BMI Calculator
Enter your height and weight in metric or imperial units to get your body mass index (BMI) and the adult category it falls into, with the formula shown.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to under 25 | Healthy weight |
| 25 to under 30 | Overweight |
| 30 to under 35 | Obesity, class 1 |
| 35 to under 40 | Obesity, class 2 |
| 40 or above | Obesity, class 3 |
Categories as published by the US CDC. The WHO uses the same thresholds for adult overweight (25 or more) and obesity (30 or more).
Your height and weight are used only inside your browser. They are not sent to our server or saved.
How to use the BMI Calculator
- Choose metric (kilograms and centimeters) or imperial (pounds, feet and inches).
- Enter your height and your weight. In imperial mode, 5 ft 7 in goes in as 5 and 7.
- Read your BMI and the category it falls into. The matching row in the table is highlighted.
- Check the formula line if you want to see the numbers used, including any unit conversion.
How BMI is calculated
Body mass index is your weight in kilograms divided by the square of your height in meters. Someone who is 1.70 m tall and weighs 65 kg has a BMI of 65 ÷ (1.70 × 1.70) = 22.5. In imperial mode, the calculator first converts pounds and inches to kilograms and meters using the exact definitions (1 lb = 0.45359237 kg, 1 in = 2.54 cm), so you get the same result either way.
The result is rounded to one decimal place, and the category is read from that rounded figure. A BMI of 24.96 is shown as 25.0 and placed in the overweight band, which matches what you see on screen.
What the categories mean, and what they do not
The table uses the adult categories published by the US Centers for Disease Control and Prevention (CDC), which apply to adults aged 20 and over. The World Health Organization (WHO) uses the same main thresholds, defining overweight in adults as a BMI of 25 or more and obesity as 30 or more.
Both organizations treat BMI as a starting point rather than a verdict. The CDC describes it as one potential health indicator that should be considered with other factors, and the WHO calls it a surrogate marker of fatness, noting that other measurements such as waist circumference can help with a diagnosis. Because the number comes from height and weight alone, it cannot tell muscle from fat or show where weight is carried. A health professional who knows your history is the right person to interpret it.
Children and teenagers
For anyone under 20, the adult table above does not apply. Children's bodies change as they grow, so their BMI is compared with others of the same age and sex. The CDC uses BMI-for-age percentiles for ages 2 to 19, and the WHO uses its growth reference for ages 5 to 19. Use a dedicated child and teen calculator, or ask a pediatrician.
Frequently asked questions
Is BMI a diagnosis?
No. It is a screening number based on height and weight. The CDC and WHO both say it should be looked at alongside other information. Only a qualified health professional can tell you what it means for your health.
Can I use this calculator for my child?
No. The categories shown are for adults aged 20 and over. Children and teenagers are assessed with age- and sex-specific percentiles or growth charts, which this tool does not provide.
Why do metric and imperial give slightly different answers for me?
They should not, as long as the inputs describe the same height and weight. If you rounded when converting, for example entering 5 ft 7 in for 170 cm (which is actually 5 ft 6.9 in), the BMI can shift by a tenth or so.
My result is right on a boundary. Which category applies?
The bands are "from" one value "to under" the next. A BMI of exactly 25.0 counts as overweight, and 24.9 as healthy weight. The calculator uses the one-decimal value shown on screen.
Is my data stored?
No. The calculation runs in your browser. Nothing is sent to our server, and the numbers are gone when you close or reload the page.
Sources: CDC: Adult BMI Categories; World Health Organization: Obesity and overweight fact sheet