What is BMI? Body mass index, or BMI, is a single number that pairs your weight with your height to flag whether you fall below, within, or above the weight range considered healthy.
The math is the same for everyone: your weight divided by your height squared. The result drops into a classification band.
Before the charts, what is BMI actually good at? It works as a fast screen, and this guide shows exactly what the number can tell you and what slips past it.
What is BMI and how is it calculated
BMI is a calculation that combines weight and height to show whether your body mass sits inside a range statistically tied to lower health risk.
The formula is short. In metric units, BMI equals weight in kilograms divided by height in meters squared. In US units, multiply your weight in pounds by 703, then divide by your height in inches squared.
The number you get falls into a band, from underweight to obesity. Doctors use it, insurers use it, and anyone who wants a quick reference uses it. The cutoffs follow the standard set by the World Health Organization.
The BMI classification table
The BMI table splits the result into six bands, from severe underweight to class III obesity, using the cutoffs adopted for adults.
| BMI range | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Class I obesity |
| 35.0 to 39.9 | Class II obesity |
| 40.0 or higher | Class III obesity |
This table applies to adults between 20 and 60. Children, teenagers, pregnant women, older adults, and athletes follow different references.
ContaCal
Count calories and macros with just 1 photo
Snap your meal and the AI instantly calculates calories, protein, carbs and fat.
How to calculate your BMI step by step
To calculate BMI, square your height, then divide your weight by that result, adding the 703 factor if you work in pounds and inches.
- Write down your height in inches (example: 67 in, which is 5 ft 7 in).
- Multiply your height by itself: 67 × 67 = 4,489.
- Write down your weight in pounds (example: 154 lb).
- Multiply weight by 703 and divide by step 2: 154 × 703 ÷ 4,489 = 24.1.
- Match the result to the table. 24.1 lands in the healthy range (18.5 to 24.9).
That number is a static snapshot. To move it, you adjust weight, and that runs through the calorie calculator that sets your daily target. For the tool itself, run an adult BMI calculator.
What BMI does not show
BMI cannot tell muscle from fat, ignores where fat sits on your body, and leaves out age, sex, ethnicity, and how active you are.
- Muscle weighs. Someone who lifts heavy can read as overweight or obese on BMI while carrying little fat, because muscle is denser.
- Where fat sits matters more than how much. Belly fat carries more risk than fat on the hips, and BMI treats them the same.
- Age shifts the picture. Older adults lose lean mass naturally, so the healthy reference creeps up. Standard BMI reads a 70-year-old like a 25-year-old.
- Ethnicity and build. People of South Asian descent, for instance, face higher metabolic risk at BMIs below the usual cutoff.
Is BMI still worth using?
Yes, as a screen. BMI stays useful for a quick read across a population, but it does not settle an individual diagnosis on its own.
So what is BMI good for? Think of it as the thermometer of weight. It signals whether something deserves a closer look, it does not explain what is happening, the way your basal metabolic rate explains how much energy you burn at rest. For public health, the simplicity of BMI is hard to beat. In a one-on-one consult, it is the first step, not the last.
So if your BMI lands in the overweight or obese band, read it as a prompt to dig deeper, not a verdict. The serious route runs through steady habits, not panic.
What to track alongside BMI
Three measures round out BMI and give a more honest picture: waist circumference, body fat percentage, and body composition.
Watch zone: a waist above 40 inches (102 cm) in men and 35 inches (88 cm) in women signals visceral fat, the kind of risk BMI alone cannot see.
- Waist circumference: the measure around your middle flags visceral fat, the risky kind. The watch zone is above 40 inches for men and 35 inches for women.
- Body fat percentage: this shows how much of your weight is fat. A bioimpedance scan delivers that number with more precision than BMI.
- Body composition: this separates lean mass from fat. A sedentary person with a normal BMI can still be low on lean mass, which BMI hides.
On the practical side, if you want to cut belly fat, it pays to watch what lands on the plate and hold a steady calorie deficit.