Health & Fitness Calculators

Know your numbers

Body mass index, calorie needs and body fat are simple numbers that help you set realistic goals and track progress. Each calculator explains the formula it uses — such as Mifflin-St Jeor for metabolism and the U.S. Navy method for body fat — along with its limitations, so you know how much weight to give the result.

These tools are for general information for healthy adults and are not a substitute for professional medical advice. Talk to your doctor before making significant changes to your diet or exercise.

Which health calculator do you need?

• Weight and body composition: BMI is a quick screen; waist-to-height ratio and body fat add information about where fat is and how much; ideal weight gives reference weights for your height.
• Nutrition: find your daily energy needs with the calorie calculator (based on your BMR), then divide them into protein, carbs and fat with the macro calculator.
• Activity: convert daily steps with steps to miles, and plan runs and races with the pace calculator.
• Pregnancy: estimate a due date and key milestones with the due date calculator.
• Sleep and stimulants: see how much caffeine is left at bedtime with the caffeine calculator.

Using health numbers well

No single number describes health. BMI ignores muscle, body fat estimates have a margin of error of a few percentage points, and calorie equations can be off by 10% for an individual. Their real value is in trends: measure the same way, at the same time of day, every few weeks, and look at the direction of change. Combine measurements — for example BMI with waist size — rather than relying on one.

• Weigh yourself in the morning after using the bathroom, before eating, and use a weekly average.
• Measure your waist at the same point each time, with the tape level.
• Adjust calorie targets based on 2–4 weeks of real results, not day-to-day swings.

Our sources

The calculators use published, peer-reviewed methods: the World Health Organization and CDC BMI categories, the Mifflin-St Jeor and Harris-Benedict equations for metabolic rate, the U.S. Navy circumference method for body fat, NICE guidance on waist-to-height ratio, Naegele’s rule for due dates and Riegel’s formula for race times. Each page names its method and explains its limits.

These tools are for general information for adults and are not medical advice. Children, pregnant people, athletes and anyone with a health condition should get personal advice from a doctor or registered dietitian.