Quick answer. BMI = weight in kg ÷ (height in m)². For adults 20+, under 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight and 30 or more is obesity. Example: 68 kg at 170 cm gives a BMI of 23.5, in the healthy range.
Health disclaimer. This calculator gives general information only and is not medical advice, diagnosis or treatment. BMI does not measure body fat directly and does not account for muscle, bone, pregnancy, ethnicity or where fat is stored. If you are worried about your weight or health, speak to a doctor or other qualified health professional.
How to use the BMI calculator
- Pick your units. Metric uses centimetres and kilograms; imperial uses feet, inches and pounds. Switching converts the numbers you already typed.
- Enter height and weight. The result updates as you type.
- Read the result. You see your BMI to one decimal place, its category, a marker on the coloured scale and the weight range that gives a healthy BMI at your height.
- Optional age and sex. These do not change the number. They add notes, for example when the adult scale does not apply.
The BMI formula
BMI = weight (kg) ÷ height (m)². In imperial units: BMI = 703 × weight (lb) ÷ height (in)². For someone 1.70 m tall weighing 68 kg: 68 ÷ (1.70 × 1.70) = 68 ÷ 2.89 = 23.5. The healthy weight range comes from the same formula run backwards: 18.5 × 2.89 = 53.5 kg up to 24.9 × 2.89 = 72.0 kg.
Adult BMI categories (WHO and CDC)
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity, class 1 |
| 35.0 to 39.9 | Obesity, class 2 |
| 40.0 and above | Obesity, class 3 (severe) |
These cut-offs are the same for women and men aged 20 and over. The calculator rounds BMI to one decimal, as CDC does, and picks the category from that rounded number so the two always agree. Some health bodies use lower action points for people of South Asian, Chinese and other Asian backgrounds, because health risks can rise at a lower BMI. Ask your doctor which thresholds apply to you.
Children and teens
For anyone aged 2 to 19, BMI is calculated the same way but judged differently. It is compared with other children of the same age and sex on growth charts and reported as a percentile (BMI-for-age). A BMI of 22 can be healthy for a 17-year-old but high for an 8-year-old. This tool is for adults 20 and older. For a child, use a paediatric BMI-for-age calculator or ask a doctor.
What BMI can and cannot tell you
- Good for: a quick, free screening number that is easy to track over time.
- Not good for: very muscular people, who can show as “overweight” with little body fat, and older adults who have lost muscle.
- Use it with other measures: waist size, blood pressure, blood tests and family history give a fuller picture.
Sources and last check
Categories last checked on 12 November 2026 against the official pages below.
- CDC: Adult BMI Categories: underweight below 18.5, healthy 18.5 to less than 25, overweight 25 to less than 30, obesity 30 or more (classes 1 to 3), for adults 20 and older. Checked 12 Nov 2026.
- WHO fact sheet: Obesity and overweight (dated 7 October 2026): adult overweight is a BMI of 25 or more and obesity 30 or more; children and adolescents are assessed by age.
- CDC: Child and Teen BMI Calculator: BMI-for-age percentiles for ages 2 to 19.
Frequently asked questions
Is this BMI calculator free?
Yes. It is free with no sign-up, and nothing you enter leaves your browser.
What is a healthy BMI?
For adults 20 and older, a BMI from 18.5 to 24.9 is classed as a healthy weight by WHO and CDC.
How is BMI calculated?
Divide your weight in kilograms by your height in metres squared. In pounds and inches, multiply weight by 703 and divide by height in inches squared.
Can I use this for my child or teenager?
No. For ages 2 to 19, BMI is judged with age- and sex-specific percentiles. This tool uses the adult categories and is meant for adults 20 and older.
Is BMI different for men and women?
The adult categories are the same for both. At the same BMI, women tend to have more body fat than men, which is one reason BMI is only a screening tool.
Why does my BMI say overweight when I am muscular?
BMI only uses height and weight, so it cannot tell muscle from fat. Athletes often score high without extra body fat.
Is this medical advice?
No. It is general information. Talk to a doctor or qualified health professional about your weight and health.