A healthy BMI is 18.5–24.9 for adults. Below 18.5 is underweight; 25–29.9 is overweight; 30+ is obese (WHO). BMI cannot distinguish muscle from fat — pair it with body fat % for accuracy. Calculate yours →
Most adults need 1,600–3,000 kcal/day depending on size and activity. Subtract 500 cal to lose ~0.5kg/week. Add 200–300 cal to build muscle. Find your number →
Healthy body fat: 6–24% for men, 14–31% for women. Athletic: 6–13% (men), 14–20% (women). More accurate than BMI for fitness goals. Measure yours →
Total Daily Energy Expenditure — the calories you burn each day from all sources. Eat below it to lose fat. Above it to gain muscle. At it to maintain weight. Calculate TDEE →
For fat loss: 35% protein / 35% carbs / 30% fat. Set protein first at 1.8–2.4g/kg bodyweight, fat at 0.8–1g/kg, then fill remaining with carbs. Set your macros →
Active adults: 1.6–2.4g per kg bodyweight daily. A 75kg person needs 120–180g/day. The RDA of 0.8g/kg is for sedentary adults only. Get your target →
Precision Health Calculators — Free Forever
FREE HEALTH
CALCULATORS
31 free, evidence-based calculators for BMI, body fat, calories, macros, TDEE, protein, hydration, sleep and more. All not medical advice. All instant. All private.
How It Works
Three steps to a complete picture of your health and nutrition.
Use the BMI and body fat calculators to understand where you are now. Two numbers. Clear picture.
Start with BMI →Calculate your TDEE, calorie target, and protein goal. These three numbers drive everything else.
Calculate TDEE →Use the macro calculator to split your calories into protein, carbs, and fat. Your complete nutrition plan.
Set Macros →Popular Health Calculators Explained
Each calculator below uses a specific clinically validated formula. Here is what each one does, why it matters, and which ones to use together for the most complete picture of your health.
Body Mass Index divides your weight (kg) by height squared (m²). The WHO uses it to screen for weight-related health risk across six categories from Severe Thinness to Obese Class III. A healthy BMI is 18.5–24.9. Important limitation: BMI cannot distinguish muscle from fat, making it misleading for athletes. Pair it with body fat percentage for accuracy.
Use the BMI Calculator →Uses the Mifflin-St Jeor equation — the most accurate BMR formula endorsed by the American Dietetic Association — multiplied by your activity level to find your Total Daily Energy Expenditure (TDEE). Eat 500 cal below this to lose ~0.5kg/week. Eat 200–300 above it to build muscle. This is the single most important number for body composition.
Use the Calorie Calculator →Macronutrients — protein, carbohydrates, and fat — each serve distinct roles. Protein (4 kcal/g) builds and preserves muscle. Carbs (4 kcal/g) fuel high-intensity exercise. Fat (9 kcal/g) supports hormone production and fat-soluble vitamin absorption. For fat loss, 35% protein / 35% carbs / 30% fat is the evidence-backed starting point. Set protein first (1.8–2.4g/kg), then adjust carbs and fat around it.
Use the Macro Calculator →Uses the US Navy circumference method — accurate within 3–4 percentage points of gold-standard DEXA scanning. For men: uses neck and waist measurements. For women: neck, waist, and hips. Healthy ranges: 14–24% for men, 21–31% for women. Research shows 39% of people with a “healthy” BMI have high body fat — this calculator catches what BMI misses.
Use the Body Fat Calculator →Total Daily Energy Expenditure is your complete calorie burn across all four components: BMR (60–70%), Thermic Effect of Food (~10%), Exercise Activity (5–30%), and Non-Exercise Activity Thermogenesis (NEAT, 15–50%). NEAT alone can vary by 2,000 cal/day between people — which explains why some people seem to “eat anything” and stay lean. Know your TDEE and you take control of your weight.
Use the TDEE Calculator →The official RDA of 0.8g/kg was designed to prevent deficiency — not to optimise muscle or body composition. A 2018 meta-analysis of 1,863 participants found protein synthesis plateaus at ~1.62g/kg during a surplus. For fat loss phases, 1.8–2.4g/kg is recommended to protect lean mass. For a 75kg person building muscle, that is 120–150g of protein daily. Use this alongside our macro calculator for a complete nutrition target.
Use the Protein Calculator →Use these 6 calculators in order for a complete picture of your health and nutrition.
IndexBody provides precision health calculators built on peer-reviewed formulas: Mifflin-St Jeor for calorie calculations, US Navy circumference method for body fat, WHO thresholds for BMI classification, and Morton et al. 2018 meta-analysis for protein targets. Every formula is documented with citations on our methodology page.
All content is reviewed by IndexBody Editorial, Sources: WHO, CDC, NIH, peer-reviewed research, for clinical accuracy against WHO, CDC, and NIH guidelines. IndexBody is free, ad-supported, and contains no paywalls. Your calculation results are processed entirely in your browser — no personal data is ever sent to a server.
All 31 Free Health Calculators
Every calculator below uses a peer-reviewed clinical formula. All are free, private, and work instantly in your browser — no account required, no data stored on servers.