Body mass index is a quick screen that relates your weight to your height. It doesn't measure body fat directly, but it's a useful, standardized starting point for a conversation about healthy weight ranges.
How BMI is calculated
In metric units, BMI is your weight in kilograms divided by the square of your height in meters. The imperial version multiplies pounds over inches-squared by 703 to reach the same number.
The calculator above accepts either unit system — switch the toggle and enter height and weight in the matching units.
Reading the categories
The World Health Organization groups adult BMI into underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (30 and above).
BMI has real limits: it can't tell muscle from fat, and it isn't tailored to age, sex, or athletic build. Use it as a rough indicator, not a diagnosis, and talk to a clinician about what's right for you.
A worked example
A person 175 cm tall weighing 70 kg has a BMI of about 22.9 — that's 70 ÷ (1.75 × 1.75) — which sits comfortably in the normal range. In imperial units the same person is roughly 5'9" and 154 lb, and 703 × 154 ÷ (69 × 69) gives the same ~22.9.
Gaining 15 kg would push that BMI to about 27.8, into the overweight band, while dropping to 55 kg would bring it to about 18.0, just under the normal range. Seeing how much weight it takes to move between categories is a useful reality check on the number.
When BMI misleads
Because BMI uses only height and weight, it can misclassify people at both extremes. A muscular athlete may register as “overweight” despite low body fat, while an older adult who has lost muscle can sit in the “normal” range while carrying excess fat. It also wasn't designed for children, pregnancy, or very short or tall statures.
Waist circumference and direct body-fat measurements capture what BMI can't, which is why clinicians treat it as a first screen rather than a verdict. If your number lands near a category boundary, that context matters more than the exact figure.