BMR Calculator
Your recommended BMR (Mifflin-St Jeor)
Best for the general population; includes age.
The older, widely recognized standard.
Uses lean body mass — needs body fat %.
Which Formula Is Right for You?
Every BMR equation is an estimate, not a lab measurement. Each one weighs your inputs slightly differently, so they rarely agree exactly. Here is how to read the trade-off.
| Formula | Best For | Key Strength | Weakness |
|---|---|---|---|
| Mifflin-St Jeor (1990) | General population, weight management | Most accurate for average adults; accounts for age | Assumes average body composition |
| Harris-Benedict (revised, 1984) | Historical comparisons, clinical estimates | Simple, familiar, widely published | Known to overestimate BMR in modern populations |
| Katch-McArdle (1975) | Lean, athletic individuals | Based on lean body mass — ignores body fat weight | Requires an accurate body fat percentage |
How Each Formula Works
The equations below all take metric inputs: weight in kilograms, height in centimeters, age in years.
Mifflin-St Jeor
Women: BMR = 10·weight + 6.25·height − 5·age − 161
Harris-Benedict (revised)
Women: BMR = 447.593 + 9.247·weight + 3.098·height − 4.330·age
Katch-McArdle
Lean body mass = weight × (1 − body fat %)
Because Katch-McArdle strips out the calories that body fat barely burns, it is the most precise option for muscular people who know their real body-fat percentage.
BMR vs TDEE
Think of BMR as the minimum fuel your body needs just to keep running while you rest in bed. Your TDEE (Total Daily Energy Expenditure) adds everything else — walking, working out, digesting food, even fidgeting. Multiply your BMR by an activity factor to get your TDEE:
| Activity Level | Factor | Example |
|---|---|---|
| Sedentary | 1.2 | Desk job, little exercise |
| Lightly active | 1.375 | Light exercise 1–3 days a week |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extra active | 1.9 | Physically demanding job plus daily training |
To lose weight you eat below TDEE; to gain, eat above it. BMR is the floor you should rarely go under.
What Affects Your BMR
- Body composition. Muscle burns more calories than fat, even at rest. This is the biggest controllable factor.
- Age. Metabolism slows as lean mass naturally declines; the chart above shows the typical curve.
- Gender. Men generally have more muscle and a higher BMR (reflected in each formula).
- Genetics & hormones. Thyroid function and genetic baseline can shift your number up or down.
- Weight. Because heavier bodies need more fuel, BMR rises with body weight.
When the numbers matter more than the guesswork
This calculator is for anyone who needs a reliable metabolic baseline before adjusting calories for cutting, lean gains, or maintenance. Dietitians, coaches, and self-trackers use it to replace rough estimates with formula-driven values. Enter your basic metrics, choose between Mifflin-St Jeor, Harris-Benedict, or Katch-McArdle, and get an instant BMR you can build a nutrition plan around. If you know your body fat percentage, the Katch-McArdle option leverages lean mass for a tighter result. No sign-up, no spreadsheet — just the number you came for.
Your inputs to your baseline
- Toggle Gender by selecting either the Female or Male button to set your metabolic formula base.
- Enter your Age in years so the calculator can apply age-related metabolic adjustments accurately.
- Input your Height and Weight using the units provided to anchor the energy-expenditure equations.
- Optionally add your Body Fat percentage if you want Katch-McArdle results based on lean mass.
- Click Calculate My BMR to instantly view results across all three formulas for side-by-side comparison.
Frequently Asked Questions
What is BMR?
BMR, or Basal Metabolic Rate, is the number of calories your body burns at complete rest to keep your heart beating, lungs working, brain active and body warm. It typically accounts for 60–75% of the calories you burn in a day — which is why knowing it matters so much for weight management.
Which BMR formula is most accurate?
For most healthy adults, the Mifflin-St Jeor equation is the current accepted standard and the one this tool recommends. If you are lean, highly muscular, and know your body-fat percentage, the Katch-McArdle formula gives a more individualised result because it is built on lean body mass.
How does age affect BMR?
BMR declines roughly 2% per decade after age 20, mainly because muscle mass shrinks as we get older. It stays fairly stable through your 30s, then the drop becomes more noticeable. Resistance training and adequate protein help slow that decline.
What is the difference between BMR and RMR?
BMR is measured under strict laboratory conditions — after sleeping, fasting, in a neutral-temperature room. RMR (Resting Metabolic Rate) is measured while awake but relaxed and is typically a bit higher. In practice the two are close, and everyday calculators use either to estimate resting calorie burn.
Why do the three formulas give different numbers?
Because each equation was derived from a different study population and weights your age, height, weight and (for Katch-McArdle) body fat differently. Disagreements of 100–200 kcal are normal. Treat the range as more useful than any single value.
Is this BMR calculator free and private?
Yes, completely free and fully private. All calculations run locally in your browser on your own device — your age, weight, height and body-fat data are never uploaded or stored anywhere.
Browse the conversion library
Every unit pair has a dedicated page with the exact factor, a full value table and a worked example.