How to use it
- Under Units, pick Metric (cm and kg) or US (feet, inches and pounds).
- Enter your Height and Weight. The result updates as you type.
- Add your Age. The adult classes are meant for ages 18 and up, and under 18 the page says the WHO classes are for adults and points to BMI-for-age charts
- Read your BMI, its class, and the weight range for BMI 18.5–24.9 at your height. Below the class table you'll also see BMI Prime and the ponderal index. Open How this was calculated to see the math with your own numbers.
- Use Copy link to keep the result. The numbers sit after a # in the link, so they never reach a server.
How the calculation works
BMI divides weight in kilograms by height in meters, squared: BMI = kg ÷ m². The World Health Organization and the CDC both give this definition.
With US units the calculator uses the same idea with a conversion factor: BMI = 703 × lb ÷ in². The 703 turns pounds per square inch into kilograms per square meter.
The healthy-range weights come from turning the formula around: weight = BMI × m², once with 18.5 and once with 24.9. Here is the full working for two people:
| Step | 70 kg, 175 cm | 154 lb, 5 ft 9 in (69 in) |
|---|---|---|
| Height squared | 1.75 × 1.75 = 3.0625 m² | 69 × 69 = 4,761 in² |
| BMI | 70 ÷ 3.0625 = 22.9 | 703 × 154 ÷ 4,761 = 22.7 |
| WHO class | Normal range | Normal range |
| Weight at BMI 18.5 | 18.5 × 3.0625 = 56.7 kg | 18.5 × 4,761 ÷ 703 = 125.3 lb |
| Weight at BMI 24.9 | 24.9 × 3.0625 = 76.3 kg | 24.9 × 4,761 ÷ 703 = 168.6 lb |
How accurate is this?
The BMI number itself is exact: weight divided by height squared. The limit is what it measures. BMI cannot tell muscle from fat, so very muscular people can land in a higher class, and it says nothing about where fat sits on the body. The WHO classes are for adults; under 18, BMI-for-age charts are used instead.
The WHO adult classes
The classes below are the WHO adult classification. They are the same for men and women and for every adult age. The WHO fact sheet on obesity puts overweight at a BMI of 25 or more and obesity at 30 or more.
The CDC uses the same cutoffs for adults aged 20 and over, worded as "18.5 to less than 25", "25 to less than 30" and so on. The CDC calls its middle class "healthy weight"; the WHO calls it the normal range.
| BMI (kg/m²) | WHO class |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Normal range |
| 25.0–29.9 | Pre-obese (overweight) |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
Lower thresholds used for some ethnic groups
The calculator shows the WHO classes for everyone and asks nothing about ethnicity. Some guidelines use lower cut-offs for some groups, because cardiometabolic risk tends to start at a lower BMI in them.
In 2004 a WHO expert consultation found that a substantial share of Asian people have a high risk of type 2 diabetes and cardiovascular disease below 25. It kept the international WHO classes but listed extra public-health action points at 23.0, 27.5, 32.5 and 37.5 kg/m², and left it to each country to decide how to use them.
NICE in the UK (guideline NG246) uses lower thresholds for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background:
| Class | WHO, used on this page | NICE, groups listed above |
|---|---|---|
| Overweight | 25.0–29.9 | 23.0–27.4 |
| Obesity | 30.0 or more | 27.5 or more |
NICE also suggests measuring waist-to-height ratio for people with a BMI under 35. The waist-to-height ratio calculator does that with the NICE classes.
What BMI tells you, and what it doesn't
BMI is a ratio of weight to height. The CDC describes it as a quick screening measure and notes that it does not measure body fat directly. It does track moderately to strongly with measures that do.
The ratio can't tell fat from muscle or bone. Two people with the same height and weight get the same BMI, even if one carries much more muscle. BMI also says nothing about where fat sits on the body.
Because the classes ignore age and sex, the same BMI may reflect different amounts of body fat in different groups. Body proportions also differ between populations. A WHO expert consultation on Asian populations kept the standard cutoffs but proposed extra points at 23, 27.5, 32.5 and 37.5 for public-health use.
The adult classes are not made for children, teens or pregnancy, which use other methods. For a reading of what a number means for one person, the CDC points to a health care provider, who looks at BMI alongside other measures.
Other ways to look at body size
The WHO fact sheet notes that other measures, such as waist circumference, can also be used. If you want an estimate that uses tape measurements, the body fat calculator applies the U.S. Navy circumference equations.
BMI is also unrelated to how much energy a body uses. For that, the TDEE calculator estimates daily calories from height, weight, age, sex and activity.
BMI Prime and the ponderal index
BMI Prime is BMI divided by 25, the top of the WHO normal range. A value of 1.00 sits exactly at that limit; 0.92 is 8% below it. It is the same information as BMI on a different scale.
The ponderal index (also called Rohrer's index) divides weight by height cubed instead of squared: kg ÷ m³. The MRC Epidemiology Unit's measurement toolkit rates it suitable for adults but notes that BMI is far more common, and adult reference values for it are not widely available. So the page shows it without classes.
For 70 kg and 175 cm, BMI is 22.9, BMI Prime is 0.91 and the ponderal index is 13.1 kg/m³.
Rounding and small differences
The calculator shows BMI to one decimal place, the precision the WHO table uses. At 175 cm, each extra kilogram adds about 0.3 to the result (1 ÷ 3.0625 = 0.33), so small weight changes move the last digit.
Metric and US results can differ slightly for the same person. That happens because 154 lb and 69 in are rounded values, not exact conversions of 70 kg and 175 cm.
Questions
- What is the BMI formula?
- Weight in kilograms divided by height in meters squared. With pounds and inches, multiply weight by 703 and divide by height in inches squared. Both give the same number for the same person.
- Is a BMI of 25 overweight?
- In the WHO adult classification, 25.0 is the first value in the pre-obese (overweight) class. The normal range ends at 24.9.
- Does BMI work the same for men and women?
- The formula and the adult classes are the same for both. Women and men with the same BMI often have different amounts of body fat, which is one of its known limits.
- Why is my BMI high when I have a lot of muscle?
- BMI only uses height and weight, so it cannot tell muscle from fat. The CDC notes this directly. A circumference-based estimate such as the U.S. Navy method measures something different.
- Can I use this calculator for a child?
- No. The adult classes are for ages 18 and up. Children and teens are assessed with age- and sex-specific percentiles, which this calculator doesn't compute.
- Are my height and weight sent anywhere?
- No. The math runs in your browser. Remember my details on this device stores them only in this browser, and Forget my details clears them.
- Do the BMI classes differ by ethnicity?
- The WHO classes on this page are the international ones. NICE in the UK uses lower thresholds (overweight from 23, obesity from 27.5) for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background. The table above shows both.