How to use the BMI Calculator
- Choose Metric to enter centimeters and kilograms, or Imperial for feet, inches and pounds.
- Enter your height and your current weight. The result updates as you type.
- Read your BMI, its WHO category and where it sits on the colored scale.
- Check the healthy weight range for your height and how far you are from it.
Features
- Metric and imperial units, with values carried over when you switch
- WHO adult categories, including the three obesity classes
- Colored gauge that shows where your BMI falls
- Healthy weight range for your exact height
- BMI Prime, your BMI relative to the upper normal limit
- Runs in your browser; nothing you enter is sent anywhere
The BMI formula
Body mass index compares your weight with your height. It is your weight in kilograms divided by the square of your height in meters:
BMI = weight (kg) ÷ height (m)²
For someone who weighs 70 kg and is 1.75 m tall, BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. With pounds and inches, the same idea needs a conversion factor: BMI = 703 × weight (lb) ÷ height (in)². Someone who is 5 ft 9 in (69 inches) and weighs 154 lb has a BMI of 703 × 154 ÷ 4,761 ≈ 22.7. This calculator converts imperial values to metric exactly, so both unit systems give the same answer for the same person.
WHO BMI categories for adults
| BMI | Category |
|---|---|
| Below 16.0 | Underweight: severe thinness |
| 16.0–16.9 | Underweight: moderate thinness |
| 17.0–18.4 | Underweight: mild thinness |
| 18.5–24.9 | Normal weight |
| 25.0–29.9 | Overweight (pre-obese) |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
The healthy weight range under the result is the span of weights that would give a BMI from 18.5 to 24.9 at your height. BMI Prime is your BMI divided by 25, the upper limit of the normal range: 0.92 means you are 8% below that limit, and 1.20 means 20% above it.
What BMI does not tell you
BMI is a quick screening number, not a diagnosis. It was designed to describe groups of people, and it has known blind spots for individuals:
- Muscle versus fat. Muscle is denser than fat, so athletes and people who lift weights can land in the overweight band while carrying little body fat. The Body Fat Calculator gives a second view.
- Where fat sits. Fat stored around the waist is more closely linked to heart disease and type 2 diabetes than fat on the hips and thighs, and BMI cannot see the difference. A waist measurement adds that information.
- Age and sex. Older adults tend to have less muscle at the same BMI, and women usually carry more body fat than men at the same BMI.
- Ethnicity. Health risks rise at lower BMI values in some populations. A WHO expert consultation suggested extra public-health action points of 23 and 27.5 for many Asian populations.
BMI for children and teens
This calculator is meant for adults aged 20 and older. Children's body composition changes as they grow and differs between boys and girls, so for ages 2 to 19 a BMI is compared with growth-chart data for the same age and sex and reported as a percentile. A child at the 85th percentile has a higher BMI than 85% of children of the same age and sex in the reference data. A pediatrician or an official growth-chart calculator is the right tool for kids.
Tips for an accurate reading
- Measure your height without shoes, standing straight with your heels against a wall.
- Weigh yourself at the same time of day, ideally in the morning, because body weight can swing by a kilogram or more over a day.
- Watch the trend over several weeks rather than reacting to one reading.
If you want to change your weight, the Calorie Calculator estimates how much energy you use each day. Results from this calculator are estimates for general information and are not medical advice.
Frequently asked questions
What is a healthy BMI for adults?
The World Health Organization classes a BMI from 18.5 to 24.9 as normal weight for adults. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obesity.
Is BMI accurate for athletes and muscular people?
Often not. BMI cannot tell muscle from fat, so very muscular people can score as overweight while having low body fat. A body fat estimate or waist measurement gives a fuller picture.
Can I use this BMI calculator for my child?
No. For children and teens aged 2 to 19, BMI has to be compared with age- and sex-specific growth charts and read as a percentile. Ask your pediatrician or use an official BMI-for-age tool.
Is the BMI formula different for men and women?
No. Adults of both sexes use the same formula and the same WHO categories, although women typically carry more body fat than men at the same BMI.
Why does the imperial formula use 703?
703 converts pounds per square inch into kilograms per square meter (it is about 0.4536 ÷ 0.0254²). This calculator does the exact conversion, so metric and imperial inputs give identical results.