Body Mass Index, or BMI, is one of the most widely used health measures in the world. It is simple to work out and helps doctors spot weight ranges that may affect health. At the same time, it has real limits, and it is easy to misread. This guide explains how BMI works, how to calculate it, what the numbers mean and what to do with your result.
This article is general information and not medical advice. Please speak to a doctor about your own health.
What Is BMI?
BMI is a ratio of weight to height. It was designed to compare weight across populations and to screen for weight categories that are linked to higher health risks. It does not measure body fat directly.
The BMI Formula
BMI = weight (kg) ÷ height (m) × height (m)
If you know your height in centimetres, divide it by 100 to get metres. For example, 170 cm is 1.70 m.
Worked Examples
- 70 kg, 170 cm: 1.70 × 1.70 = 2.89. BMI = 70 ÷ 2.89 = 24.2 (normal weight).
- 55 kg, 165 cm: 1.65 × 1.65 = 2.7225. BMI = 55 ÷ 2.7225 = 20.2 (normal weight).
- 85 kg, 175 cm: 1.75 × 1.75 = 3.0625. BMI = 85 ÷ 3.0625 = 27.8 (overweight).
You can skip the maths with our free BMI and body fat calculator.
BMI Categories for Adults
The World Health Organization uses these categories for adults aged 18 and over:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obese |
Is the Same BMI Healthy for Everyone?
No. BMI is a general guide, and several things can make it less accurate:
- Muscle mass: muscle weighs more than fat, so athletes and very muscular people can have a high BMI without excess body fat.
- Age: older adults may lose muscle, so a normal BMI can hide a high body fat percentage.
- Body shape and fat distribution: fat around the waist carries more risk than fat elsewhere, and BMI does not show where fat is stored.
- Ethnic background: health risks can appear at lower BMI values in some groups. For people of South Asian background, some health guidelines suggest taking extra care at lower BMI levels than the standard cut-offs, so a doctor may interpret your number differently.
- Children and teenagers: they use age-specific growth charts, not these adult categories. Pregnant women should not use BMI to judge their weight.
What to Look at Besides BMI
- Waist measurement: a larger waist can signal higher health risk even when BMI looks normal.
- Body fat percentage: gives a clearer picture of body composition. See our body fat calculator, which uses the US Navy method.
- Blood pressure, blood sugar and cholesterol: these tests show how your body is actually doing.
- Energy, sleep, fitness and eating habits: day-to-day signs matter as much as a single number.
What to Do With Your Result
- If your BMI is in the normal range, keep up balanced meals, regular movement and good sleep.
- If it is below 18.5, speak to a doctor, since being underweight can also affect health.
- If it is 25 or above, do not panic. Small, steady habits such as walking more, eating more vegetables and cutting sugary drinks are a good start. A doctor or dietitian can give advice that fits you.
- Recheck occasionally, for example every few months, rather than every day. Weight naturally moves a little.
Common BMI Mistakes
- Using pounds or inches in a metric formula without converting.
- Treating BMI as a final judgement of health.
- Comparing your number with someone of a different age, sex or build.
- Trying extreme diets to change a number quickly.
When to See a Doctor
Please speak to a healthcare professional if your BMI is very low or high, if your weight changes suddenly without a clear reason, or if you have other symptoms or health conditions. Do not start a strict diet or intense exercise programme without advice, especially if you have medical conditions.
Frequently Asked Questions
Is BMI accurate?
It is a reliable quick screen for large groups, but it can be misleading for individuals, especially athletes and older adults.
What is a good BMI for my age?
The adult categories apply to everyone over 18, but doctors may interpret the result differently depending on your age, build and health.
Can I be healthy with a high BMI?
Some people are, but a high BMI is linked with higher risks for several conditions, so it is worth discussing with a doctor.
Does BMI work for children?
Not in the same way. Children are assessed with growth charts that compare them with other children of the same age and sex.
This article is for educational purposes only and does not replace advice from a qualified healthcare professional.
