Estimates only, not medical advice. These are population formulas, and any one person can be well above or below them. If you are pregnant or breastfeeding, under 18, have diabetes, kidney disease, an eating disorder or another medical condition, or take medicine that affects weight, talk to your doctor or a registered dietitian before changing how you eat.
How BMI is calculated
Body mass index is your weight divided by the square of your height. It was designed to compare groups of people, and doctors use it as a quick first screen for weight-related health risk.
BMI = weight in kg ÷ (height in m)²
BMI = 703 × weight in lb ÷ (height in inches)²
The calculator converts US units with the exact definitions (1 lb = 0.45359237 kg, 1 in = 2.54 cm), so the result matches the metric formula to the decimal. The healthy weight range is the weight that would give a BMI of 18.5 to 24.9 at your height.
Worked example: 5 ft 9 in and 170 lb
5 ft 9 in is 69 inches. 703 × 170 = 119,510, and 69² = 4,761, so the BMI is 119,510 ÷ 4,761 = 25.1. That is just inside the overweight band. At 5 ft 9 in, a BMI of 18.5 to 24.9 works out to about 125 to 169 lb, so this person is about 1 lb over the healthy range. In metric: 77.1 kg ÷ 1.753² = 25.1. This example is loaded when the page opens.
CDC adult categories (20 and over)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity, class 1 |
| 35 to 39.9 | Obesity, class 2 |
| 40 or more | Obesity, class 3 (severe) |
Source: CDC, Adult BMI Categories. The CDC uses these for adults regardless of age, sex or race. The calculator rounds BMI to one decimal before picking the category, as the CDC's own calculator shows it.
What BMI cannot tell you
The CDC is clear that BMI does not measure body fat directly, cannot tell fat from muscle or bone, and says nothing about where fat sits. In its guidance for practitioners it gives these examples:
- Athletes and muscular people may have a high BMI because of muscle, not fat.
- Older adults tend, on average, to carry more body fat than younger adults at the same BMI.
- Women on average have more body fat than men at the same BMI.
- Ethnicity changes the link between BMI and risk. A 2004 WHO expert consultation found that many Asian populations carry higher risk of type 2 diabetes and heart disease at BMIs under 25, and proposed extra action points at 23 and 27.5.
So treat BMI as a starting point for a conversation, not a verdict. A waist measurement, blood pressure, blood sugar and cholesterol give a much fuller picture, and your doctor can put them together.
Children and teens
For ages 2 to 19, BMI is compared with growth charts for the same age and sex to give a percentile, and the adult categories above do not apply. If you enter an age under 20, the calculator still shows the BMI number but sends you to the CDC Child and Teen BMI Calculator for the category.
Want to know what to eat for your goal? The calorie calculator works out a daily target from the same height and weight.
Sources checked October 2026.
Questions people ask
What is a healthy BMI?
For adults 20 and over, the CDC classes a BMI of 18.5 to 24.9 as a healthy weight. Under 18.5 is underweight, 25 to 29.9 is overweight, and 30 or more is obesity, split into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 or more). The same cut-offs apply to men and women of any adult age.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703, then divide by your height in inches squared. For 170 lb at 5 ft 9 in (69 in): 170 × 703 = 119,510, and 69 × 69 = 4,761, so BMI = 119,510 ÷ 4,761 = 25.1. In metric units it is weight in kg divided by height in meters squared.
Is BMI accurate for athletes and muscular people?
Often not. BMI cannot tell muscle from fat. The CDC notes that muscular people and highly trained athletes can have a high BMI because of extra muscle, not extra fat. If that is you, a waist measurement or a body fat test tells you more.
Does BMI work the same for older adults and for women?
The cut-offs are the same, but the meaning shifts. The CDC says that, on average, older adults have more body fat than younger adults at the same BMI, and women have more body fat than men at the same BMI. So a "healthy" BMI in an older adult can still hide low muscle and high fat.
Is the BMI scale different for Asian people?
The US categories are the same for everyone. A 2004 World Health Organization expert consultation found that many Asian populations face higher risk of type 2 diabetes and heart disease at BMIs below 25, and suggested extra public health action points at 23 and 27.5. Some countries use lower cut-offs as a result. If you are of Asian descent, ask your doctor which cut-offs to use.
How is BMI worked out for children and teens?
For ages 2 to 19, the same BMI number is compared with other children of the same age and sex, giving a BMI-for-age percentile. The adult categories do not apply. Use the CDC Child and Teen BMI Calculator, which reports the percentile and the matching category.