BMI calculator
Body mass index is a quick screening number that relates your weight to your height. Enter your details below to get your BMI and the category it falls into — then read on for what BMI actually measures, and the important things it leaves out.
A screening tool, not a diagnosis. See the important note below.
How to use this calculator
Pick your units, enter your height and weight, and press calculate. The result is a single number plus the standard category it falls into. Switch between metric and imperial freely — the math is the same, only the units change.
How BMI is calculated
BMI is weight divided by height squared. In metric units:
In imperial units, a conversion factor of 703 keeps the result on the same scale:
The standard adult categories, defined by the World Health Organization, are: under 18.5 = underweight, 18.5 to 24.9 = healthy weight, 25.0 to 29.9 = overweight, and 30.0 and above = obesity. These ranges apply to most adults aged 20 and over.
A worked example
For someone 170 cm tall weighing 68 kg: height in metres is 1.70, and 1.70² = 2.89. So BMI = 68 ÷ 2.89 ≈ 23.5, which sits in the healthy-weight range. In imperial, that's about 5 ft 7 in and 150 lb: 703 × 150 ÷ 67² ≈ 23.5 — the same answer, as it should be.
What BMI does and doesn't tell you
BMI is popular because it's simple and needs only two easy measurements. But that simplicity is also its limit: it can't tell muscle from fat, doesn't account for where fat is stored, and doesn't reflect age, sex, ethnicity, or fitness. A muscular athlete can register as "overweight" while carrying very little fat, and BMI can miss health risks in people with a "normal" number but high abdominal fat. It also isn't appropriate as-is for children, pregnant people, or some athletes. Think of BMI as one rough screening signal — a reason to look closer, never a verdict on its own.
Frequently asked questions
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so very muscular people can have a high BMI despite low body fat. For athletes and highly active people, measures like body-fat percentage or waist circumference usually give a more meaningful picture.
Does BMI work for children?
Not with the adult categories above. Children and teens are assessed using age- and sex-specific BMI percentiles rather than fixed cutoffs, because healthy ranges change as they grow. A pediatric provider should interpret a child's BMI.
What's a "healthy" BMI?
For most adults the healthy-weight range is 18.5 to 24.9. But "healthy" depends on far more than one number — including muscle, fat distribution, fitness, and family history. Use BMI as a starting point, not the whole story.