Free BMI Calculator
Work out your body mass index, see your adult weight category and the healthy weight range for your height.
Enter your weight and height and the calculator divides your weight in kilograms by your height in metres squared. It shows the result with the adult category and the weight range that falls inside a BMI of 18.5 to 24.9 for your height.
How to use the BMI Calculator
- Choose metric or US units and enter your own measurements.
- Read the result and the supporting numbers shown beneath it.
- Use the explanation below to see what the number means, and talk to a doctor about anything that worries you.
About the BMI Calculator
Work out your body mass index, see your adult weight category and the healthy weight range for your height.
Health numbers are easier to understand when you can see how they are worked out. These calculators use published formulas and explain what the result does and does not tell you. They give general estimates and are not medical advice.
What is BMI?
Body mass index (BMI) is a single number that compares your weight with your height. It grew out of the work of the Belgian mathematician Adolphe Quetelet in the 1800s, and the name BMI was adopted in the 1970s. Doctors and public health teams use it because it is cheap, fast and works for large groups of people.
It is a screening tool. It can flag that a closer look at weight and health might be useful, but it cannot tell you how much of your weight is fat, where it sits on your body, or how healthy you are.
The BMI formula
In metric units the formula is BMI = weight in kilograms ÷ (height in metres)². In US units it is BMI = 703 × weight in pounds ÷ (height in inches)². Both give the same answer, so you can use either set of units in the calculator above.
- Worked example: a person who weighs 70 kg and is 175 cm (1.75 m) tall has a BMI of 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9.
- The same person in US units is 154 lb and 68.9 in: 703 × 154 ÷ 68.9² = 22.8, the small difference coming from rounding the inputs.
BMI categories for adults
These are the categories used by the World Health Organization and the US Centers for Disease Control and Prevention for adults aged 20 and over.
| BMI | 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 |
What BMI does not tell you
BMI ignores what your weight is made of. Muscle is denser than fat, so a strength athlete can have a high BMI and low body fat. The reverse also happens: an older adult who has lost muscle can have a normal BMI and still carry a lot of body fat.
- It does not show where fat is stored. Fat around the waist is linked with higher health risk than fat on the hips and thighs.
- It is less reliable in older adults, in people who are pregnant, and in people who are very tall or very short.
- Risk levels differ between ethnic groups, and some health bodies use lower cut-offs for people of Asian descent.
- For children and teenagers, BMI must be compared with age and sex growth charts, not the adult table.
What to do with your result
If your BMI is inside the healthy range, there is nothing to fix. If it is above or below, treat it as a prompt to look at the bigger picture: your waist measurement, your blood pressure and blood tests, your eating, sleep and activity, and any family history. A doctor can put those together in a way no calculator can.
If you want a different angle, try the body fat calculator or the ideal weight calculator, and the calorie calculator if you are planning a change.
Getting started with the BMI Calculator
Using the bmi calculator takes a minute or two. Your entries are saved in this browser, so you can come back later and carry on.
Tips for better results
Use BMI as a starting point, not a verdict. Pair it with your waist measurement and how you feel, and talk to a doctor before making health decisions from a single number.
Use measurements you have taken recently, not remembered ones.
These tools give general estimates and are not medical advice. Check anything important with a doctor or registered dietitian.
Privacy and where your data goes
Everything you type into the bmi calculator stays on your own device. Zapxa does not receive your numbers or documents. If you clear your browser data, your saved entries will be removed, so download a PDF of anything you want to keep.
Frequently asked questions
What is a healthy BMI?
For adults, health bodies such as the WHO and CDC define a healthy weight as a BMI from 18.5 to 24.9. 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?
Not always. BMI uses only height and weight, so a muscular person can score as overweight without carrying extra fat. A waist measurement or body fat estimate gives more context.
Can I use BMI for children and teenagers?
Not with the adult categories. For ages 2 to 19, BMI is compared with growth charts for the same age and sex (percentiles), which a doctor or school nurse can do.
Does BMI work the same for every ethnic group?
Risk at a given BMI differs between groups. Some health bodies suggest lower cut-offs for people of Asian descent, so ask a doctor how to read your result.
Related Health & Age tools
- BMR Calculator — Estimate the calories your body burns at rest each day with the Mifflin-St Jeor and Harris-Benedict formulas.
- Calorie Calculator — Estimate daily calories to maintain, lose or gain weight from your age, size and activity level.
- Ideal Weight Calculator — Compare the Devine, Robinson, Miller and Hamwi ideal body weight formulas with your healthy BMI range.
- Body Fat Calculator — Estimate body fat percentage from neck, waist and hip measurements using the U.S. Navy method.
- Pregnancy Due Date Calculator — Estimate a due date from the first day of your last period, with trimester dates and weeks pregnant.