What is BMI?
BMI is a height-and-weight calculation used as a broad adult screening guide. It can help flag a possible weight range, but it does not measure health, muscle or body fat directly.
BMI stands for body mass index. It compares an adult’s weight with their height using a simple formula: weight in kilograms divided by height in metres squared.
BMI is useful because it is quick and easy to calculate, but it is only a broad screening tool. It does not show how much muscle, fat or bone someone has, and it cannot diagnose whether someone is healthy or unhealthy on its own.
Why BMI matters
BMI is often used as a first check because it can quickly place an adult into a broad weight range. NHS adult BMI tools show categories such as underweight, healthy weight, overweight and obese, but the result still needs context.
For example, a muscular person may have a high BMI without having a high body fat percentage. Someone else may have a BMI in the healthy range but still have other health risks that BMI does not detect.
Check your BMI estimate
Use the BMI calculator with metric or UK imperial units, then read the result with the limitations in mind.
How BMI is calculated
The standard BMI formula uses kilograms and metres.
BMI = weight in kg ÷ (height in metres × height in metres)If you enter stones, pounds, feet or inches, the calculator first converts those values into kilograms and metres, then applies the same formula.
Worked example
Suppose an adult weighs 70kg and is 1.75m tall.
1.75 × 1.75 = 3.0625
70 ÷ 3.0625 = 22.9A BMI of 22.9 sits in the broad adult healthy-weight range, but it should still be interpreted alongside other factors such as waist measurement, activity, health history and professional advice where needed.
Adult BMI categories
These are broad adult screening categories commonly used in UK health guidance. They are not a full assessment of individual health.
| BMI range | Broad category | How to read it |
|---|---|---|
| Below 18.5 | Underweight | May indicate a low weight for height and may need professional context. |
| 18.5 to 24.9 | Healthy weight range | A broad range associated with lower weight-related risk for many adults. |
| 25 to 29.9 | Overweight range | May suggest increased risk, especially if waist measurement is also high. |
| 30 or more | Obesity range | May indicate higher health risk, but individual assessment still matters. |
When BMI can be misleading
- Higher muscle mass: athletes or strength-trained people can have a higher BMI because muscle adds weight.
- Pregnancy: adult BMI categories are not designed to assess pregnancy weight changes.
- Children and teenagers: under-18s need age-and-sex-specific BMI tools, not adult thresholds.
- Body fat distribution: BMI does not show where body fat is carried, so waist measurement can add useful context.
- Health conditions: some conditions, medicines or eating-disorder concerns make calculator results inappropriate without professional support.
BMI FAQs
Is BMI a diagnosis?
No. BMI is a screening calculation. It can help flag a broad weight range, but it does not diagnose health, fitness, nutrition status or disease risk by itself.
What does BMI stand for?
BMI stands for body mass index. It is calculated from weight and height.
Is BMI accurate if I lift weights?
It may be less useful if you have a lot of muscle, because BMI does not separate muscle from fat. A body-fat estimate or waist measurement may add context.
Can children use adult BMI categories?
No. Children and teenagers need age-and-sex-specific BMI interpretation, so adult thresholds should not be used for under-18s.
Sources and notes
- NHS adult BMI calculator guidance: NHS — calculate BMI for adults
- NICE guidance on identifying and assessing overweight, obesity and central adiposity: NICE NG246