BMR Calculator

What this tool does

The BMR Calculator estimates the calories your body burns at complete rest using three published equations side by side. Add your body fat percentage to unlock Katch-McArdle, which is based on lean mass, and multiply the result by standard activity factors to get your daily calorie needs.

Basal metabolic rate is the energy your body uses just to stay alive: breathing, circulating blood, maintaining temperature and running your organs while you lie still. For most adults it accounts for 60 to 70 percent of daily calorie expenditure, so it is the foundation of any calorie plan. Because measuring BMR directly needs a laboratory and a fasting, rested subject, everyday estimates rely on predictive equations derived from population studies.

This tool shows three of them at once. Mifflin-St Jeor (1990) is the equation most dietetics bodies currently recommend for healthy adults. The revised Harris-Benedict (Roza and Shizgal, 1984) is the classic alternative and tends to read slightly higher. Katch-McArdle uses lean body mass (370 + 21.6 × LBM in kg), so it suits people who know their body fat percentage. Results are then multiplied by activity factors from 1.2 to 1.9 to estimate total daily energy expenditure.

How to use BMR Calculator

1. Pick your units and sex, then enter your age.
2. Enter weight and height.
3. Optionally add your body fat percentage to enable the Katch-McArdle result.
4. Choose which formula should be the main result under Main formula.
5. Compare all equations in the result cards; the gap between them shows the natural uncertainty.
6. Read the activity table to find daily calories for your typical week and download it if you want to keep it.

Tips for better results

• If you are lean and muscular, Katch-McArdle with a reliable body fat figure often fits better than weight-based equations.
• Pick the activity level that matches an average week, not your most active day.
• Recalculate after every 4 to 5 kg of weight change, because BMR falls as body mass falls.

Common use cases

• Setting a starting calorie budget before tracking food intake.
• Comparing how much each equation differs for the same person.
• Estimating energy needs for a client brief or a nutrition assignment.
• Understanding why intake below BMR is rarely a sensible long-term target.

Common mistakes to avoid

• Confusing BMR with total daily calories; BMR excludes all movement and digestion.
• Eating at BMR to lose weight quickly, which usually leaves no room for activity and is hard to sustain.
• Selecting Katch-McArdle with a guessed body fat percentage, which can be less accurate than Mifflin-St Jeor.
• Expecting the estimate to match a metabolic test exactly; individuals vary by around 10 percent.