Adult BMI Calculator
Enter your height and weight to see your Body Mass Index, its World Health Organization category, and the weight range linked to a healthy BMI at your height.
Calculate your BMI
General information, not a diagnosis. If you are worried about your weight, speak to a doctor or registered dietitian.
How BMI is calculated
Body Mass Index compares your weight with your height. It is the same formula for men and women:
BMI = weight (kg) ÷ height (m)²
Imperial measurements are converted exactly first (1 lb = 0.45359237 kg and 1 inch = 2.54 cm). The result is shown to one decimal place, and the category is based on that displayed value, so the number and the category always agree. The healthy-weight range shown with your result is the range of weights that gives a BMI from 18.5 up to the top of the healthy band at your height.
Worked example
Someone 5 ft 7 in tall (170.18 cm) who weighs 145 lb (65.77 kg) has a BMI of 65.77 ÷ (1.7018 × 1.7018) = 22.7, which is in the healthy range. At that height, a healthy BMI corresponds to about 53.6–72.1 kg (118–159 lb).
Adult BMI categories
| Category | WHO | Lower thresholds (NICE) |
|---|---|---|
| Underweight | below 18.5 | below 18.5 |
| Healthy weight | 18.5–24.9 | 18.5–22.9 |
| Overweight | 25–29.9 | 23–27.4 |
| Obesity | 30 or more | 27.5 or more |
The World Health Organization further divides obesity into class 1 (30–34.9), class 2 (35–39.9) and class 3 (40 or more). In the UK, NICE advises the lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, because the risk of conditions such as type 2 diabetes rises at a lower BMI in these groups.
What BMI can and cannot tell you
- It is a quick screening number, useful for spotting whether weight might be affecting health.
- It does not measure body fat. Muscle, bone and body shape are not taken into account, so muscular people can score high and some older adults can score “healthy” while carrying more fat than it suggests.
- Waist size matters too. NICE suggests keeping your waist to less than half your height. A 170 cm adult would aim for a waist under 85 cm.
- It is not used for children, teenagers or pregnancy. Under 20, BMI is compared with growth charts for age and sex. During pregnancy, your midwife or doctor will guide you.
Whatever the number, steady habits make the biggest difference. Read our guides to sustainable weight loss, the risks of rapid weight loss and foods rich in fiber, or browse our diet plans.
Sources
- World Health Organization: obesity and overweight fact sheet.
- NICE NG246: overweight and obesity management (ethnicity-adjusted thresholds and waist-to-height ratio).
- CDC: adult BMI calculator and categories.
- CDC: BMI calculator for children and teens.
Frequently Asked Questions
BMI is your weight in kilograms divided by your height in metres squared. For someone 170 cm tall weighing 65 kg: 65 ÷ (1.70 × 1.70) = 22.5. In imperial units the calculator converts pounds and inches to kilograms and metres first.
The World Health Organization classes an adult BMI of 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight and 30 or more as obesity. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE in the UK advises lower thresholds: overweight from 23 and obesity from 27.5, because health risks rise at lower BMIs in these groups.
No. BMI compares weight with height. It cannot tell muscle from fat, so muscular people can have a high BMI without excess fat, and older adults can have a “healthy” BMI while carrying more fat than it suggests. Waist size and a health professional’s assessment add important context.
The calculation is the same, but adult categories do not apply under the age of 20. Children and teenagers are assessed with BMI-for-age growth charts that compare them with others of the same age and sex. Ask a doctor or use a dedicated child and teen BMI calculator.
No. BMI categories are not used to judge weight during pregnancy. Your midwife or doctor will usually use your pre-pregnancy BMI to guide advice.