How to use the BMI calculator
- Choose Metric (centimetres and kilograms) or US units (feet, inches and pounds).
- Enter your height and your weight. The result updates as you type.
- Read your BMI, your weight category, and the weight range that gives a healthy BMI at your height. If you are outside that range, the calculator shows how much you would need to gain or lose to reach it.
How BMI is calculated
Body mass index divides your weight by the square of your height:
The factor 703 converts pounds and inches to the metric result, so both versions give the same number. The calculator also shows BMI Prime (your BMI divided by 25, the top of the healthy range, so 1.00 is the upper limit) and the ponderal index (weight ÷ height³), which changes less with height and is sometimes used for very tall or short people.
Worked example
These are the calculator's default inputs: a height of 175 cm and a weight of 70 kg.
- Height in metres: 1.75 m, squared: 1.75 × 1.75 = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9, inside the healthy range of 18.5 to 24.9
- Healthy weight at this height: 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg
In US units, 5 ft 9 in and 154 lb gives 703 × 154 ÷ 69² = 22.7. It differs slightly from the metric example because 5 ft 9 in is 175.3 cm and 154 lb is 69.9 kg.
BMI categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class I |
| 35 to 39.9 | Obesity class II |
| 40 or more | Obesity class III |
These are the World Health Organization's categories for adults aged 20 and over. Some health services use a lower threshold, such as 23 or 27.5, for people of South Asian, Chinese and some other backgrounds, because health risks start to rise at a lower BMI.
What BMI does not tell you
BMI is a quick screening number, not a diagnosis. It does not measure body fat directly, so it can overstate fatness in muscular people and understate it in older adults who have lost muscle. It also says nothing about where fat is stored; waist size is a useful second check. Children and teenagers need age- and sex-specific BMI charts instead of the adult categories, and BMI does not apply during pregnancy. For a fuller picture, use the body fat calculator alongside it.
Frequently asked questions
What is a healthy BMI?
For most adults a BMI from 18.5 to 24.9 is considered healthy. Below 18.5 is underweight and 25 or more is overweight. Your doctor may suggest a different target based on your age, background and health.
Is BMI accurate for athletes?
Not always. Muscle weighs more than fat for the same volume, so a muscular athlete can have a BMI in the overweight range while having little body fat. A body fat measurement or waist size gives a better picture.
Why does my BMI differ between metric and US units?
It shouldn't by more than rounding. The two formulas are equivalent; a small difference usually means the height or weight you entered is not an exact conversion, for example 5 ft 9 in is 175.3 cm, not 175 cm.
Can I use this calculator for children?
No. Children's BMI is compared with growth charts for their age and sex, because body fat changes as they grow. Ask a doctor or use a child BMI percentile chart.
Related calculators
- Body Fat Calculator — Estimate your body fat percentage with the U.S. Navy tape method and compare it with a BMI-based estimate.
- Ideal Weight Calculator — Ideal body weight from the Robinson, Miller, Devine and Hamwi formulas, next to the healthy BMI range.
- Calorie Calculator — Daily calories to maintain your weight, plus gradual targets for losing or gaining, using the Mifflin-St Jeor equation.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.