The body, energy and fitness formulas behind our health calculators. Every example is recomputed and matches the calculator. These are population estimates for general information, not medical advice: a clinician can interpret your own numbers.
Body measurements
| Measure | Formula | Example |
|---|---|---|
| Body mass index (BMI) | BMI = kg / m² |
70 kg at 1.75 m: 22.86 |
| Weight range for a BMI of 18.5 to 25 | 18.5 × m² to 25 × m² |
At 1.75 m: 56.7 to 76.6 kg |
| Body surface area (Mosteller) | BSA = √(cm × kg / 3600) |
175 cm, 70 kg: 1.84 m² |
| Waist-to-hip ratio | waist / hip |
82 cm waist, 100 cm hips: 0.82 |
| Waist-to-height ratio | waist / height |
A common guide is to keep it under 0.5 |
BMI bands for adults are under 18.5 underweight, 18.5 to 24.9 healthy range, 25 to 29.9 overweight and 30 or more obese. BMI ignores muscle and fat distribution, so it is a screening number, not a diagnosis. Try the BMI Calculator, Healthy Weight Calculator, Ideal Weight Calculator, Body Surface Area Calculator, Waist-to-Hip Ratio Calculator, Waist-to-Height Ratio Calculator, Body Fat Calculator, Army Body Fat Calculator, Lean Body Mass Calculator, FFMI Calculator, Body Type Calculator and Overweight Calculator.
Energy and nutrition
| Measure | Formula | Example |
|---|---|---|
| Basal metabolic rate (Mifflin-St Jeor) | 10·kg + 6.25·cm − 5·age + 5 (men), … − 161 (women) |
Man, 30 years, 80 kg, 180 cm: 1,780 kcal a day |
| Total daily energy expenditure (TDEE) | TDEE = BMR × activity |
Activity 1.55 (moderate): 2,759 kcal |
| Daily deficit for a weekly loss | kg per week × 7700 / 7 |
0.5 kg a week: about 550 kcal a day |
| Macro grams from a calorie target | protein, carbs: kcal × share / 4, fat: kcal × share / 9 |
2000 kcal at 30% protein, 40% carbs, 30% fat: 150 g, 200 g, 66.7 g |
Energy per gram is 4 kcal for protein and carbohydrate, 9 for fat and 7 for alcohol. Try the BMR Calculator, TDEE Calculator, Calorie Calculator, Calorie Deficit Calculator, Macro Calculator, Protein Calculator, Carbohydrate Calculator, Fat Intake Calculator, Daily Water Intake Calculator, Caffeine Calculator, Alcohol Units Calculator, Calories Burned Calculator and Calories to Joules Converter. The guide How Calorie Needs Are Calculated explains how they fit together.
Fitness and vitals
| Measure | Formula | Example |
|---|---|---|
| Estimated maximum heart rate | 220 − age |
Age 40: 180 beats a minute |
| Target heart rate (Karvonen) | (max − rest) × intensity + rest |
Max 180, rest 60, 70%: 144 beats a minute |
| One-rep max (Epley) | weight × (1 + reps / 30) |
100 kg for 5 reps: 116.7 kg |
| One-rep max (Brzycki) | weight × 36 / (37 − reps) |
Same lift: 112.5 kg |
| Mean arterial pressure | diastolic + (systolic − diastolic) / 3 |
120/80 mmHg: 93.3 mmHg |
Try the Target Heart Rate Calculator, One Rep Max Calculator, VO2 Max Calculator, Running Pace & Split Calculator, Steps to Calories & Miles Calculator, Wilks & DOTS Calculator and Mean Arterial Pressure Calculator.
Common mistakes
- Mixing units. BMI needs kilograms and metres; with pounds and inches multiply by 703.
- Treating the deficit rule as exact. The 7,700 kcal per kilogram figure is a rule of thumb, and metabolism adapts as weight drops.
- Trusting
220 − age. Individual maximum heart rate varies by 10 beats or more either way.