What is a healthy BMI?

The WHO defines 18.5 to 24.9 as the healthy weight range for adults. Some health authorities use lower thresholds for people of South and East Asian descent.

Is BMI accurate for athletes?

Often not. BMI cannot distinguish muscle from fat, so people with substantial muscle mass regularly score in the overweight or obese range while being nothing of the sort.

BMI Calculator

Calculate body mass index from height and weight in either metric or imperial units.

Draft — figures checked, awaiting an editorial read. Not indexed.

BMI—

WHO category: —

Formula: Metric: BMI = kg ÷ m². Imperial: BMI = 703 × lb ÷ in².

BMI is a population screening statistic, not a medical diagnosis, and it does not account for body composition, age, sex, or ethnicity. Nothing on this page is medical advice — discuss any health concern with a qualified clinician.

What this calculates

Body mass index is weight divided by height squared, and it places the result into one of the World Health Organization's four bands. It is a population-level screening measure, and it is worth being clear about what that means: BMI was designed to describe groups, not to diagnose individuals. It says nothing about body composition, so it cannot distinguish muscle from fat, and it systematically misclassifies people at both ends of the athletic spectrum. It is a rough indicator worth knowing and a poor basis for any decision on its own.

How it works

In metric units, BMI is mass in kilograms divided by height in metres squared. In imperial units the same relationship needs a conversion factor: pounds divided by inches squared, multiplied by 703. The WHO categories are below 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 and above obesity. Some health authorities apply lower thresholds for people of South and East Asian descent, where cardiometabolic risk rises at lower BMI values.

Worked example

Someone 5 feet 10 inches tall weighing 170 pounds: height is 70 inches, so BMI = 703 × 170 ÷ (70 × 70) = 703 × 170 ÷ 4900 = 24.4, which falls in the healthy weight band. The same person in metric is 177.8 cm and 77.1 kg: 77.1 ÷ 1.778² = 77.1 ÷ 3.161 = 24.4 — the same figure, as it should be.

Common mistakes

Treating BMI as a measure of health rather than a screening statistic. A muscular athlete frequently reads as "overweight" and a sedentary person with low muscle mass can read as "healthy" while carrying substantial visceral fat. Second, forgetting to square the height. Third, comparing BMI across very different heights — the squared term does not model tall and short bodies equally well, which is a known limitation of the index rather than a fault in the arithmetic.

Common questions

What is a healthy BMI?: The WHO defines 18.5 to 24.9 as the healthy weight range for adults. Some health authorities use lower thresholds for people of South and East Asian descent.
Is BMI accurate for athletes?: Often not. BMI cannot distinguish muscle from fat, so people with substantial muscle mass regularly score in the overweight or obese range while being nothing of the sort.