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BMI Calculator

Body mass index with category and healthy weight range

  • Runs in your browser
  • No sign-up
  • Free forever
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How to use the BMI Calculator

  1. 1

    Choose Metric for centimetres and kilograms, or Imperial for feet, inches and pounds or stones.

  2. 2

    Enter your height and weight. Your BMI and category appear as you type.

  3. 3

    Read where you sit on the colour scale and the healthy weight range for your height, shown in both unit systems.

  4. 4

    Optionally add your age and sex to see notes on how to read the result. They do not change the number.

  5. 5

    Press Copy link to save or share the result, or Reset to start again.

Features

  • BMI to one decimal with the WHO adult category, from underweight to obesity class III
  • Metric and imperial units, including stones and pounds, kept in sync as you switch
  • A colour scale showing where your BMI falls
  • Healthy weight range for your height in kilograms and pounds, and how far you are from it
  • BMI Prime, your BMI divided by the upper healthy limit of 25
  • Notes for children, older adults and differences between sexes when you add age and sex
  • The formula shown with your numbers
  • Calculated in your browser; nothing you enter is sent anywhere

What BMI tells you

Body mass index relates weight to height with one number. It was designed to compare populations, and it is still the first check doctors use because it needs nothing but a tape measure and scales. For most adults a BMI between 18.5 and 24.9 is associated with the lowest health risks; higher and lower values are a prompt to look more closely, not a verdict.

The formula

BMI = weight (kg) ÷ height (m)²

The healthy weight range for a height comes from rearranging the same formula: 18.5 × height² to 24.9 × height². At 175 cm that is 56.7 kg to 76.3 kg, or about 125 lb to 168 lb. BMI Prime is BMI ÷ 25. Categories are applied to the BMI rounded to one decimal, as published charts do, so 24.96 counts as overweight while 24.94 is healthy.

To convert heights and weights between units, use the Unit Converter. For quick percentage questions, such as how much a 5% change in weight is, try the Percentage Calculator.

Frequently asked questions

How is BMI calculated?
BMI is your weight in kilograms divided by the square of your height in metres. Someone 1.75 m tall weighing 72 kg has a BMI of 72 ÷ 1.75² = 23.5. In imperial units the same result comes from 703 × pounds ÷ inches²; this calculator converts to metric first.
What are the BMI categories?
For adults the World Health Organization uses underweight below 18.5, healthy weight 18.5 to 24.9, overweight 25 to 29.9, and obesity from 30, split into class I (30 to 34.9), class II (35 to 39.9) and class III (40 and above). Some health services use lower thresholds for people of South Asian, Chinese and Black African or Caribbean background, often 23 and 27.5.
Is BMI accurate?
BMI is a quick screening measure for groups, not a diagnosis for one person. It does not distinguish muscle from fat or say where fat is stored. Muscular people can score as overweight while being lean, and older adults can score as healthy while carrying more fat. Waist measurement and a conversation with a clinician add the context BMI lacks.
Can I use this for children?
Not reliably. For ages 2 to 19, BMI is compared against growth charts for the same age and sex, giving a percentile rather than a category. This calculator uses the adult categories only, and shows a note when you enter an age under 20.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value below 1.00 is within that limit, and 1.20 means 20% above it. It makes the distance from the healthy range easy to read without remembering the thresholds.
Is my data stored?
No. The calculation runs in your browser. Your last measurements are remembered on this device only. They are put into a link only when you press Copy link, so they leave your browser only if you share that link.

Last updated .