BMR calculator

How this calculator works

Basal metabolic rate is the energy your body burns at complete rest, just to keep you alive. Breathing, pumping blood, repairing cells: all of it runs on calories even when you do nothing. BMR is the biggest share of what you burn in a day. This tool runs three published equations at once, so you compare them side by side instead of trusting one number.

Mifflin-St Jeor is the default. A 2005 systematic review tested the common equations against measured metabolism and found Mifflin-St Jeor landed within 10 percent of the real value for more people than any other. Know your body fat percentage? Katch-McArdle often does better, because it works from lean mass rather than total weight.

Here is a real one. A 30 year old man, 180 cm, 75 kg. Mifflin-St Jeor gives 1,730 calories a day. Harris-Benedict gives 1,787. His resting metabolic rate lands near 1,903, and at moderate activity he needs about 2,682 calories to hold his weight.

The three formulas

Each equation uses weight in kilograms (W), height in centimetres (H), and age in years (A).

Katch-McArdle skips the sex split because lean mass already carries most of that difference.

BMR and RMR are close, but not identical

Resting metabolic rate is BMR measured without the strict fasting and overnight-rest conditions a true BMR test demands. Loosen those controls and the number drifts up, usually by 5 to 10 percent. This tool reports both and estimates RMR at about 1.1 times your BMR. For planning meals or a deficit, the gap is small enough that people use the two words interchangeably.

What changes your BMR

A few things move the number, and some of them no equation can see:

• Muscle raises it. Muscle burns more at rest than fat does.
• Age lowers it, mostly by way of lost muscle.
• A bigger body costs more to run, so size raises it.
• Hormones, thyroid function, and genetics push it either way.

Even the best equation only lands within 10 percent of measured metabolism for most people, so read any estimate as a starting point, not a verdict.

What about metabolic age?

Metabolic age takes your BMR, compares it to the average for different ages, and reports the age your number matches. Body-composition scales love the metric. The science does not back it: there is no agreed definition or validated method, and two devices will hand you two different ages. We leave it out on purpose, because a number that precise would oversell a fuzzy idea. Your actual BMR and body composition tell you more.

BMR by age

Metabolism drifts down as you get older, mostly because muscle fades. The table below runs Mifflin-St Jeor for two reference adults so you can see the slope, not guess at an average.

From BMR to daily calories

BMR is what you burn doing nothing. To estimate what you actually burn, multiply it by an activity factor.

That total is your TDEE. For the full breakdown across every activity level, use the TDEE calculator.

Frequently asked questions

Which BMR formula is most accurate? Mifflin-St Jeor is the most accurate for most people. Katch-McArdle does better if you know your body fat percentage, because it works from lean mass.

What is the difference between BMR and RMR? BMR is measured under strict fasting and rest; RMR skips those controls and runs about 5 to 10 percent higher. In everyday use the two are close.

How do I turn BMR into daily calories? Multiply BMR by an activity factor from 1.2 to 1.9. That total is your TDEE.

This tool is an estimate, not medical advice. For decisions about your diet or health, talk to a doctor or a registered dietitian.

Sources

• Mifflin MD et al., 1990, American Journal of Clinical Nutrition
• Roza AM, Shizgal HM, 1984, revised Harris-Benedict equation
• Frankenfield D et al., 2005, comparison of predictive equations