How to Calculate and Understand Your BMI

BMI shows up on almost every health form, but the number alone doesn't mean much without context. Here's how it's calculated, what the standard ranges represent, and where it falls short.

What BMI actually measures

Body Mass Index is a simple ratio of weight to height, used as a quick screening tool to categorize whether someone's weight falls within a typical range for their height. It doesn't measure body fat directly, muscle mass, or overall health — it's a starting point clinicians use, not a full picture on its own.

The BMI formula

Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = weight (lb) ÷ height (in)² × 703

For example, someone weighing 70 kg at 1.75 m would calculate: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 ≈ 22.9.

Standard BMI ranges

These are the commonly used World Health Organization categories for adults:

Below 18.5 — underweight
18.5–24.9 — normal weight
25.0–29.9 — overweight
30.0 and above — obesity

These ranges are population-level guidelines, not individual medical assessments.

Why BMI is a screening tool, not a diagnosis

BMI doesn't distinguish between muscle and fat — a very muscular person can show a high BMI despite low body fat, and someone with low muscle mass can show a "normal" BMI despite high body fat. It also doesn't account for age, sex, or where fat is distributed on the body, all of which affect health risk. For these reasons, BMI is best used as a quick screening number, with any real health concerns discussed with a healthcare provider who can consider your full picture.

How to calculate your BMI

Enter your height and weight into the BMI Calculator to get your BMI and category instantly, in either metric or imperial units.

FAQ

Is BMI different for children? Yes — children and teens are assessed using age- and sex-specific percentile charts, not the fixed adult ranges above, because growth patterns change significantly with age.

Can I have a "normal" BMI and still have a health risk? Yes. Because BMI doesn't measure body composition, it's possible to have a typical BMI alongside other risk factors, or the reverse. It's one data point, not the full story.

Calculate your BMI now

Try the free BMI Calculator for an instant result. See more calculators for related tools.