BMI Calculator

What BMI is and how it is built

BMI stands for body mass index. It is a single number that compares your weight with your height. The U.S. Centers for Disease Control and Prevention (CDC) defines it as body weight in kilograms divided by the square of height in meters, and the World Health Organization (WHO) gives the same formula in its fact sheet.

Squaring the height is the key step. A person who is 1.65 m tall has a height-squared value of 1.65 × 1.65 = 2.7225. Dividing weight by that number gives BMI. WHO's 2000 report uses the example of an adult who weighs 70 kg and is 1.75 m tall, which gives a BMI of 22.9.

The number is then compared with fixed cut-points. WHO's Technical Report 894 lists six classes for adults, and the CDC lists the same boundaries with different names. Both sets of names are shown in the table on this page.

The WHO table says the BMI values are age-independent and the same for both sexes. The CDC likewise applies its adult categories to people 20 and older "regardless of age, sex, or race".

This site's calculator is for adults only. WHO's fact sheet says that for children, age needs to be considered, and that the BMI categories used to define overweight and obesity vary by age and sex across adults, adolescents, children and infants.

Worked examples

These three cases use the same steps as the calculator on this page. Categories come from the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC).

Computed with the same steps as the calculator on this page.

For case 1 the calculator also shows the weights that give a BMI from 18.5 up to under 25 at that height: 18.5 × 2.7225 = 50.4 kg up to 25 × 2.7225 = 68.1 kg. In case 2 the same range is 125 lb up to under 169 lb. This is arithmetic on the cut-points of the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), not a personal target.

How WHO and the CDC compare

For adults the two organizations use the same boundaries: 18.5, 25, 30, 35 and 40. The main difference is vocabulary. The World Health Organization (WHO) 2000 table calls 18.50 to 24.99 the "normal range" and 25.00 to 29.99 "pre-obese". The U.S. Centers for Disease Control and Prevention (CDC) calls the same ranges "healthy weight" and "overweight".

The decimals also look different. WHO prints 24.99 and 29.99, where the CDC writes "less than 25" and "less than 30". Both describe the same boundaries, so a BMI of exactly 25.0 is in the overweight group under both.

WHO's current fact sheet states only two thresholds for adults: overweight at a BMI of 25 or more and obesity at 30 or more. The full six-class table on this site comes from the 2000 report, so we cite it with its year. We could not find a current WHO web page that prints the whole table.

WHO's report also explains why it splits obesity at 35. It says the classification adds a subdivision at BMI 35.0 to 39.9 because management options for obesity differ above a BMI of 35.

What BMI cannot tell you

BMI is a screening measure. The U.S. Centers for Disease Control and Prevention (CDC) says it does not distinguish between fat, muscle and bone mass, and that it does not show what types of fat a person has or where in the body fat is carried. The CDC suggests considering BMI together with medical history, health behaviors, physical exam findings and laboratory results.

The World Health Organization (WHO) makes similar points. Its 2000 report says a given BMI may not correspond to the same degree of fatness across populations. It adds that the percentage of body fat increases with age up to 60 to 65 years in both sexes, and is higher in women than in men of equivalent BMI.

A separate WHO expert consultation looked at Asian populations. Its abstract says the proportion of Asian people with a high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs lower than 25. It also says the BMI at which observed risk begins varies from 22 to 25 across Asian populations. The group did not redefine cut-offs for each population and kept the WHO cut-offs as the international classification. It named 23.0, 27.5, 32.5 and 37.5 only as "potential public health action points". We read the abstract, not the full paper.

None of the sources we read gives a corrected BMI table for older adults or for people with a lot of muscle. The sensible reading is that the result is a starting point for a conversation, not a verdict.

Edge cases and what to discuss with a professional

Unit conversion uses the exact definitions 1 inch = 2.54 cm and 1 pound = 0.45359237 kg, so a US entry gives the same BMI as the metric entry for the same body. When you switch units on the form, the calculator converts your numbers and shows one decimal place.

The category is chosen from the unrounded BMI, but the headline shows one decimal. For example, at 170 cm, 72.15 kg gives 72.15 ÷ 2.89 = 24.965. The headline reads 25.0, yet the category is still normal range (World Health Organization, WHO) and healthy weight (Centers for Disease Control and Prevention, CDC), because the value is below 25. At 72.3 kg the BMI is 25.017, the headline also reads 25.0, and the category is overweight.

The calculator accepts heights of 100 to 230 cm (about 39.4 to 90.5 inches) and weights of 20 to 300 kg (about 44.1 to 661 pounds). Values outside these limits show a message asking you to check your entries.

Questions to bring to a health care professional: what BMI means for you given your muscle, age and background; whether other measures such as blood pressure or blood tests matter more in your case; and how any change should be made safely. The CDC says that if you have questions about your BMI, you should talk with your health care provider. This page is general information, not medical advice.