How to use the BMI calculator
- Choose your units — US units (feet, inches, pounds) or metric (centimeters, kilograms).
- Enter your height without shoes. In US units, 5 feet 9 inches is entered as 5 and 9.
- Enter your current weight, ideally measured in the morning.
Open the table below the result to see the weight boundaries for every category at your height.
The BMI formula
Body mass index compares weight with height:
BMI = weight (kg) ÷ height (m)² or, in US units, BMI = 703 × weight (lb) ÷ height (in)²
The factor 703 converts pounds and inches into the same number you would get from kilograms and meters. The index was devised by the Belgian mathematician Adolphe Quetelet in the 1830s and was named "body mass index" by the physiologist Ancel Keys in 1972 as a practical stand-in for body fat in population studies.
Worked example: a person who is 5 ft 9 in (69 inches) tall and weighs 160 lb has a BMI of 703 × 160 ÷ 69² = 112,480 ÷ 4,761 ≈ 23.6, which is in the healthy range. At that height the healthy range runs from about 125 lb (BMI 18.5) to about 169 lb (BMI 24.9). In metric, 70 kg at 1.75 m gives 70 ÷ 1.75² = 70 ÷ 3.0625 ≈ 22.9.
Adult BMI categories
These are the categories the U.S. Centers for Disease Control and Prevention (CDC) uses for adults 20 and older:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to under 25 | Healthy weight |
| 25 to under 30 | Overweight |
| 30 to under 35 | Obesity, class 1 |
| 35 to under 40 | Obesity, class 2 |
| 40 or higher | Obesity, class 3 (sometimes called severe obesity) |
BMI Prime is simply your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 sits exactly at that limit, 1.20 is 20% above it, and anything below about 0.74 (a BMI of 18.5) is in the underweight range.
What BMI can't tell you
BMI is a screening number, not a measure of body fat or health. Keep these limits in mind:
- Muscle versus fat. BMI cannot tell them apart. Muscular athletes are often classed as overweight even with low body fat. A body fat estimate adds useful context.
- Age. Older adults tend to lose muscle, so the same BMI can mean more body fat at 70 than at 30.
- Sex. At the same BMI, women generally carry more body fat than men.
- Ethnicity. A 2004 World Health Organization expert consultation noted that many Asian populations face higher risks of type 2 diabetes and heart disease at BMIs below 25, and some guidelines use lower cutoffs for people of Asian descent.
- Where fat is stored. Fat around the abdomen carries more risk than fat elsewhere. U.S. guidelines flag a waist over 40 inches (102 cm) in men or 35 inches (88 cm) in non-pregnant women as a sign of higher risk.
Children, teens and pregnancy
The adult categories above do not apply to anyone under 20. For ages 2 to 19, BMI is compared with others of the same age and sex using the CDC BMI-for-age growth charts, and the result is a percentile — for example, a BMI at or above the 95th percentile falls in the obesity category. Use the CDC's child and teen BMI tool or ask your pediatrician.
BMI is also not meaningful during pregnancy, when weight gain is expected. Doctors use pre-pregnancy BMI to recommend how much weight to gain.
When to talk to a professional
If your BMI is in the underweight or obesity range, or your weight has changed without explanation, talk to your doctor. A clinician can look at the bigger picture — waist size, blood pressure, blood sugar, cholesterol, family history and activity — that a single number cannot capture.
Important: results are estimates for general information only and are not medical advice. They cannot replace an assessment by a doctor, registered dietitian or other qualified health professional.
Frequently asked questions
What is a healthy BMI for adults?
For adults 20 and older, a BMI from 18.5 to just under 25 is considered the healthy weight range. Your doctor may set a different target based on your age, health conditions and body composition.
Why does the US formula multiply by 703?
One pound is 0.4536 kg and one inch is 0.0254 m, so 0.4536 ÷ 0.0254² ≈ 703.07. Multiplying by 703 turns pounds per square inch into the same kilograms-per-square-meter value the metric formula gives.
Is BMI accurate for athletes?
Often not. BMI counts all weight the same, so people with a lot of muscle can land in the overweight or even obesity range while having little body fat. A body fat measurement or waist size gives a better picture for them.
Can I use this calculator for my child?
No. Children and teens need an age- and sex-specific percentile from the CDC BMI-for-age charts. The adult categories on this page would give a misleading answer.
Is BMI the same for men and women?
The adult formula and category cutoffs are the same for both sexes. The interpretation differs a little, because women typically have a higher percentage of body fat than men at the same BMI.
Last updated: October 8, 2026