What BMI Measures, and Where It Falls Apart
Two adults who both stand 1.80 m and weigh 90 kg get exactly the same BMI of 27.78. One of them might row competitively. The formula cannot tell them apart, and that single blind spot is behind most of the argument about the index. What BMI does, it does cheaply and consistently, which is why screening still starts there. The BMI calculator shows the working and the category side by side.
Formula and worked example
BMI is your weight in kilograms divided by your height in metres squared. At 80 kg and 1.80 m, that is 80 ÷ 3.24 = 24.69, and in imperial units the same answer comes from 703 × pounds ÷ inches squared. Your two results may differ slightly. Rounded inputs are usually the reason.
Adult categories
From age 20 the CDC sorts adults into four bands. Below 18.5 is underweight, 18.5 to just under 25 is healthy weight, 25 to just under 30 is overweight, and 30 or above is obesity. Read the unrounded figure. A display showing 25.0 may be hiding 24.96, and that rounding moves you a whole category. The bands and the wording come from the CDC’s adult BMI calculator and interpretation page.
Fix a height and the bands turn into weights. At 1.80 m, the healthy-weight band runs from 59.94 kg to just under 81.00 kg. A 21 kg spread tells you how loose the calculation is, and how little the endpoints mean as targets. The ideal weight calculator prints one figure per formula where this prints a band. Both conventions hide the same imprecision.
What BMI leaves out
Go back to the two people at 1.80 m and 90 kg. BMI sees the two numbers you typed and nothing else, so your muscle and your fat arrive on the same line. It also cannot locate abdominal fat, judge your fitness, or read your symptoms, your history and your blood tests.
A lower score does not automatically mean a better one. Age, ethnicity and body composition all change how a clinician reads the figure, and some guidelines use lower action thresholds for particular ethnic groups. Do not subtract a number from your own score and call it adjusted; NICE guidance on BMI and waist-to-height ratio sets out the distinctions.
Using the result
Check your units before you check your category. Then compare readings taken under similar conditions, because one morning’s number is one morning, for the reasons the guide to scale fluctuations works through.
A waist-to-height measurement adds the thing BMI leaves out, which is where your weight sits, and a tape-based body-fat estimate carries its own error. Your doctor makes the diagnosis; these tools only describe your dimensions.
Take an unexplained result to a doctor, particularly if your weight is changing for reasons you cannot place. These bands describe adults from age 20 upward, though what a given BMI means at 30 and at 70 are different questions. Children are assessed on growth charts instead, and pregnancy changes how body weight should be read altogether.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.