BMR Calculator

What the BMR Calculator Does

This tool works out your Basal Metabolic Rate: the energy your body uses at rest to keep your heart beating, lungs breathing and organs running, before any activity. It also multiplies that figure for a sedentary and a moderately active day to estimate daily calorie needs.

It is the quick way to find your resting calorie baseline — the starting point for understanding how much you burn and, from there, how much you might eat to maintain, lose or gain weight.

How It Works

The tool uses the Mifflin–St Jeor equation, which combines your weight, height and age, with a small adjustment for sex. Weight has the largest effect, height a smaller positive one, and age a negative one, since metabolism tends to slow with age. The result is your resting calorie burn for a day.

To turn the resting figure into daily needs, it multiplies the BMR by an activity factor — about 1.2 for a sedentary day and 1.55 for a moderately active one. Those multipliers account for the energy used in movement on top of the resting baseline.

Methodology

1. Read your details. Take your age, weight in kilograms, height in centimetres and sex.
2. Apply Mifflin–St Jeor. Combine weight, height and age, adding 5 for men or subtracting 161 for women.
3. Get the resting figure. The result is your Basal Metabolic Rate — calories burned at rest per day.
4. Estimate daily needs. Multiply by an activity factor for sedentary and moderate days.

Mifflin–St Jeor BMR

The only difference between the men's and women's formula is the constant at the end. The Mifflin–St Jeor equation is the modern standard, more accurate for most people than older formulas.

Assumptions

• Inputs are metric — weight in kilograms and height in centimetres — and the result is calories (kcal) per day.
• The Mifflin–St Jeor equation estimates an average person's resting burn; individual metabolism varies with body composition, genetics and health.
• The activity multipliers are rough averages; a real day's burn depends on how much you actually move.

Standards & references

• Mifflin–St Jeor equation — The modern standard for estimating BMR, published in 1990. It predicts resting metabolic rate within about 5% for most people, more accurately than the older Harris–Benedict equation.
• Basal Metabolic Rate — The energy the body uses at complete rest to maintain basic functions. It is the largest part of daily calorie use for most people, before activity is added.
• Activity multipliers — BMR times an activity factor estimates total daily energy use — roughly 1.2 for sedentary, 1.375 light, 1.55 moderate, 1.725 active and 1.9 very active.

Accuracy & Limitations

The Mifflin–St Jeor equation is the most reliable simple BMR estimate for the general population, with an average error of about 5% — better than older formulas. For most people it is a sound resting baseline.

It is still an estimate. Two people of the same age, weight, height and sex can have different real metabolic rates because of body composition, genetics and health, so treat the figure as a guide, not an exact measurement.

It tends to underestimate BMR for very muscular people, since muscle burns more at rest than fat and the equation does not measure body composition. Athletes may have a higher real BMR than the formula suggests.

The activity multipliers are broad averages. A real day varies, so the daily-need figures are approximate. For a precise resting rate, a metabolic test is needed; this gives a good everyday estimate.

Real-World Use Cases

When to use it — and when not to

For a precise resting rate, a metabolic (calorimetry) test measures it directly. For full daily calorie targets across activity levels and weight goals, a calorie calculator builds on this BMR. For medical needs, consult a professional.

Frequently Asked Questions

References

• Basal metabolic rate (Wikipedia)
• Harris–Benedict / Mifflin–St Jeor equations (Wikipedia)
• Resting metabolic rate prediction (Mifflin et al., 1990 — PubMed)

Related Tools