160 − 161
= 1,340.25 calories/day
Example 2: Calculating BMR for a man
A 40-year-old man, 178 cm tall, weighing 82 kg.
BMR = (10 × 82) + (6.25 × 178) − (5 × 40) + 5
= 820 + 1,112.5 − 200 + 5
= 1,737.5 calories/day
Example 3: How age alone changes BMR
The same man from Example 2, recalculated at age 60 instead of 40, with identical height and weight.
BMR = (10 × 82) + (6.25 × 178) − (5 × 60) + 5
= 820 + 1,112.5 − 300 + 5
= 1,637.5 calories/day
Twenty years of aging, with no change in height or weight, reduces this individual’s calculated BMR by 100 calories per day — reflecting the formula’s built-in assumption that metabolic rate declines gradually with age, largely due to typical reductions in lean muscle mass over time.
Example 4: Comparing two people of the same weight but different height
Two 30-year-old women, both weighing 60 kg, but one is 155 cm tall and the other is 175 cm tall.
Shorter (155 cm): BMR = (10×60) + (6.25×155) − (5×30) − 161 = 600 + 968.75 − 150 − 161 = 1,257.75
Taller (175 cm): BMR = (10×60) + (6.25×175) − (5×30) − 161 = 600 + 1,093.75 − 150 − 161 = 1,382.75
Despite identical weight and age, the taller woman’s calculated BMR is 125 calories higher — a direct result of height’s positive coefficient in the formula, reflecting that a taller frame generally requires more baseline energy to maintain.
Step-by-Step: How to Use a BMR Calculator
- Enter your weight in kilograms and height in centimeters (convert from pounds and inches if needed).
- Enter your age and sex.
- Apply the Mifflin-St Jeor formula, using the appropriate constant for your sex.
- Review your BMR result — remember this is your calorie floor, not a target to eat, since it doesn’t include any activity.
- Multiply by an activity factor (1.2 for sedentary, up to 1.9 for very active) to convert BMR into a full TDEE figure that reflects your actual daily calorie needs.
Why Mifflin-St Jeor Replaced the Older Harris-Benedict Formula
The Harris-Benedict equation, originally published in 1919 and revised in 1984, was the standard BMR formula for decades before Mifflin-St Jeor was published in 1990 using more contemporary data. Research comparing the two has generally found Harris-Benedict tends to overestimate BMR, particularly in sedentary individuals, by a meaningful margin — commonly cited estimates put the overestimation at 5-15% compared to Mifflin-St Jeor’s roughly 5% average margin of error. This is why most modern nutrition professionals and calculators default to Mifflin-St Jeor as the more accurate choice for the general population, even though Harris-Benedict remains in circulation on some older tools and references.
BMR vs. TDEE: Understanding the Difference
A BMR calculator answers a narrower question than most people actually need answered. BMR represents calories burned at complete rest — no walking, no digestion, no exercise. TDEE (Total Daily Energy Expenditure) builds on top of BMR by adding the thermic effect of food, non-exercise daily movement, and deliberate exercise, producing the number that actually reflects how many calories you burn in a typical day. Using your BMR alone as a calorie target — rather than your full TDEE — would significantly underestimate your actual needs for anyone who isn’t completely sedentary, which in practice is nearly everyone.
Limitations of the Mifflin-St Jeor BMR Formula
Like any population-based equation, Mifflin-St Jeor has real limitations for individuals whose body composition differs meaningfully from the general population it was derived from. It doesn’t directly account for muscle mass, meaning a highly muscular individual’s actual BMR may run higher than the formula predicts, since muscle tissue burns more calories at rest than fat tissue. For those with a known body fat percentage, the Katch-McArdle formula (based on lean body mass rather than total weight) can provide a more individualized estimate, though it requires an additional measurement Mifflin-St Jeor doesn’t need.
Using a BMR Calculator to Set Realistic Nutrition Goals
A BMR calculator is most useful as the first step in a larger process, not as a standalone number to fixate on.
A practical BMR calculator workflow
- Calculate BMR first, then immediately convert it to TDEE. A BMR calculator result on its own significantly understates real daily needs for anyone who isn’t completely sedentary, so applying an activity multiplier right away avoids the common mistake of eating too close to the BMR figure alone.
- Recalculate after a meaningful weight change. Since weight is a direct input, a BMR calculator run again after losing or gaining 10+ pounds will produce a noticeably different result, keeping calorie targets aligned with your current body rather than an outdated one.
- Use BMR as a sanity check for very low-calorie approaches. If a proposed diet plan sets daily intake below your calculated BMR calculator result, that’s a signal the plan may be too aggressive, since eating below resting needs for extended periods carries real metabolic and health risks.
- Compare Mifflin-St Jeor against Katch-McArdle if you know your body fat percentage. A BMR calculator offering both formulas lets athletic or highly muscular individuals cross-check whether the standard formula is under- or overestimating their true resting needs.
Frequently Asked Questions
No — BMR is your resting calorie floor, not your maintenance calorie target. To find maintenance calories, multiply your BMR by an activity factor reflecting your typical daily movement and exercise, producing your TDEE.
The constant difference (+5 for men, −161 for women) reflects average differences in body composition between sexes — men generally carry a higher proportion of muscle mass relative to fat at the same height, weight, and age, and muscle tissue has a higher resting energy demand than fat tissue.
Yes, according to the formula — the −5×age term means BMR calculated by this equation declines gradually every year, largely reflecting the average tendency to lose lean muscle mass with age even when total body weight remains stable.
Less so than for the general population — since Mifflin-St Jeor doesn’t directly account for body composition, highly muscular individuals may have a true BMR somewhat higher than the formula predicts, and the Katch-McArdle formula (using lean body mass) can provide a more tailored estimate in these cases.
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In summary, a BMR calculator built on the Mifflin-St Jeor equation gives you a scientifically grounded estimate of your body’s baseline calorie needs at rest, and understanding that this figure is a starting point — not your actual daily calorie target — is essential before applying an activity multiplier to find your true TDEE.
About the author: Mathew is a Financial Tools & Calculation Specialist focused on building and fact-checking online calculators across health, fitness, and nutrition topics.
Note: This calculator and article are provided for general educational and informational purposes only and do not constitute medical or nutritional advice. Consult a doctor or registered dietitian before making significant changes to your diet, especially if you have an existing health condition.