Health Calculators Backed by Authoritative Sources
Health calculators are everywhere online, but most don't tell you where their formulas come from. That's a problem — slight changes in formulas produce significantly different recommendations, and "fitness math" is rarely standardized across the internet. Every calculator on this page lists its source.
The BMI Calculator uses the World Health Organization's official BMI categories (under 18.5 = underweight, 18.5-24.9 = normal, 25-29.9 = overweight, 30+ = obese). The TDEE Calculator uses the Mifflin-St Jeor equation, which the American College of Sports Medicine recognizes as the most accurate basal metabolic rate formula for non-athletic populations. The Body Fat Calculator uses the U.S. Navy circumference method, used by the Department of Defense for fitness assessments.
Body Composition and Energy Calculators
Body Mass Index (BMI) is the simplest screening tool for weight categories. Calculate yours with the BMI Calculator. BMI has known limitations — it doesn't distinguish muscle from fat, so athletes often score "overweight" despite being lean. For more accuracy, pair it with the Body Fat Calculator.
Total Daily Energy Expenditure (TDEE) tells you how many calories you burn per day, accounting for your basal metabolic rate, activity level, and the thermic effect of food. The TDEE Calculator is the foundation for any weight loss, maintenance, or muscle gain plan. To gain weight, eat above your TDEE; to lose, eat below.
Calorie and macronutrient needs go beyond just calories — protein, carbs, and fats each serve different functions. The Calorie & Macro Calculator provides specific gram targets based on your goals (weight loss, maintenance, muscle gain) and ACSM's protein recommendations (0.8-2.2g per kg of body weight depending on activity).
Ideal weight ranges vary by methodology. The Ideal Weight Calculator shows you the Hamwi, Devine, Robinson, and Miller formulas — all four widely used in clinical settings — so you can see the realistic range, not just one number.
Daily Wellness Calculators
Hydration matters more than most people realize. Even mild dehydration (1-2% of body weight) impairs cognitive performance, mood, and physical capacity. The Water Intake Calculator bases recommendations on body weight, activity level, and climate — not the simplistic "8 glasses a day" myth.
Sleep operates in 90-minute cycles, and waking mid-cycle (especially during deep sleep) causes grogginess no amount of coffee can fix. The Sleep Calculator shows you ideal bedtimes for any wake-up time, and ideal wake-up times for any bedtime, all aligned to complete sleep cycles.
Screen time is a modern wellness concern. The Screen Time Calculator helps you track and reduce digital consumption, with specific recommendations for adults and children based on AAP guidelines.
Reproductive Health Calculators
The Ovulation Calculator identifies your fertile window using the standard luteal-phase calculation (ovulation occurs ~14 days before your next period). The Due Date Calculator uses Naegele's rule (LMP + 280 days), which has been the standard pregnancy dating method since the 19th century. The Pregnancy Week Calculator tracks your current week and trimester from any reference date.
Athletic Performance
Runners and athletes can use the Pace Calculator to plan training intensities, predict race times, and split long runs into target paces. The calculator handles all distances (5K, 10K, half marathon, marathon, custom) and all units (km, miles).