Home › BMI Calculator
BMI Calculator
Your Body Mass Index, category, and healthy weight range — in your choice of units.
Please enter your weight and height.
| Healthy weight for your height |
How to use the BMI calculator
Choose your unit system with the toggle at the top. Imperial asks for your weight in pounds and your height in feet and inches — the way most people in the US measure. Metric asks for kilograms and centimetres. Enter your numbers and press Calculate BMI. You will get your BMI value, the category it falls into, and the weight range that would put you in the healthy band for your exact height.
What BMI actually measures
Body Mass Index is a single number that relates your weight to your height. It was designed in the 1830s by a Belgian mathematician as a way to describe populations, not individuals, and it has stuck around because it is cheap, fast, and needs nothing but a scale and a tape measure. The formula divides your weight by the square of your height, which roughly corrects for the fact that taller people weigh more simply because they are bigger.
The standard adult categories are: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, and 30 and above obese. These cut-offs come from large population studies linking BMI ranges to health risks.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
The formula, and a worked example
In metric, BMI = weight (kg) ÷ height (m)². In imperial, BMI = weight (lb) ÷ height (in)² × 703, where the 703 factor simply converts the units. Say you weigh 160 pounds and stand 5 feet 9 inches, which is 69 inches. Then BMI = 160 ÷ (69 × 69) × 703 = 160 ÷ 4761 × 703 ≈ 23.6, which sits comfortably in the healthy range.
Where BMI gets it wrong
Because BMI only knows your weight and height, it makes some predictable mistakes. A muscular rugby player and an inactive person of the same height and weight get identical scores, even though their body composition is completely different. BMI also treats a 25-year-old and a 75-year-old the same, ignoring the muscle loss that comes with age. And it says nothing about where fat sits — visceral fat around the organs is far riskier than fat on the hips, but BMI cannot see the difference. For a fuller picture, measures like waist circumference or body-fat percentage add useful context.
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is classed as healthy. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
Is BMI accurate for everyone?
No. It does not distinguish muscle from fat, so very muscular people can read as overweight. It also ignores age, sex, and fat distribution. Treat it as a rough screening tool.
How is BMI calculated?
BMI equals weight in kilograms divided by height in metres squared. In imperial units it is weight in pounds divided by height in inches squared, multiplied by 703.
Does BMI work for children?
Not with adult categories. Children and teens use age- and sex-specific percentiles. This calculator is intended for adults 18 and over.