How BMI is calculated
Body mass index compares your weight with your height. It is the same calculation for everyone over 2 years old.
BMI = weight (kg) ÷ height (m)²
BMI = 703 × weight (lb) ÷ height (in)²
Adult BMI categories
These categories from the CDC apply to adults aged 20 and over, whatever their sex.
| BMI | Category |
|---|---|
| Under 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 more | Obesity, class 3 |
Worked example
You are 5 ft 9 in tall and weigh 160 lb.
- Height in inches: 5 × 12 + 9 = 69 in.
- BMI: 703 × 160 ÷ 69² = 112,480 ÷ 4,761 = 23.6.
- That falls in the healthy weight range. At 69 in, a BMI of 18.5 to 24.9 is about 125 to 169 lb.
What BMI doesn’t tell you
- Muscle versus fat. Athletes can have a high BMI with little body fat. A body fat estimate gives a different view.
- Where fat is stored. Fat around the waist carries more health risk than fat elsewhere.
- Age and background. Body composition changes with age and differs between people.
Frequently asked questions
What is a healthy BMI?
For adults aged 20 and over, the CDC classes a BMI of 18.5 to under 25 as a healthy weight. Under 18.5 is underweight, 25 to under 30 is overweight, and 30 or more is obesity.
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared. In pounds and inches, multiply your weight by 703 and divide by your height in inches squared. A 160 lb person who is 5 ft 9 in (69 in) has a BMI of 703 × 160 ÷ 69² = 23.6.
Is BMI accurate?
BMI is a useful, quick screening tool for groups of people, but it doesn’t measure body fat directly. It can overestimate fat in muscular people and underestimate it in older adults who have lost muscle. A doctor looks at BMI alongside other measures such as waist size, blood pressure and blood tests.
Is BMI different for men and women?
The adult categories are the same for men and women. For children and teens, BMI is compared with others of the same age and sex using growth charts, and the result is a percentile.
What should I do if my BMI is outside the healthy range?
A BMI result on its own isn’t a diagnosis. If you are concerned about your weight or health, talk to a doctor, who can look at your full picture and suggest what, if anything, to change.
Sources
- Centers for Disease Control and Prevention: Adult BMI categories
- Centers for Disease Control and Prevention: Child and teen BMI calculator
BMI is a screening measure, not a diagnosis. This calculator is for general information and is not medical advice; talk to a doctor about your health. Read the full disclaimer and how we check our formulas.