BMI, or body mass index, is a screening measure calculated from height and weight. It can be a starting point for a general discussion, but it does not measure body composition, fitness, diet, symptoms, medical history, or an individual’s full health picture. A BMI number is not a diagnosis and should not be used by itself to make a health decision.
Understand what the calculation does
For adults, BMI is calculated by dividing weight in kilograms by height in metres squared. In formula form: BMI = weight (kg) ÷ height (m)². When measurements are entered in pounds and inches, a calculator can perform the unit conversion and arithmetic for you.
The calculation produces a number from two inputs only. It can be useful for a broad population-level screening discussion, yet two people with the same number can have different body composition, circumstances, and health needs. Interpret any result cautiously and in context.
Use current, accurate measurements
Use a recent height and weight, and check the units before you calculate. A mistake between centimetres and metres, or between pounds and kilograms, can produce a very different number. If you are using a height from an old form or a scale reading taken under unusual conditions, record that limitation rather than treating the result as precise.
For young people, pregnancy, older age, athletic training, disability, illness, medication, and many other circumstances, general adult screening interpretations may not fit the situation. Use age-appropriate and context-appropriate guidance rather than applying an adult result to everyone.
Worked example: check the arithmetic, not a health verdict
Suppose an adult records a height of 1.70 metres and a weight of 70 kilograms. The arithmetic is 70 ÷ (1.70 × 1.70), which gives approximately 24.2. You can enter the same values into the BMI Calculator to check the calculation.
That number is only the output of the formula. It does not identify a cause of weight change, assess someone’s wellbeing, or determine what treatment, diet, exercise, or medical action is appropriate. If the result concerns you, discuss the full context with a qualified healthcare professional instead of relying on a single online calculation.
Use the BMI Calculator carefully
- Confirm that the calculator is set to the units you intend to use.
- Enter a current height and weight without guessing at conversions.
- Review the displayed inputs before reading the result.
- Keep the result in context and avoid treating it as a diagnosis.
- Use an authoritative health resource or qualified professional for questions about your own health.
What BMI cannot tell you
BMI does not directly measure muscle, fat distribution, bone density, metabolism, blood pressure, nutrition, or mental wellbeing. It cannot explain why a person’s weight changed or predict an individual outcome. An apparently precise decimal does not make the underlying assessment more complete.
Do not use a general BMI calculation to judge someone else’s health, appearance, effort, or personal circumstances. Health information is private, and casual comparisons can be inaccurate as well as unhelpful.
Use authoritative health guidance for interpretation
For general background, read the CDC’s adult BMI calculator guidance, which describes BMI as a screening measure and provides context for adult use. Official or clinical guidance may differ by age group and country, so use a source appropriate to the person and situation involved.
If you have a health concern, a major or unexplained change in weight, eating difficulties, a question about a child or teenager, or uncertainty about interpreting a result, seek individual advice from a qualified healthcare professional. A browser-based tool cannot assess personal risk or provide medical care.
Limits and responsible use
This guide provides general educational information only. It does not provide medical diagnosis, treatment, nutrition advice, exercise advice, or an assessment of any person. Do not use a BMI result as the sole basis for a health, insurance, employment, or eligibility decision.
For urgent symptoms or a medical emergency, contact the appropriate local emergency service or a qualified healthcare provider rather than using an online calculator.
Useful next steps
Use the BMI Calculator only to check the general arithmetic, then read the CDC guidance for general context. For unrelated everyday calculations, the Average Calculator and Age Calculator are available in the ToolMixo Calculators hub.