How Is BMI Calculated?

Body mass index, or BMI, is one number from two inputs: your weight and your height. It is a quick screen, not a full picture of health. Here is how the math works, and what the result does and does not say.

The short version: square your height, then divide your weight by that figure. The metric and imperial versions differ only by one constant. Below is each formula with a worked example.

The BMI formula

BMI uses weight divided by height squared. The metric form is the original. The imperial form adds a constant of 703 to convert pounds and inches into the same scale.

Metric: BMI = weight (kg) / height (m)²

Imperial: BMI = 703 × weight (lb) / height (in)²

Both give the same result for the same person. Pick the units you already have. The only common slip is squaring the wrong number, so keep height and weight in the right spots.

A worked example, step by step

Take a person who weighs 80 kg and stands 1.80 m tall. Work the metric formula first.

1. Square the height: 1.80 × 1.80 = 3.24.
2. Divide the weight: 80 / 3.24 = 24.69.

Their BMI is about 24.7. Now the same person in imperial units: 176 lb and 71 in tall.

1. Square the height: 71 × 71 = 5041.
2. Multiply weight by 703: 703 × 176 = 123,728.
3. Divide: 123,728 / 5041 = 24.5.

Both land near 24.7, with a tiny gap from rounding the conversions. That is the whole calculation. If you measured height in centimeters, divide by 100 first: 180 cm becomes 1.80 m. The most common slip is leaving height in centimeters, which makes the result far too small.

What the ranges mean

The World Health Organization sets standard adult BMI bands, and the CDC uses the same cutoffs. They are reference points, not a diagnosis.

• Below 18.5: underweight range.
• 18.5 to 24.9: normal range.
• 25.0 to 29.9: overweight range.
• 30.0 and above: obese range.

Our example BMI of 24.7 sits at the top of the normal band. The bands are the same for adult men and women. They do not apply directly to children, who use age and sex percentiles instead.

What BMI does not tell you

BMI counts only weight and height. It cannot tell muscle from fat. A muscular athlete and a sedentary person can share the same BMI while their bodies differ a lot. That is the main limit to keep in mind.

It also ignores where weight sits, your age, and your bone density. So BMI works best as a first screen across a population, not a verdict on one person. A health professional can read it alongside other measures like waist size, blood work, and activity level.

Why BMI is still used

Given those limits, why keep it? Because it is cheap, fast, and consistent. Anyone can measure height and weight, and the formula gives the same answer everywhere. That makes BMI useful for comparing large groups and tracking trends over years.

For one person, the more telling signal is often the change over time rather than the single number. A steady reading across months can be more reassuring than any one band. Pair it with how you feel, how you move, and what a check-up shows, and the number does its job: a starting point, nothing more.

Common mistakes to avoid

• Mixing units. Do not divide pounds by meters. Stay fully metric or fully imperial.
• Using centimeters in the metric formula. Convert to meters first, so 175 cm becomes 1.75 m.
• Forgetting the 703 constant in the imperial formula. Without it the answer is far too small.
• Squaring the weight instead of the height. Only the height gets squared.

Frequently asked questions

Is BMI the same for men and women? Yes. Adults use the same formula and the same ranges regardless of sex.

Why do the metric and imperial answers differ slightly? Rounding. Converting kilograms to pounds and meters to inches loses a decimal or two, so the results land very close but not identical.

Does BMI work for children? Not directly. Children use BMI-for-age percentiles that account for growth, so the adult bands do not apply.

Is a higher or lower BMI better? Neither on its own. BMI is a screening number, and the healthy range is broad. It is one data point to discuss with a professional, not a goal by itself.

Is BMI accurate for athletes? Often not. Muscle weighs more than fat for its size, so a fit, muscular person can read in the overweight band while carrying little fat. For them, a body fat measure tells a clearer story than BMI alone.

How often should I check it? There is no need to track it daily. Weight shifts with water, food, and time of day. A reading every few weeks or months shows the trend without the noise.