BMI Calculator
Check your body mass index in under a minute with this BMI calculator.
Your numbers
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Field Notes that go deeper
The science behind the BMI calculator
Show the formulas, method and sources
BMI is a measure of weight relative to height. When you enter your numbers, the tool divides your weight in kilograms by your height in meters, squared. Then it places the result in one of four adult categories. BMI is a screening measure, so it cannot diagnose anything.
Formulas
Metric
BMI = weight (kg) / height (m)^2
US units
BMI = 703 x weight (lb) / height (in)^2
Adult categories (age 20 and older)
Below 18.5: underweight18.5 to below 25: healthy weight25 to below 30: overweight30 and above: obesity
Weight band for BMI 18.5 to 24.9
Lowest = 18.5 x height (m)^2Highest = 24.9 x height (m)^2
How it works, step by step
- First, the calculator converts your height and weight to meters and kilograms.
- Then it divides weight by height squared and rounds the result to one decimal place.
- Next, it matches the result to the CDC adult categories.
- Finally, it works out the weights at your height that fall in the healthy weight category.
Worked example
A person who weighs 80 kg and is 180 cm tall has a BMI of 80 divided by 1.8 squared, which comes to 24.7. That falls in the healthy weight category.
Accuracy and limits
The CDC calls BMI a quick, low-cost screening measure for underweight, overweight and obesity. It adds that you should consider the number with other factors when you judge one person's health.
BMI does not separate fat, muscle and bone. Because of that, someone with a lot of muscle can have a high BMI without carrying much fat, and the reverse can also happen.
The CDC categories apply to adults aged 20 and older. Younger people need a different approach, which this tool does not cover.
- Because BMI is one ratio, it says nothing about where you carry weight, or about fitness, diet and blood tests.
- If your result falls outside the healthy category, take it as a prompt to look closer. It gives no verdict.
- When the number worries you, a clinician can look at the whole picture.
Sources
- Centers for Disease Control and Prevention. Adult BMI categories (page dated March 19, 2024). Used for: The adult BMI cut points and the description of BMI as a screening measure.
- Centers for Disease Control and Prevention. About body mass index (BMI) (page dated December 16, 2025). Used for: What BMI measures and that it does not separate fat, muscle and bone.
Sources and math last reviewed: .
Questions people ask
Is BMI accurate?
It is accurate as arithmetic and useful as a quick screen. It is a weak guide to one person's health, though. The CDC says BMI does not distinguish fat, muscle and bone, and you should read it with other factors.
What is the difference between BMI and body fat?
BMI uses only height and weight, so it ignores body composition. Body fat tries to estimate the share of your weight that is fat. If you lift and carry extra muscle, your BMI can read high while your body fat stays moderate. The body fat calculator gives a second view.
What BMI should I aim for?
The healthy weight category runs from 18.5 to below 25. It works as a reference band for adults, so it does not set a target for everyone. Your own healthy weight depends on more than height, so talk with a clinician before you set a goal.
Why does the calculator ask for my age?
The CDC categories used here apply to adults aged 20 and older. When you enter a younger age, the tool stops and tells you the categories do not apply.
Can my BMI be high if I lift weights?
Yes. Muscle is denser than fat, and BMI counts all weight the same way. If you train regularly, pair BMI with a waist measurement or a body fat estimate before you draw conclusions.
These calculators give general estimates for adults and are not medical advice. For decisions about your own health, talk with a doctor or a registered dietitian. How we write and check things