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BMI Calculator (Body Mass Index)

BMI: -
Calculation explained

How this result was calculated

The formula, your substituted values, and the evaluated result are shown below.

Visual analysis

How the result changes when the first input changes

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This calculator applies the body mass index relationship. Enter values in the stated units and review the result. Check assumptions and use appropriate professional guidance for safety-critical decisions.
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About this calculator

Calculate body mass index using clearly labelled inputs.

Calculator guide

How the BMI calculator works

Body mass index (BMI) compares your weight with your height. It is the same formula doctors and the World Health Organization use for adult screening:

BMI = weight (kg) ÷ height (m)²

Enter your weight in kilograms and your height in metres, not centimetres. If you know your height as 175 cm, enter 1.75. The result is a single number, usually between about 15 and 40 for adults.

Worked example

Suppose you weigh 70 kg and are 1.75 m tall.

  1. Square the height: 1.75 × 1.75 = 3.0625
  2. Divide the weight by that: 70 ÷ 3.0625 = 22.9

A BMI of 22.9 sits in the healthy weight range for adults. Type those two values into the calculator above and you will get the same answer. The working panel shows each step so you can check it.

What your result means

BMI (adults)WHO category
Below 18.5Underweight
18.5 – 24.9Healthy weight
25.0 – 29.9Overweight
30.0 and aboveObese

For people of Asian background, many health services use lower thresholds (overweight from about 23, obese from about 27.5), because health risks start rising at a lower BMI.

Where BMI falls short

BMI is a quick screening number, not a diagnosis. It cannot tell muscle from fat, so a fit, muscular person can score as "overweight" while being very healthy. It also says nothing about where fat is carried. Waist measurement is a useful second check. BMI isn't suitable for children and teenagers, who are assessed with age-and-sex percentile charts, and it is less reliable during pregnancy and in older adults.

If your result worries you, treat it as a reason to talk to your GP, not as a verdict.

Common mistakes

Frequently asked questions

Is a BMI of 25 bad?

25 is the start of the "overweight" band, but on its own it isn't a problem. Look at it together with waist size, fitness, blood pressure and your doctor's advice.

Why does my BMI differ from my smart scale?

Many scales estimate body fat percentage, which is a different measurement. BMI only uses height and weight, so the two numbers aren't comparable.

Can I use this for my child?

No. Children's BMI has to be compared against growth charts for their age and sex. Ask your GP or child health nurse.

Written by Sam Alex, civil engineer. About the author