What Is a Healthy BMI — and How Many Calories You Need

How to read your BMI, what the categories mean, the limits of the number, and how daily calorie needs are estimated.

Body mass index (BMI) is a quick way to see whether your weight is in a healthy range for your height. It is a screening tool, not a diagnosis, but it is a useful starting point.

How BMI is calculated

BMI is your weight in kilograms divided by your height in metres squared. Someone 1.75 m tall weighing 68 kg has a BMI of 68 ÷ (1.75 × 1.75) = 22.2. The BMI calculator works it out in metric or imperial units.

The categories

The World Health Organization bands for adults are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30 or more obese. They are the same for men and women.

Where BMI falls short

BMI does not tell muscle from fat, so very muscular people can read as overweight, while older adults may carry less muscle than the number suggests. For children and teenagers, doctors use age- and sex-specific percentiles instead. Waist measurement adds useful context about where fat is stored.

How many calories you need

Daily calories start from your resting metabolism — the energy your body uses at rest — estimated with the Mifflin-St Jeor equation from your weight, height, age and sex. That figure is multiplied by an activity factor, from sedentary to very active, to estimate what you burn in a day.

Using the numbers

To maintain weight, eat around your daily calorie estimate; a moderate deficit or surplus shifts weight slowly and sustainably. Treat all of this as a guide, and speak to a professional for personal health decisions.