BMI Calculator
Use this BMI calculator in metric or imperial units to see your body mass index, weight category and healthy weight range for adults.
Formula & worked example
Formula: BMI = weight (kg) ÷ height (m)². Healthy weight range = 18.5 to 24.9 × height².
Example: 70 kg at 1.70 m: BMI 24.2 (healthy weight). Healthy range for that height is about 53.5–72.0 kg.
Method & source: Adult BMI categories used by the NHS and World Health Organization. See how we test our calculators.
Next step: try exact age or price per unit.
What Is a BMI Calculator?
A BMI calculator works out your body mass index – your weight in kilograms divided by your height in metres squared – and places you in a standard adult weight category. It is a quick screening number used by doctors, nurses and health services.
This BMI calculator works in metric or imperial units, including stones and pounds, and also shows the healthy weight range for your height. It is intended for adults aged 18 and over.
How Our BMI Calculator Works
Choose your units, enter your height and weight and confirm you are an adult.
- Formula: BMI = weight (kg) ÷ height (m) × height (m).
- Categories: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 or more obese.
- Obesity classes: class 1 (30–34.9), class 2 (35–39.9) and class 3 (40+).
- Healthy range: the weights that would give a BMI of 18.5 to 24.9 at your height.
- Visual scale: a colour bar shows where your result sits.
Height and weight are checked for sensible ranges so a typing slip does not give a misleading result.
Why BMI Is Useful – and Its Limits
BMI is popular because it is simple, but it is only a starting point.
- It gives a quick, consistent measure to compare with standard categories.
- Health professionals use it alongside other checks such as waist size and blood pressure.
- It helps you set a realistic healthy weight range for your height.
- It can flag when a conversation with a doctor or nurse may be worthwhile.
- It tracks change over time using the same method.
BMI does not measure body fat directly and does not separate muscle from fat, so it can mislead for athletes, older adults and some ethnic groups. It is not a diagnosis.
Worked walkthrough: a BMI example
Someone who is 1.70 m tall and weighs 70 kg has a BMI of 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 ≈ 24.2, which falls in the healthy weight range of 18.5 to 24.9. For that height, weights from about 53.5 kg to 72.0 kg give a BMI in the healthy range. In imperial units, 5 ft 7 in and 11 stone convert to about 1.70 m and 69.9 kg and give a BMI of around 24.1. A weight of 90 kg at 1.75 m would give a BMI of about 29.4, in the overweight range.
Practical tips for bmi calculations
- Measure your height without shoes and your weight at a consistent time of day for comparable results.
- Treat BMI as one clue, not a verdict. Waist size, fitness, diet and medical history all matter for health.
- Muscular people can have a high BMI without excess body fat; older adults can have a normal BMI with low muscle mass.
- If you are pregnant, under 18, or have a medical condition that affects weight, speak to a health professional instead of relying on this result.
- If your result is outside the healthy range, a doctor or nurse can advise on what, if anything, to do – the calculator cannot give medical advice.
Frequently Asked Questions (FAQs)
Divide your weight in kilograms by your height in metres squared. A person who is 1.70 m and 70 kg has a BMI of 70 ÷ 2.89 ≈ 24.2.
For adults, 18.5 to 24.9 is usually described as a healthy weight. Below 18.5 is underweight, 25 to 29.9 overweight and 30 or above obese.
Yes. Switch to imperial units and enter feet and inches for height, and stone and pounds for weight. The tool converts them for you.
No. It does not distinguish muscle from fat and may not suit athletes, older people, pregnant women or people of some ethnic backgrounds, for whom lower thresholds are sometimes used.
No. Children and teenagers are assessed with age- and sex-specific growth charts, so this tool is only for adults aged 18 and over.
Not medical advice. BMI is a general screening measure for adults and does not diagnose any condition. It does not account for muscle, age, ethnicity or pregnancy. Speak to a doctor, nurse or other health professional about your own health. See our full disclaimer.