BMI Calculator

How to use this calculator

Choose metric or imperial units, enter your height and weight, and press Calculate BMI. The tool returns your body mass index to one decimal place and the standard category it falls into — underweight, healthy, overweight or obese. BMI is a quick screening number used by health services worldwide to flag whether weight may be a health concern relative to height. It is a starting point for a conversation with a professional, not a full health assessment on its own.

The formula explained

BMI relates weight to the square of height:

metric: kg ÷ m² · imperial: 703 × lb ÷ in²

Squaring the height is what makes BMI roughly comparable between tall and short people of similar build. The imperial version uses the same idea with a conversion factor of 703 so that pounds and inches give the same scale as kilograms and metres. The standard adult bands are: below 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, and 30 or above obese.

A worked example

Someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9, which sits in the healthy range. The same person measured as 5 ft 9 in (69 in) and 154 lb gives 703 × 154 ÷ 69² ≈ 22.7 — the small difference is just rounding between unit systems.

Frequently asked questions

How is BMI calculated?

Weight in kilograms divided by height in metres squared — or 703 × pounds ÷ inches² in imperial.

What is a healthy BMI?

For most adults, 18.5 to 24.9 is considered the healthy range.

Is BMI accurate for everyone?

It is a general screen and ignores muscle, build and age, so treat it as a guide rather than a diagnosis.