How to Calculate BMI (Body Mass Index)
BMI (Body Mass Index) is a quick screening number that compares your weight to your height. This guide shows the formula for both metric and imperial units, walks through a worked example, and lists the standard adult categories.
The BMI formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: pounds and inches are converted to kilograms and metres first, then the same formula is used — so a metric and an imperial input for the same person always give the same BMI.
Worked example
Metric: 70 kg at 1.75 m tall.
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
Imperial: 154 lb at 5 ft 8 in (68 in) tall.
Converted to metric: 69.85 kg and 1.727 m, giving a BMI of 23.4 — close to the metric example above, since both describe a similar build.
Adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These are the standard adult ranges. They don't apply the same way to children and teens (2–19), who are assessed against age- and sex-specific percentile charts instead.
What BMI doesn't measure
BMI uses only height and weight, so it can't tell the difference between muscle and fat. A muscular person can land in "overweight" while carrying little body fat, and someone with low muscle mass can land in "normal" while carrying more fat than is healthy. Treat BMI as a quick screening figure, not a complete health assessment.
Calculate your own
The calculator accepts either metric or imperial units, and also shows your BMI Prime, an ideal-weight estimate, and — if you add your age and sex — how much you'd need to gain or lose to reach the normal range.
Frequently asked questions
What is the BMI formula?
Metric: BMI = weight in kilograms divided by height in metres squared. Imperial measurements are converted to metric first, then the same formula is used, so the result is identical either way.
What counts as a healthy BMI?
For most adults, the World Health Organization defines 18.5 to 24.9 as normal weight. BMI is a general screening figure, not a diagnosis, and it doesn't account for muscle mass, age, or body composition.
Does BMI work the same way for children?
No. For ages 2 to 19, the fixed adult cutoffs don't apply. Children's BMI is compared against CDC growth-chart percentiles for their exact age and sex instead, since a healthy BMI changes as kids grow.
Is BMI accurate for athletes or very muscular people?
Not always. Muscle weighs more than fat, so a very muscular person can show a high BMI while carrying low body fat. BMI is best treated as a starting point, not the full picture.