How it works
Body mass index (BMI) compares your weight with your height. It is a quick screening number used by doctors and the World Health Organization (WHO) for adults aged 18 and over.
BMI = weight (kg) ÷ height (m)²Worked example. 70 kg and 175 cm: 1.75² = 3.0625, and 70 ÷ 3.0625 = 22.9 — a healthy weight. The healthy range at 175 cm is 18.5 × 3.0625 = 56.7 kg to 24.9 × 3.0625 = 76.3 kg.
WHO adult categories
| BMI | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class I |
| 35 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
BMI does not measure body fat directly. Very muscular people may score “overweight” while being healthy, and older adults may have less muscle at a normal BMI. Children and teenagers need age-specific charts, and the categories do not apply during pregnancy.
Calories: BMR and daily needs
Your basal metabolic rate (BMR) is the energy your body uses at rest. We use the Mifflin–St Jeor equation, which studies have found to be among the most accurate for adults:
BMR = 10 × kg + 6.25 × cm − 5 × age + 5 (men) or − 161 (women)Daily calories = BMR × an activity factor: 1.2 (little exercise), 1.375 (light), 1.55 (moderate), 1.725 (hard) or 1.9 (very hard). A 30-year-old man of 70 kg and 175 cm has a BMR of 1,649 kcal and needs about 2,556 kcal a day with moderate exercise. Eating about 500 kcal less per day leads to roughly 0.5 kg of weight loss per week for many people.
Body fat (optional)
With a tape measure you can estimate body fat using the U.S. Navy circumference method (neck and waist for men; neck, waist and hips for women). It is an estimate, typically within a few percentage points of lab methods.
Need to convert pounds or feet? Use the unit converter, or switch the calculator to imperial units.