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BMI Calculator

Body Mass Index (BMI) compares your weight with your height to give a single number that screens for weight categories that may affect health. Use this calculator for a quick check in metric (cm and kg) or imperial (feet...

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Check your BMI

Clinical Body Mass Analysis

About the BMI Calculator

Body Mass Index (BMI) compares your weight with your height to give a single number that screens for weight categories that may affect health. Use this calculator for a quick check in metric (cm and kg) or imperial (feet, inches and pounds) units, and see where you fall between underweight and obese.

How to use the BMI Calculator

  1. Choose Metric or Imperial at the top of the calculator.
  2. Enter your Height (in cm, or in Ft and In) and your Weight (in kg, or in Lbs).
  3. Press the calculate button to see your BMI, your category and the weight range that corresponds to a BMI between 18.5 and 24.9 for your height.

Formula and method

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = 703 × weight (lb) ÷ height (in)²

The categories follow the World Health Organization adult classification.

WHO adult BMI categories
BMI Category
Below 18.5 Underweight
18.5 to 24.9 Normal weight
25.0 to 29.9 Overweight
30.0 to 34.9 Obesity class I
35.0 to 39.9 Obesity class II
40.0 and above Obesity class III

Worked example

A person who is 175 cm (1.75 m) tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, which is in the normal range. For the same height, a BMI between 18.5 and 24.9 corresponds to roughly 57 to 76 kg.

Tips

  • BMI is a screening tool, not a diagnosis. It cannot tell muscle from fat, so muscular athletes can be classed as overweight.
  • It is not designed for children and teenagers, who are assessed with age- and sex-specific growth percentiles.
  • Some health bodies use lower cut-offs for people of Asian descent, so ask your doctor which thresholds apply to you.
  • Waist circumference and body fat percentage give useful extra context alongside BMI.

Frequently asked questions

What is a healthy BMI?
For most adults a BMI from 18.5 to 24.9 is classed as normal weight. Being inside that range does not guarantee good health, and being slightly outside it is not automatically a problem, so use it as a starting point for a conversation with a health professional.
Is BMI accurate for athletes?
Not always. Muscle is denser than fat, so a very muscular person can have a high BMI without carrying excess body fat. Body fat percentage or waist measurements are better indicators in that case.
Does BMI change with age?
The formula is the same for all adults, but older adults tend to lose muscle and height, so BMI can under-estimate body fat. Doctors often accept a slightly higher BMI as healthy in later life.
Can I use this BMI calculator for my child?
No. Children are compared with growth charts for their age and sex. Ask your paediatrician or use a child-specific BMI percentile chart.