BMI Explained: What It Does and Doesn't Tell You

Body Mass Index, or BMI, is one of the most widely used health numbers in the world, and one of the most misunderstood. It's quick to calculate and useful as a rough screening tool, but it's also easy to over-read. This guide explains how BMI works, what the categories mean, and what a BMI number can't tell you.

This is general information, not medical advice. If you have concerns about your weight or health, talk to a doctor or another qualified health professional.

What BMI is

BMI compares your weight with your height. It was designed to describe weight patterns across large populations, not to diagnose an individual. It has one big advantage: it needs only two measurements that almost everyone knows.

How it's calculated

Metric: BMI = weight in kilograms ÷ (height in metres)²

Imperial: BMI = 703 × weight in pounds ÷ (height in inches)²

A worked example: a person who weighs 70 kg and is 175 cm tall. Height in metres is 1.75, and 1.75² is 3.0625. So BMI = 70 ÷ 3.0625 = 22.9.

The same calculation in imperial units gives a matching result: someone weighing 154 lb at 69 inches has a BMI of 703 × 154 ÷ 4,761 = 22.7.

The standard categories

The World Health Organization's categories for adults are:

BMI	Category
Below 18.5	Underweight
18.5 to 24.9	Normal weight
25.0 to 29.9	Overweight
30.0 and above	Obese

Using the same 175 cm height, the "normal" range corresponds to a weight of roughly 56.7 to 76.3 kg. You can work this out for any height by multiplying 18.5 and 24.9 by the height in metres squared. Our BMI Calculator shows your result, the category and the healthy weight range for your height, in metric or imperial units.

Why BMI is useful

• It's simple and free. No equipment beyond a scale and a ruler.
• It's good at the extremes. A BMI well above or below the normal range is associated with higher health risks across populations, which is why doctors use it as a first check.
• It tracks change over time. If your height is stable, BMI is a quick way to see whether your weight is moving.

What BMI can't tell you

This is the important part. BMI is a ratio of weight to height, and nothing else.

• It doesn't distinguish muscle from fat. A muscular athlete can have a BMI in the "overweight" range with very little body fat. Muscle is denser than fat.
• It ignores where fat is stored. Fat around the waist and organs carries more health risk than fat elsewhere, and BMI can't see that.
• It doesn't account for age and sex. Body composition naturally differs, and older adults often have more fat at the same BMI.
• It's different for different populations. Health risks at a given BMI vary between ethnic groups, and some health bodies use lower cut-offs for certain populations.
• It isn't designed for children. Children and teenagers are assessed using growth charts that compare them with others of the same age and sex.
• It can't tell you about fitness, diet, sleep, blood pressure or blood sugar, all of which matter as much for health.

A person with a "normal" BMI can still have poor health, and a person with a higher BMI can be metabolically healthy. BMI is a starting point for a conversation, not a verdict.

Other measures that add context

Doctors often combine BMI with other measurements:

• Waist circumference, which reflects fat stored around the abdomen
• Waist-to-hip ratio
• Body fat percentage
• Blood pressure, cholesterol and blood sugar
• Fitness and activity levels

If your BMI is outside the normal range, these give a fuller picture.

How to use your result sensibly

1. Treat it as a screening number, not a score to chase.
2. Look at the trend. One reading matters less than the direction over months.
3. Consider your build. If you're very muscular, BMI will overstate your fat.
4. Check with a professional if you're outside the normal range, or if your weight is changing without explanation.
5. Focus on habits, such as movement, sleep, balanced eating and not smoking, which improve health whatever the number says.

Related health calculators

BMI tells you where you are; other calculators help you plan. The BMR Calculator estimates how many calories your body uses at rest, and the Calorie Calculator turns that into a daily target based on activity. Our guide to BMR and daily calorie needs explains how the two fit together.

The short version

BMI is weight divided by height squared. It's a quick, imperfect screening tool: useful for spotting broad patterns, poor at describing an individual's body. Use it as one piece of information, look at trends, and speak to a health professional about anything that worries you.