What BMI actually measures
BMI — Body Mass Index — is your weight divided by the square of your height. That's the entire definition. It doesn't measure body fat, fitness, or health directly. What it does is scale your weight to your height, so a tall person and a short person carrying a proportionally similar amount of mass land on a similar number.
It was designed in the 1830s by a Belgian statistician, Adolphe Quetelet, as a population-level tool — a way to describe the "average" build of a group. It only became a personal screening number in the 20th century. That history matters: BMI is very good at describing populations and only roughly informative about any one individual.
How to calculate your BMI
Work in metric if you can — the formula is cleaner. Convert first if you think in imperial: 1 lb = 0.45 kg, and 1 inch = 2.54 cm. Height in feet and inches becomes total inches first, then metres.
Worked example. Take someone who is 5 ft 8 in tall and weighs 165 lb.
- Height: 5 ft 8 in = 68 in = 1.73 m
- Height squared: 1.73 × 1.73 = 2.99
- Weight: 165 lb = 74.8 kg
- BMI: 74.8 ÷ 2.99 = 25.0
A BMI of 25.0 sits right on the line between the healthy and overweight categories — a good illustration of why the number is a prompt for context, not a verdict.
The BMI categories
The World Health Organization and the CDC use the same adult cut-offs. They apply to men and women aged 20 and over, and are read the same way regardless of age.
| Category | BMI range | What it flags |
|---|---|---|
| Underweight | Below 18.5 | Possible undernutrition or an underlying condition |
| Healthy weight | 18.5 – 24.9 | Lowest average risk band for weight-related conditions |
| Overweight | 25.0 – 29.9 | Raised average risk; worth looking at trend and waist |
| Obesity | 30.0 and above | Higher average risk; often split into class I, II, III |
Turning your BMI into a healthy weight range
The more useful direction is backwards: fix your height, then solve the formula for the weight range that lands you in the healthy band. Multiply 18.5 and 24.9 each by your height in metres squared.
For the 5 ft 8 in person above, height squared is 2.99, so the healthy range is 18.5 × 2.99 to 24.9 × 2.99 — about 122 to 164 lb. That band, not the single BMI figure, is what's worth writing down. The BMI calculator shows your personal range automatically and marks where you fall on the scale.
What BMI gets wrong
BMI only knows your total weight. It can't tell muscle from fat, or where fat is stored — and both of those matter more for health than the raw number.
- Muscle reads as excess weight. Athletes and regular lifters are frequently pushed into the overweight or obese band despite low body fat.
- It ignores fat distribution. Waist circumference — fat carried around the middle — predicts metabolic risk better than BMI alone. A waist over 40 in (men) or 35 in (women) is the common threshold.
- It shifts with age. Older adults lose muscle and gain fat at the same weight, so an unchanged BMI can hide a real change in body composition.
- Population differences. Some groups — including many people of South Asian descent — face weight-related risk at lower BMI values, and some health bodies use a lower overweight threshold as a result.
- It says nothing about fitness. Blood pressure, blood sugar, activity level, and cardiorespiratory fitness all sit outside the number.
If your BMI is 25 or higher, or you carry weight around your waist, a body fat percentage estimate adds the context BMI misses. Our guide to calculating body fat percentage walks through the tape method and how the two numbers compare.
BMI for children and teens
For anyone under 20, the adult categories don't apply. Children's body composition changes constantly with growth, and it differs by age and sex. BMI is still calculated the same way, but the result is plotted on age-and-sex percentile charts — so a child's BMI is reported as a percentile ("85th percentile for age"), not a fixed category.
Frequently asked questions
Is BMI accurate?
It's accurate as a description of the population and a rough screen for an individual. For most people who aren't very muscular, a high BMI does track with higher body fat. It becomes unreliable for athletes, older adults, and anyone whose build sits far from average — which is why it's a starting point, not a diagnosis.
What is a healthy BMI?
For adults, a BMI between 18.5 and 24.9 is classified as a healthy weight. The band spans a wide weight range at any given height, so being anywhere inside it is fine — there's no advantage to being near the bottom of it.
Does BMI differ for men and women?
The formula and the categories are identical. Women carry more essential body fat than men at the same BMI, but the cut-offs aren't adjusted for that — another reason to treat the number as a screen rather than a precise measure of fatness.
My BMI says overweight but I'm fit — what should I do?
Check a second number. Estimate your body fat percentage and measure your waist. If both are in a healthy range and your bloodwork is normal, a BMI just over 25 driven by muscle isn't a concern. If your waist is high, the BMI is likely picking up something real.