How it works
Think of your body as an engine that never fully shuts off. Even when you are lying perfectly still, your heart pumps blood, your lungs expand, and your cells constantly repair themselves. The BMR Calculator measures the fuel required to keep that idling engine running. It does not account for walking, digesting food, or working out — it is strictly the baseline energy needed to stay alive in a resting, fasted state.
The tool uses the Mifflin-St Jeor equation, introduced in 1990, which replaced the older Harris-Benedict standard. It calculates your resting calories from four physical inputs: sex, age, weight, and height. The resulting number is the absolute floor of your daily energy expenditure. Every additional activity you do stacks on top of this baseline.
The formula
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + sex constant
The sex constant is +5 for males and −161 for females. If you enter US units (pounds and inches), the calculator converts them to metric before applying the equation.
Worked example
Let us calculate the BMR for a 30-year-old male who weighs 75 kg and is 178 cm tall.
Step 1: Multiply weight by 10
75 kg × 10 = 750
Step 2: Multiply height by 6.25
178 cm × 6.25 = 1,112.5
Step 3: Multiply age by 5
30 years × 5 = 150
Step 4: Add the first two results, subtract the third, and add the male constant (+5)
750 + 1,112.5 − 150 + 5 = 1,717.5
Rounded to the nearest whole number, this person's BMR is 1,718 calories per day. That is the energy their body would consume over 24 hours of complete rest, without so much as getting up for a glass of water. If they were female, the only change would be swapping the +5 constant for −161, which would yield 1,551.5 (roughly 1,552 calories).
| Factor | Value | Contribution |
|---|---|---|
| Weight (75 kg × 10) | 750 | Largest driver |
| Height (178 cm × 6.25) | 1,112.5 | Secondary driver |
| Age (30 × 5) | −150 | Minor deduction |
| Sex constant (male) | +5 | Small adjustment |
| Total BMR | 1,718 | Resting baseline |
Things to watch
BMR is an estimate, not professional medical advice — individual metabolic rates vary based on genetics, body composition, and hormonal factors that no formula can fully capture.
A few specifics to keep in mind:
- Muscle changes the floor. The Mifflin-St Jeor equation uses total body weight, not lean mass. Two people can weigh 75 kg, but the one with higher muscle mass will actually burn more at rest because muscle tissue requires more energy to maintain than fat tissue.
- Age matters more than you think. Metabolic rate declines roughly 1–2% per decade after age 20, largely due to natural muscle loss. Your BMR at 50 will be lower than at 30 even if your weight stays identical.
- Do not confuse BMR with TDEE. A BMR of 1,718 is not your daily eating target. If you sit at a desk all day, your actual burn might be around 2,060 (BMR × 1.2). The BMR Calculator gives you the resting baseline only — useful for understanding your metabolic floor, not for setting a meal plan.
- Extreme body sizes reduce accuracy. The equation was validated on populations within normal weight ranges. For individuals with severe obesity or very low body fat, standard BMR formulas can overestimate or underestimate true resting energy expenditure by 10% or more.