BMI calculator (body mass index)
Body Mass Index (BMI) estimates how your weight compares to your height using a single number. It is a fast, free screening tool — not a diagnosis of body fat or health. Doctors, insurers, and drug labels use it as a starting point, which is why the same number can decide whether you qualify for treatments like GLP-1 weight-loss medications.
How it works
The formula is weight divided by height squared. In metric units: BMI = weight(kg) / height(m)^2. In US units it is the same relationship rescaled by 703, the constant that converts pounds-per-inch-squared into the metric result: BMI = 703 x weight(lb) / height(in)^2. The result maps to WHO/CDC adult categories: underweight below 18.5, healthy weight 18.5 to 24.9, overweight 25.0 to 29.9, and obesity 30.0 and above. Obesity is further split into class 1 (30.0 to 34.9), class 2 (35.0 to 39.9), and class 3 (40.0 and above). These bands apply to adults 20 and older; children and teens use age- and sex-specific percentiles instead.
Worked example
An adult who weighs 80 kg and is 1.75 m tall: 1.75^2 = 3.0625, and 80 / 3.0625 = 26.1. A BMI of 26.1 falls in the overweight range (25.0 to 29.9). The same person in US units — 176 lb and 69 inches (5 ft 9 in) — computes as 703 x 176 / 69^2 = 123,728 / 4,761 = 26.0, the same answer apart from rounding.
Important
This calculator gives an estimate for screening only and is not a diagnosis or a substitute for a clinician's evaluation. The categories apply to adults 20 and older; do not use them for children, teens, or during pregnancy. BMI measures weight relative to height, not body fat — it can overstate risk in muscular people (athletes, weightlifters) and understate it in older adults who have lost muscle, and the standard cutoffs may not fit every ethnic group equally (some guidelines lower the overweight threshold to a BMI of 23 for people of South Asian descent). Two people at the same BMI can carry very different amounts of fat and be at very different risk. A number in or out of a given range does not by itself mean you are healthy or ill, nor does it decide whether a medication is right or safe for you. On the drug-eligibility point: FDA labels for semaglutide 2.4 mg (Wegovy) and tirzepatide (Zepbound) indicate them for adults with a BMI of 30 or higher, or 27 or higher with at least one weight-related condition — but that threshold is a starting criterion, not an approval; only a prescriber can weigh your full history, contraindications, and other measures. Enter your own height and weight accurately, and discuss the result with a healthcare professional before acting on it.