BMI Explained: How to Calculate and Interpret It
Body Mass Index (BMI) is a quick screening number that relates your weight to your height. Health bodies use it because it is cheap and fast to apply across whole populations — but it was never meant to be a precise verdict on any one person. Here is how it works, how to read the categories, where it misleads, and what to pair it with.
What BMI is — and is not
BMI is a single number that estimates whether your weight sits in a healthy range for your height. Clinicians and health agencies use it for population screening, not diagnosis: it flags who might benefit from a closer look, and nothing more.
On its own it says nothing about fitness, body composition or health. Treat it as one data point among several, not a scorecard.
The BMI formula
BMI = weight (kg) / height (m) squared. For example, 74 kg at 1.78 m gives 74 / (1.78 x 1.78) = 23.4.
Using pounds and inches? Multiply the result by 703: BMI = (lb / in squared) x 703. The BMI Calculator accepts either unit system and shows your category instantly.
The categories
- Below 18.5 — underweight
- 18.5 to 24.9 — normal (healthy) range
- 25 to 29.9 — overweight
- 30 and above — obese
A worked example
Say you weigh 82 kg and stand 1.75 m. Square the height: 1.75 x 1.75 = 3.0625. Divide: 82 / 3.0625 = 26.8. That lands in the 25-29.9 band — "overweight" by the screening definition — but read the next section before drawing any conclusion.
Where BMI misleads
BMI cannot tell muscle from fat. A muscular athlete can read as "overweight" while carrying very little fat, and an older person who has lost muscle can read as "normal" while carrying too much. It also ignores where fat sits — fat around the organs matters more than the single number suggests — and does not adjust for age, sex or ethnicity, though some health systems apply adjusted cut-offs.
What to use alongside it
For a fuller view, pair BMI with a healthy-weight range for your height and a sense of your daily energy needs. The Ideal Weight Calculator gives a target range, and the BMR & Calorie Calculator estimates how much energy your body burns each day — more actionable than BMI alone. A simple waist measurement is another useful check.
Frequently asked questions
Is BMI accurate for athletes? Often not — high muscle mass inflates BMI without the health risk it usually implies.
What counts as a "healthy" BMI? The 18.5-24.9 band is the usual adult screening range, but it is a guide, not a target to chase.
Does BMI work for children? Not the adult version — children are assessed on age- and sex-specific percentile charts instead.
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Last updated: August 27, 2026