BMI Calculator: What It Is and How to Use It
Body mass index (BMI) is weight divided by height squared, used as a quick screen for whether weight is in a healthy range for height. It doesn't measure body fat directly, so treat it as a starting point for a conversation with a clinician rather than a diagnosis.
The Formula
The metric formula divides weight in kilograms by the square of height in meters (kg/m²). In imperial units, it is weight in pounds divided by height in inches squared, multiplied by 703.
A person who weighs 154 pounds and is 5 feet 6 inches (66 inches) tall has a BMI of (154 ÷ (66 × 66)) × 703 ≈ 24.9, at the upper end of the healthy-weight range.
The index was devised in the 1830s by the Belgian statistician Adolphe Quetelet and named "body mass index" in the 1970s. It works well as a population-level indicator but has real limits for individuals (see below).
BMI Categories
The CDC and the World Health Organization use the same adult categories (ages 20 and over):
- Underweight: below 18.5
- Healthy weight: 18.5 to less than 25
- Overweight: 25 to less than 30
- Obesity: 30 or above, which the CDC divides into class 1 (30 to under 35), class 2 (35 to under 40) and class 3, or severe obesity (40 and above)
Using the Calculator
The BMI Calculator takes weight in kilograms and height in centimeters. If you measure in pounds and inches, convert first (1 lb = 0.4536 kg, 1 in = 2.54 cm) or use the imperial formula above: 175 pounds at 5 feet 8 inches (68 inches) gives (175 ÷ 68²) × 703 ≈ 26.6, in the overweight range.
If you track BMI over time, weigh yourself at the same time of day under similar conditions so the trend isn't just day-to-day water fluctuation.
The Ideal Weight Calculator gives a reference weight in pounds or kilograms from height, sex and frame size using the Robinson, Miller, Hamwi or Devine formula, a rough anchor rather than a target.
Limitations
BMI doesn't distinguish muscle from fat. A muscular athlete with low body fat can register as overweight or obese, while someone with little muscle and high body fat can fall in the healthy range despite a poor body composition.
Age, sex and ethnicity also change how BMI relates to risk. In adults 65 and over, a 2014 meta-analysis of 32 studies found the lowest mortality at BMIs in the mid-20s, with risk rising mainly below about 21 and above about 33. People of Asian descent tend to develop type 2 diabetes at lower BMIs, which is why the American Diabetes Association has since 2015 used a BMI of 23, rather than 25, as the diabetes-screening threshold for Asian American adults.
BMI also ignores where fat is stored. Abdominal fat carries greater risk than fat on the hips and thighs, so two people with the same BMI can have different risk profiles; clinicians often measure waist circumference alongside BMI.
Using BMI to Track Change
BMI is most useful for following change over months rather than as a one-off verdict.
If you are aiming to lose weight, the CDC notes that people who lose about 1 to 2 pounds a week are more likely to keep it off. One pound changes BMI by about 0.16 at 5 feet 6 inches (less for taller people), so going from a BMI of 28 to under 25 at that height means losing about 19 pounds, roughly 10 to 19 weeks at that pace. At a BMI around 30, a 5% to 10% weight loss lowers BMI by about 1.5 to 3 points.
BMI is a screening estimate, not medical advice; a clinician can interpret it alongside your other measurements and history.
Related guide: how to calculate a calorie deficit.