To calculate BMI (body mass index), divide your weight in kilograms by the square of your height in metres: BMI = kg ÷ m². For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which sits in the healthy range of 18.5–24.9. If you work in pounds and inches, there is an equivalent formula using a conversion factor of 703.
This guide shows both formulas step by step, works through examples in each system, gives you the standard category chart, and explains how to find the healthy weight range for your height. If you just want your number now, the BMI Calculator does all of it instantly — this article is for understanding what the number means and where it comes from.
What BMI is — and what it isn’t
BMI is a screening tool, not a diagnosis. It is a simple ratio of weight to height, designed to flag whether a person’s weight is in a range usually associated with lower health risk.
It does not measure body fat, muscle mass, bone density, fat distribution or overall health directly. Two people with the same BMI — one an athlete, one sedentary — can have very different amounts of body fat. Keep that in mind when you read your result.
The metric formula
BMI = weight (kg) ÷ height (m)²
In steps:
- Measure your height in metres.
- Square it (multiply it by itself).
- Divide your weight in kilograms by that result.
Worked example: A person who weighs 82 kg and is 1.80 m tall: 1.80² = 3.24, and 82 ÷ 3.24 = 25.3, which is in the overweight range (just over the threshold — see the chart below).
The imperial formula
BMI = (weight (lb) ÷ height (in)²) × 703
In steps:
- Convert your height to inches.
- Square it.
- Divide your weight in pounds by that result.
- Multiply by 703.
Worked example: A person who is 5′8″ (68 inches) and weighs 150 lb: 68² = 4,624, 150 ÷ 4,624 = 0.03244, and 0.03244 × 703 = 22.8, which is in the healthy range.
Why 703? It is the factor that converts pounds per square inch into kilograms per square metre (1 kg = 2.20462 lb and 1 m = 39.3701 inches, and the ratio works out to about 703).
BMI category chart
| Category | BMI range | What it means |
|---|---|---|
| Underweight | Below 18.5 | Below the healthy range |
| Healthy | 18.5–24.9 | Generally associated with lower health risk |
| Overweight | 25.0–29.9 | Above the healthy range |
| Obesity class I | 30.0–34.9 | Obesity, class I |
| Obesity class II | 35.0–39.9 | Obesity, class II |
| Obesity class III | 40.0+ | Sometimes called severe obesity |
These thresholds apply to adults aged 20 and over. Children and teenagers use age- and sex-specific percentile charts, not this adult table.
How to find the healthy weight range for your height
Reverse the formula: Healthy weight = BMI × height², so the lower end is 18.5 × height² and the upper end is 24.9 × height².
Worked example: For a height of 1.70 m: 1.70² = 2.89, so the lower end is 18.5 × 2.89 = 53.5 kg, and the upper end is 24.9 × 2.89 = 72.0 kg. The healthy range for that height is therefore roughly 53.5–72.0 kg.
What BMI doesn’t tell you
- Muscle vs. fat: muscle weighs more than fat, so muscular people can have a high BMI without excess body fat.
- Fat distribution: where fat sits matters — abdominal fat carries more risk than fat elsewhere, but BMI does not distinguish.
- Age, sex and ethnicity: the same BMI can mean different things across groups, and thresholds are less reliable for older adults, athletes and pregnant people.
Use BMI as a starting point, not a verdict. If your BMI is outside the healthy range, a clinician can look at waist circumference, blood pressure, blood tests and lifestyle to give a fuller picture.
Frequently asked questions
What is a good BMI?
For most adults, 18.5–24.9 is considered the healthy range. Below 18.5 is underweight, 25.0–29.9 is overweight, and 30.0 or above is in the obesity range.
Is BMI accurate?
It is useful as a screening tool for populations and as a first check for individuals, but it is not a diagnosis. It does not measure body fat directly, so athletes, muscular people and older adults may get misleading results.
How do I calculate BMI without a calculator?
In metric: divide your weight in kilograms by your height in metres squared. In imperial: divide your weight in pounds by your height in inches squared, then multiply by 703.
Do children use the same BMI chart?
No. Children and teenagers are assessed with age- and sex-specific percentile charts, because their BMI changes as they grow. The adult chart in this article does not apply to them.
Work out your number with our BMI Calculator, and if you want to estimate daily calorie needs, try our BMR Calculator. For the official categories and guidance, see CDC’s guide to adult BMI.
