BMI calculator

How to use it

1. Choose your units. Pick US units to type pounds, feet and inches, or metric for kilograms and centimeters. Switching converts the values you already entered.
2. Enter weight, height and age. Weigh yourself without shoes, ideally in the morning. Age is optional, but under 18 the adult categories do not apply, so the calculator shows the number without a label.
3. Read the result. You get your BMI, its category, the healthy range for your height, how far you are from it and BMI Prime. Copy the result if you want to keep a record.

How BMI is calculated

Body mass index is weight divided by the square of height. In metric units it is kg ÷ m². In US units the same result comes from 703 × lb ÷ in², where 703 is the conversion factor between the two systems, rounded from 703.07. The calculator converts pounds and inches with the exact international definitions (1 lb = 0.45359237 kg, 1 in = 2.54 cm), so its result can differ from the 703 shortcut in the second or third decimal.

The value is shown with one decimal. When rounding would push it across a boundary, the calculator shows two decimals instead: a BMI of 24.96 is still in the healthy range, while "25.0" would wrongly suggest overweight. Categories are decided on the exact value, with the upper limit excluded, as the CDC does (healthy weight is "18.5 to less than 25").

BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 sits right on the border with overweight; 0.90 means you are 10% below it; 1.20 means 20% above it.

WHO categories for adults

These cut-offs are the same for men and women and for all adults from 18 up. They come from population studies: as BMI rises above 25, the average risk of type 2 diabetes, high blood pressure and cardiovascular disease rises too. They describe risk across many people, not the health of one person.

For people of Asian descent, a WHO expert consultation published in The Lancet in 2004 found that health risk starts to rise at lower BMI values, and suggested 23 and 27.5 as additional points for public health action. It kept the international cut-offs for comparison, and so does this calculator. If that applies to you, read a BMI of 23 to 25 as a reason to check other measures, such as waist size.

Worked examples

What BMI does not tell you

BMI does not separate fat from muscle, bone and water. A muscular athlete can be classed as overweight without excess fat, while an older adult who has lost muscle can have a normal BMI and still too much body fat. It also says nothing about where fat is stored: fat around the waist carries more risk than fat on the hips.

• Waist circumference adds information about abdominal fat.
• Blood pressure, blood sugar and cholesterol say more about real risk than weight alone.
• During pregnancy BMI is not a useful measure of current weight.
• Under 18, BMI is read against growth charts by age and sex, not with the adult cut-offs.

To estimate how many calories you need to keep or change your weight, continue with the calorie calculator. To check an age in years for a growth chart, the age calculator does it in one step.

Sources

• World Health Organization, Obesity: preventing and managing the global epidemic, Technical Report Series 894 (2000): adult BMI classes.
• World Health Organization, fact sheet "Obesity and overweight": adult thresholds of 25 and 30; BMI-for-age for children and teenagers (WHO Growth Reference, 2007).
• Centers for Disease Control and Prevention (CDC), "About Adult BMI": formula with the 703 factor, the 150 lb at 5 ft 5 in example and the healthy range from 18.5 to less than 25.
• WHO Expert Consultation, "Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies", The Lancet 2004;363:157-163.
• Definitions of the international yard and pound (1959): 1 lb = 0.45359237 kg, 1 in = 2.54 cm.

Sources checked on October 14, 2026.

Frequently asked questions