Health & Fitness Calculators
Free health and fitness calculators — TDEE, calorie, BMI, body fat, and more. Cited formulas, instant results in your browser, no signup. Estimates only — not medical advice.
Calculators for the everyday "how many calories should I eat?" and "how do I structure a cut or bulk?" questions, built around peer-reviewed formulas with the citations spelled out so you know exactly how the number was produced. The TDEE Calculator uses the Mifflin-St Jeor equation (1990) — the formula registered dietitians cite as more accurate than the older Harris-Benedict — and shows BMR alongside cut, maintain, and bulk targets in one card. Every calculation runs locally in your browser; your sex, age, height, and weight never leave your device. These tools provide population-level estimates with roughly ±10% accuracy and are not a substitute for individualized advice from a registered dietitian or physician.
The Health & Fitness Calculators Guide
How to use TDEE in a real diet plan
TDEE — Total Daily Energy Expenditure — is the calorie target that maintains your current weight. To lose fat, eat below it; to gain muscle, eat above it. The catch is that TDEE isn't fixed: it changes with bodyweight (less mass to maintain = lower BMR), with activity (skipping the gym for two weeks drops your real expenditure), and with diet history (chronic deficits trigger metabolic adaptation). The right workflow is: calculate TDEE, pick a deficit or surplus (a 500 kcal deficit projects roughly 0.45 kg / 1 lb of weight loss per week), eat at that calorie target for 14-21 days, weigh yourself daily, and look at the 14-day moving average. If your weight is moving faster or slower than expected, adjust your daily calories by 100-200 kcal and run another 14-day block. Recalculate TDEE every 4-6 weeks or every 5-10 lb of change.
Mifflin-St Jeor vs Harris-Benedict — which formula?
Most online calculators use one of three formulas. Harris-Benedict (1919, revised 1984) is the oldest and over-estimates BMR by 5-10% on modern populations because mid-20th-century body composition norms differed. Mifflin-St Jeor (1990) was developed specifically to correct that gap; it's the formula clinical dietitians and the American Dietetic Association recommend, and it's what this calculator uses. Katch-McArdle is the most accurate of the three but requires you to know your body fat percentage (it computes BMR from lean body mass, not total weight) — useful for athletes who've measured body fat with DEXA, less useful for most people who haven't. If you're not an athlete and don't have a DEXA scan, Mifflin-St Jeor is the right default and what registered dietitians use professionally.
Why these are estimates — and what to do about it
Population-level formulas can't account for individual variation. Two people with identical age, sex, height, and weight can have measured BMRs that differ by 10-15% due to thyroid function, muscle mass distribution, fidgeting (NEAT — Non-Exercise Activity Thermogenesis), and genetics. The formula gives you a starting point, not a verdict. The right move is to treat the calculator's TDEE as a hypothesis: eat at it for 14 days, weigh daily, and look at the trend. If your weight is stable, your real maintenance is exactly what the calculator said; if you're losing 1 lb/week without trying, your real TDEE is roughly 500 kcal higher; if you're gaining despite eating at the deficit, your real TDEE is lower. Adjust accordingly. Don't pile multiple formulas against each other — pick one and let measured weight change calibrate it.