Belgian mathematician Adolphe Quetelet created the weight-to-height formula in the 1830s as part of statistical research, not as a health test — it was called the Quetelet Index. American physiologist Ancel Keys renamed it Body Mass Index in 1972, and health organizations like the CDC and WHO later adopted it as a standard screening measure.

What is considered a healthy BMI?

The CDC classifies 18.5 to 24.9 as the healthy weight range for adults. Below 18.5 is considered underweight, 25 to 29.9 is overweight, and 30 or above is classified as obesity.

Does BMI account for muscle mass?

No — BMI only uses weight and height, so it can't tell the difference between muscle and fat. Very muscular people, including many athletes, often land in the "overweight" or "obese" range on BMI alone despite having low body fat.

Is the BMI formula the same for men and women?

Yes, the calculation itself doesn't change based on sex — it's the same formula and the same category ranges for all adults.

Is BMI calculated differently for children?

Yes. Adult BMI categories use fixed cutoffs, but for children and teens BMI is plotted on age- and sex-specific growth charts instead, since healthy body composition changes a lot during growth.

Why does my BMI calculator ask for pounds and inches instead of kg and cm?

Some calculators default to imperial units because that's the standard in the US. The result is identical either way as long as you enter consistent units — a metric-only calculator works exactly the same once your weight and height are converted.

How to Calculate BMI - The formula, the categories, and a shortcut

Step	Details
01	Get your weight and height in the right units Use kilograms and meters for the metric formula, or pounds and inches for the imperial one — mixing units is the most common source of a wrong result.
02	Square your height Multiply your height by itself. In meters this gives you a small decimal (like 1.7 × 1.7 = 2.89); in inches it gives a much larger number (like 67 × 67 = 4,489).
03	Divide your weight by that number Metric: weight in kg ÷ height in meters squared. Imperial: weight in pounds ÷ height in inches squared, then multiply by 703 to get a comparable result.
04	Compare the result to the BMI category chart The CDC splits adult BMI into four main ranges — under 18.5, 18.5–24.9, 25–29.9, and 30 or above — each with its own label.
05	Skip the math with a free calculator Enter your weight and height into SNAP's free BMI calculator and it does the formula and category lookup for you instantly.

Step

Details

Get your weight and height in the right units Use kilograms and meters for the metric formula, or pounds and inches for the imperial one — mixing units is the most common source of a wrong result.

Square your height Multiply your height by itself. In meters this gives you a small decimal (like 1.7 × 1.7 = 2.89); in inches it gives a much larger number (like 67 × 67 = 4,489).

Divide your weight by that number Metric: weight in kg ÷ height in meters squared. Imperial: weight in pounds ÷ height in inches squared, then multiply by 703 to get a comparable result.

Compare the result to the BMI category chart The CDC splits adult BMI into four main ranges — under 18.5, 18.5–24.9, 25–29.9, and 30 or above — each with its own label.

Skip the math with a free calculator Enter your weight and height into SNAP's free BMI calculator and it does the formula and category lookup for you instantly.

BMI is just a formula — weight and height in, one number out — so calculating it by hand only takes a few seconds once you know which version to use.

Just want the number?

Skip the formula entirely and use SNAP's free BMI calculator — enter your weight and height in either unit system and it returns your BMI and category instantly, no sign-up needed. Keep reading below if you want to understand the formula behind it.

What is BMI?

Body Mass Index (BMI) is a simple ratio of weight to height, used as a quick screening number for whether someone's weight falls in a typical range for their height. It doesn't measure body fat directly and doesn't look at muscle, bone density, or where fat is distributed — it's a starting point, not a diagnosis.

The formula itself is nearly 200 years old. Belgian mathematician Adolphe Quetelet devised weight ÷ height² in the 1830s — not to screen for obesity, but as part of his statistical work on the "average man." It was known as the Quetelet Index until 1972, when American physiologist Ancel Keys renamed it Body Mass Index in a study comparing it to other ways of estimating body fat. Health organizations like the CDC and WHO later adopted it as the standard screening categories used today.

The BMI formula

There are two versions of the formula depending on which units you're using:

System	Formula
Metric	weight (kg) ÷ [height (m)]²
Imperial	weight (lb) ÷ [height (in)]² × 703

Both formulas give the same BMI number for the same person — the ×703 in the imperial version exists purely to convert pounds and inches into the same scale as kilograms and meters.

Worked example

Someone who weighs 70 kg and is 1.75 m tall: square the height (1.75 × 1.75 = 3.0625), then divide the weight by that (70 ÷ 3.0625 ≈ 22.9). A BMI of 22.9 falls in the CDC's healthy weight range.

BMI categories

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obesity

Where BMI falls short

BMI can't tell muscle from fat, which is why very muscular people — bodybuilders, many athletes — often get classified as "overweight" or "obese" despite having low body fat. It also doesn't account for where fat is stored, bone density, or age-related changes in body composition, so it's best treated as a quick starting point rather than a complete picture of health.

Official documentation

Adult BMI Calculator — CDC