BMI Calculator
BMI Reference
| Category | BMI Range |
|---|---|
| Underweight | < 18.5 |
| Normal | 18.5 - 24.9 |
| Overweight | 25.0 - 29.9 |
| Obese | 30.0+ |
Body Mass Index (BMI) is a screening number that relates your weight to your height, giving a quick estimate of whether you fall in the underweight, normal, overweight, or obese range. It was devised in the 1830s by Belgian mathematician Adolphe Quetelet and remains the most widely used population-level measure of body weight status. Because BMI uses only height and weight, it does not distinguish muscle from fat, so it works best as a starting point rather than a final diagnosis.
Formula
BMI = weight (kg) / height (m)²
- weight
- Body weight in kilograms (kg)
- height
- Standing height in metres (m); centimetres are divided by 100
- BMI
- Body Mass Index, expressed in kg/m²
How it works
- Choose your unit system with the Imperial/Metric toggle. Imperial accepts height in feet and inches plus weight in pounds; Metric accepts height in centimetres and weight in kilograms.
- Enter your height and weight. Imperial values are converted internally to metric (inches to centimetres at 2.54 cm per inch, pounds to kilograms at 0.453592 kg per pound) before the BMI is computed.
- Read your BMI value in the result card and your category below it. The reference table and colour gauge show where you sit across the Underweight (< 18.5), Normal (18.5-24.9), Overweight (25.0-29.9), and Obese (30.0+) bands.
Worked examples
An adult who is 175 cm tall and weighs 70 kg wants to find their BMI.
- Convert height to metres: 175 cm ÷ 100 = 1.75 m.
- Square the height: 1.75 × 1.75 = 3.0625 m².
- Divide weight by height squared: 70 ÷ 3.0625 = 22.86 kg/m².
BMI ≈ 22.9, which falls in the Normal range (18.5-24.9).
A person who is 5 ft 9 in tall and weighs 200 lb checks their BMI using imperial units.
- Convert height: (5 × 12 + 9) × 2.54 = 69 × 2.54 = 175.26 cm = 1.7526 m.
- Convert weight: 200 × 0.453592 = 90.72 kg.
- Compute BMI: 90.72 ÷ (1.7526 × 1.7526) = 90.72 ÷ 3.0716 = 29.53 kg/m².
BMI ≈ 29.5, which falls in the Overweight range (25.0-29.9), just below the obese threshold of 30.
Frequently asked questions
- What is a healthy BMI range?
- For most adults a BMI from 18.5 to 24.9 is classified as the normal (healthy) range. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 or above is obese.
- Is BMI accurate for athletes and very muscular people?
- Not always. BMI counts only total weight, so dense muscle can push a lean, muscular athlete into the overweight or obese band even with low body fat. For these individuals a body-fat or waist-based measure is more informative.
- What is the difference between metric and imperial BMI?
- There is no difference in the result. The formula is defined in metric units (kg ÷ m²), so the imperial inputs on this calculator are first converted to kilograms and metres, meaning both systems produce the same BMI for the same body.
- Does BMI apply to children, teenagers, or pregnant people?
- The fixed adult categories used here are intended for non-pregnant adults aged 20 and over. Children and teens are assessed with age- and sex-specific BMI percentiles, and BMI is not a meaningful measure during pregnancy.