Body Mass Index, usually shortened to BMI, is a simple number worked out from your height and weight. Health professionals use it as a quick screening tool to estimate whether an adult is underweight, at a healthy weight, overweight, or living with obesity. BMI does not measure body fat directly and it does not diagnose any medical condition, but it can be a useful first step in understanding whether your weight may be affecting your health.
Table of Contents
What is BMI?
BMI compares your weight with your height. In general, a higher BMI suggests a person is carrying more weight for their height, and a lower BMI suggests they are carrying less. Because the calculation is quick and uses only height and weight, it is widely used in clinics and in public health research. Your result is placed into a standard category, shown in the chart below, so you can see where you fall.
The BMI formula
Metric formula
BMI = weight (kg) ÷ height (m)²
In words: divide your weight in kilograms by your height in metres squared (your height in metres multiplied by itself).
Worked example: An adult weighs 70 kg and is 1.75 m tall.
Step 1: Square the height: 1.75 × 1.75 = 3.0625
Step 2: Divide the weight by that number: 70 ÷ 3.0625 = 22.857, which rounds to 22.9
A BMI of 22.9 falls in the healthy range.
Imperial formula
If you use pounds and inches, use this version:
BMI = 703 × weight (lb) ÷ height (in)²
Worked example: An adult weighs 154 lb and is 68 inches tall.
Step 1: Square the height: 68 × 68 = 4624
Step 2: Multiply 703 by the weight: 703 × 154 = 108262
Step 3: Divide: 108262 ÷ 4624 = 23.41, which is approximately 23.4
A BMI of 23.4 also falls in the healthy range.
Useful conversions
- Kilograms from pounds: kg = lb × 0.4536. For example, 154 × 0.4536 = 69.85 kg (about 69.9 kg).
- Metres from inches: metres = inches × 0.0254. For example, 68 × 0.0254 = 1.7272 m (about 1.73 m).
- Metres from centimetres: metres = cm ÷ 100. For example, 175 cm ÷ 100 = 1.75 m.
BMI chart: adult categories
The table below shows the standard World Health Organization (WHO) BMI categories for adults.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These ranges are a guide for most adults. They are not the right way to assess children, teenagers, or every individual adult, as explained in the limitations section below.
How to measure your height and weight accurately
- Weigh yourself at a consistent time. Morning, after using the toilet and before breakfast, is often best. Wear light clothing and no shoes.
- Use a flat, hard surface for the scales. Place the scales on a hard floor, not carpet, stand still in the centre, and check the reading. Repeat once to confirm it.
- Measure height without shoes. Stand with your back against a wall, heels together, and your head level, looking straight ahead.
- Keep your back and heels against the wall. Ask someone to place a flat object, such as a hardback book, flat on top of your head at a right angle to the wall, mark the wall lightly, then measure from the floor to the mark with a tape measure.
- Record both numbers carefully and use the same units throughout the formula — either kilograms and metres, or pounds and inches.
Limitations of BMI
BMI is useful for screening, but it has important limits:
- Muscle and athletes: BMI cannot tell muscle from fat. A very muscular person or athlete may have a high BMI without having excess body fat.
- Pregnancy: BMI categories should not be used in the usual way during pregnancy, because weight naturally changes. Pregnant readers should follow the advice of their midwife or doctor.
- Children and teenagers: Children are still growing, so adult cut-offs do not apply. For children and teenagers, BMI must be interpreted using BMI-for-age percentiles or growth charts for their age and sex.
- Older adults: Older people may lose muscle and height, so the same BMI can mean something different from in a younger adult. Health, strength, and medical history matter too.
- Body composition and where fat is carried: BMI does not show how fat is distributed around the body, and risk can vary between people with the same BMI.
When to talk to a doctor
Consider speaking to a doctor or another qualified healthcare professional if:
- your BMI is below 18.5 or 30.0 or above;
- you have gained or lost a lot of weight without trying;
- you are worried about your weight, eating, or exercise habits;
- you have a long-term condition such as diabetes, heart disease, or high blood pressure; or
- you are planning major changes to your diet or activity and want advice that is safe for you.
A professional can look at your BMI alongside other checks, such as waist measurement, blood pressure, blood tests, and your medical history.
Frequently asked questions
What is a healthy BMI for an adult?
For most adults, a BMI between 18.5 and 24.9 is classed as the healthy weight range by the WHO. It is a guide, not a guarantee of good or poor health on its own.
Is BMI the same for men and women?
The same formula and the same adult categories are used for men and women. However, body composition can differ between individuals, so a professional may consider other factors alongside BMI.
Can I use the adult BMI chart for my child?
No. Children and teenagers should not be judged against adult ranges. Their BMI should be checked using BMI-for-age percentiles or growth charts, ideally with a healthcare professional.
What should I do if my BMI is outside the healthy range?
Do not panic and do not start an extreme diet. Take accurate measurements, check the calculation again, and consider speaking to a doctor, who can assess your overall health and discuss safe, gradual next steps if they are needed.
Medical disclaimer
Disclaimer: This article is general information only and is not medical advice, diagnosis, or treatment. BMI is a screening measure and one result cannot tell you everything about your health. Always consult a qualified healthcare professional about your own health, weight, or medical concerns, and seek urgent help from your local emergency services in an emergency.