BMI: a quick check on body size
Body mass index (BMI) relates your weight to your height to give a quick read on body size in adults. Doctors, the CDC and the World Health Organization use it to screen for underweight, overweight and obesity. This calculator gives your BMI, your category and the weight range that matches a healthy BMI for your height, in pounds or kilograms.
This tool provides general information and does not replace advice from a doctor, who can interpret your BMI in the context of your overall health.
The formula
BMI = weight (kg) ÷ height (m)², or in US units, BMI = 703 × weight (lb) ÷ height (in)².
Height is squared, so a small error in height changes the result noticeably. The healthy weight range comes from reversing the formula: 18.5 × height² to 24.9 × height².
Worked example: 170 lb at 5 ft 9 in
- Height in inches: 5 × 12 + 9 = 69 in, squared = 4,761.
- BMI: 703 × 170 ÷ 4,761 ≈ 25.1, just inside the overweight range.
- Lowest healthy weight: about 125 lb (BMI 18.5).
- Highest healthy weight: about 169 lb (BMI 24.9), so this person is about 1.4 lb above the range.
BMI categories for adults
| 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 and above | obesity class 3 (severe) |
BMI by weight at 5 ft 9 in
| Weight | BMI | Distance from the healthy range |
|---|---|---|
| 120 lb | 17.7 | 5.3 lb below |
| 140 lb | 20.7 | in range |
| 155 lb | 22.9 | in range |
| 170 lb | 25.1 | 1.4 lb above |
| 190 lb | 28.1 | 21.4 lb above |
| 210 lb | 31.0 | 41.4 lb above |
What BMI does not tell you
- Muscle versus fat: muscular athletes can have a high BMI with little body fat.
- Where fat is stored: abdominal fat carries more risk. Measure your waist too; risk rises above 40 in for men and 35 in for women.
- Differences between groups: older adults often lose muscle, so a normal BMI can hide low muscle mass, and some ethnic groups, including many people of South Asian descent, face higher risk at lower BMIs.
- Children and teens: their BMI is compared with age- and sex-specific growth charts, not adult categories. BMI is also not used during pregnancy.
Using your result
A BMI outside the healthy range is a reason to talk with your doctor, not a diagnosis. Together with waist size, blood pressure, blood sugar, cholesterol, activity and family history, it helps estimate overall risk and set a realistic goal if needed. To dig deeper, estimate your body fat percentage and daily calorie needs with our other health calculators.