What BMI tells you
Body Mass Index is weight divided by height squared. It was developed in the 1830s by the statistician Adolphe Quetelet to describe populations, and today doctors use it as a fast first screen for weight-related health risk. A higher BMI is linked, on average, with higher risk of type 2 diabetes, high blood pressure and heart disease — but it is not a diagnosis, and two people with the same BMI can be very different in health.
The formula
Metric: BMI = weight (kg) / height (m)² US: BMI = 703 × weight (lb) / height (in)²
CDC adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to < 25 | Healthy weight |
| 25 to < 30 | Overweight |
| 30 to < 35 | Obesity class 1 |
| 35 to < 40 | Obesity class 2 |
| 40 or higher | Obesity class 3 (severe) |
Worked example
Someone who is 5 ft 5 in (65 inches) and weighs 160 lb: BMI = 703 × 160 / 65² = 112,480 / 4,225 ≈ 26.6, which is in the overweight range.
The healthy range for 65 inches runs from BMI 18.5 to just under 25, which is 111 to 150 lb, so this person is about 10 lb above it. BMI Prime is 26.6 / 25 = 1.07.
In metric, 72.5 kg at 1.65 m gives 72.5 / 1.65² = 72.5 / 2.7225 ≈ 26.6 — the same answer.
Limitations
- Muscle vs. fat: athletes and very muscular people can have a high BMI with low body fat.
- Age and ethnicity: older adults tend to have more body fat at the same BMI; people of Asian descent may face higher risk at lower BMIs, and some guidelines use lower cut-offs.
- Pregnancy: BMI isn't meaningful during pregnancy.
- Under 20: this calculator uses adult categories only.
Sources: Centers for Disease Control and Prevention, Adult BMI Categories and About Body Mass Index.
Frequently asked questions
Is the BMI calculator different for women and men?
No. For adults 20 and older, the CDC uses the same BMI formula and the same categories for women and men. Women tend to carry more body fat than men at the same BMI, which is one reason BMI is a screening number rather than a measure of body fat. Children and teens are assessed with sex- and age-specific percentiles instead.
What is a healthy BMI?
For adults, the CDC defines a healthy weight as a BMI from 18.5 to less than 25. Below 18.5 is underweight, 25 to under 30 is overweight, and 30 or higher is obesity, which is further divided into three classes.
Why can BMI be misleading?
BMI only uses height and weight, so it cannot tell muscle from fat or show where fat is stored. Muscular athletes may be labeled overweight, while older adults who have lost muscle may have a "normal" BMI with excess body fat. Waist circumference and the body fat calculator add useful context.
Can I use this for a child?
No. For ages 2 to 19, BMI has to be compared with CDC growth charts for the child's exact age and sex, because healthy body composition changes as children grow. Use the CDC Child and Teen BMI Calculator or ask a pediatrician.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 is exactly at the limit; 0.90 is 10% below it and 1.20 is 20% above it. It makes the distance from the healthy range easy to read.
This calculator is for general information and screening only and is not medical advice. Talk to a doctor or registered dietitian before making changes based on the result, especially if you are pregnant, under 18, or managing a health condition.