What is BMI?
Body Mass Index (BMI) is your weight in kilograms divided by the square of your height in meters. It gives a single number that health professionals use to screen adults for weight categories that may be linked to health problems. It's quick and needs no equipment, but it's an estimate: it doesn't measure body fat directly.
The BMI formula
BMI = weight (kg) ÷ height (m)²
In imperial units the formula needs a conversion factor:
BMI = 703 × weight (lb) ÷ height (in)²
This calculator takes metric inputs. If you know your measurements in pounds and feet, convert them first: multiply pounds by 0.4536 to get kilograms, and inches by 2.54 to get centimeters (5 ft 9 in is 69 in, or 175.3 cm). The weight converter and length converter can do the conversion for you.
Worked examples
Metric: 70 kg and 175 cm tall.
- Convert height to meters: 175 cm = 1.75 m.
- Square it: 1.75 × 1.75 = 3.0625.
- Divide weight by that: 70 ÷ 3.0625 = 22.9, a healthy weight.
Imperial: 160 lb and 5 ft 9 in (69 in) tall.
- Square the height: 69 × 69 = 4,761.
- Multiply weight by 703: 160 × 703 = 112,480.
- Divide: 112,480 ÷ 4,761 = 23.6, also a healthy weight.
BMI categories for adults
These are the World Health Organization adult categories, also used by the U.S. CDC:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity, class 1 |
| 35.0 to 39.9 | Obesity, class 2 |
| 40.0 and above | Obesity, class 3 (severe) |
This calculator reports the four main categories (Underweight, Normal weight, Overweight and Obese) and rounds BMI to one decimal place.
For many Asian populations, a WHO expert consultation published in The Lancet in 2004 found that health risks rise at lower BMIs, and proposed additional action points at 23 and 27.5.
What BMI doesn't tell you
BMI is useful because it's simple, but that simplicity has limits:
- Muscle counts as weight. Athletes and people with a lot of muscle can have a BMI in the overweight range while carrying little body fat.
- It ignores where fat is stored. Fat around the abdomen carries more health risk than fat elsewhere. A waist measurement adds that information: the U.S. National Institutes of Health treats a waist over 40 inches (102 cm) for men or 35 inches (88 cm) for women as a higher risk.
- Age and sex aren't taken into account. Older adults tend to have more body fat than younger adults at the same BMI, and women tend to have more than men.
- It doesn't apply to children, or during pregnancy. Children are assessed with age- and sex-specific percentiles instead.
A BMI outside the healthy range is a reason to talk to a doctor, not a diagnosis on its own.
Related calculators
- Body fat calculator: estimates body fat percentage, which BMI can't measure.
- BMR calculator: how many calories your body burns at rest.
- Calorie calculator: daily calories to maintain, lose or gain weight.
- Ideal weight calculator: weight ranges for your height using common formulas.
This calculator is for general information and doesn't replace advice from a healthcare professional.
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