Body Mass Index — BMI — is the single most quoted number in personal health. Doctors mention it, fitness apps ask for it, and insurance forms want it. But what does it actually measure, and how do you work yours out? This guide covers the simple formula, the official categories, and what your result really tells you.
What Is BMI?
BMI is a screening measure that relates your weight to your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet and is still used worldwide because it needs only two numbers and gives a rough sense of whether your weight falls in a healthy range for your height.
Important: BMI is a screening tool, not a diagnosis. It can't tell muscle from fat, and it doesn't account for age, sex, or body composition. Think of it as a first checkpoint — useful, but not the whole story (more on its limits below).
The BMI Formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Worked example (metric): A person who weighs 70 kg and is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
Worked example (imperial): A person who weighs 154 lb and is 69 inches tall: 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7.
Don't want to do the arithmetic by hand? Our free BMI calculator handles both metric and imperial in seconds.
BMI Categories: What the Numbers Mean
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
These cut-offs come from the World Health Organization and are used by health agencies across the US, UK, and EU. A BMI of 22.9 — from our example above — sits comfortably in the healthy range.
What BMI Doesn't Tell You
BMI has well-known blind spots, and knowing them keeps you from misreading your result:
- Muscle weighs more than fat. Athletes and regular lifters can score "overweight" while being very lean.
- It ignores fat distribution. Belly fat carries more health risk than fat on hips and thighs, but BMI treats them the same.
- Age and sex matter. Older adults naturally carry more fat at the same BMI, and women typically carry more essential fat than men.
- It's less accurate at extremes of height. Very short or very tall people can get slightly skewed readings.
For a fuller picture, pair your BMI with a second measure — body fat percentage or simply your waist circumference.
What To Do With Your Result
- Healthy range (18.5–24.9): maintain with balanced eating and regular activity.
- Above 25: a modest calorie deficit and more daily movement move the number steadily — crash diets rarely stick.
- Below 18.5: focus on nutrient-dense foods and strength training rather than just "eating more."
If your BMI is well outside the healthy range, or you have conditions like diabetes or high blood pressure, talk to a clinician before making big changes — a number on a chart is a starting point, not a treatment plan.
The Bottom Line
BMI = weight ÷ height². It takes ten seconds, costs nothing, and gives you a useful first read on your weight status. Calculate yours with our free BMI calculator, note which category you land in, and remember: it's one data point in a much bigger health picture.
Frequently Asked Questions
How do you calculate BMI?
Divide your weight in kilograms by your height in metres squared (kg/m²). For example, a person weighing 70 kg at 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
What is a healthy BMI range?
The standard healthy range is 18.5 to 24.9. Below 18.5 is classified as underweight, 25 to 29.9 as overweight, and 30 or above as obese.
Is BMI accurate for everyone?
No. BMI does not distinguish muscle from fat, so very muscular people can score high while being lean. It also does not account for age, sex, or where fat is carried on the body.
What should I do with my BMI result?
Treat it as a screening starting point, not a diagnosis. Pair it with a waist measurement, and talk to a clinician for advice about your own health.
Try it yourself: use our free BMI Calculator to get your number in seconds — no sign-up needed.