For adults aged 18 and over. BMI is a screening measure, not a diagnosis.
How to use the BMI Calculator
- 1Choose Metric or Imperial with the Units toggle.
- 2Enter your height in centimeters, or in feet and inches.
- 3Enter your weight in kilograms or pounds.
- 4Read your BMI, its WHO category and the healthy weight range for your height.
- 5Check the colored scale bar to see where your BMI sits between underweight and obesity.
How BMI is calculated
Body Mass Index compares your weight with your height. In metric units the formula is BMI = weight in kilograms ÷ (height in meters)². In imperial units it is BMI = 703 × weight in pounds ÷ (height in inches)². Both give the same number, and the calculator converts between them for you.
For example, a person who is 180 cm tall and weighs 85 kg has a BMI of 85 ÷ 1.8² = 85 ÷ 3.24 ≈ 26.2. Someone who is 5 ft 9 in (69 inches) and weighs 180 lb has a BMI of 703 × 180 ÷ 69² ≈ 26.6. Both results fall in the overweight range.
BMI categories for adults
The World Health Organization uses the same cut-off points for all adults aged 18 and over, regardless of sex: underweight is below 18.5, a healthy weight is 18.5 to 24.9, overweight is 25 to 29.9, and obesity is 30 or above. Obesity is sometimes divided further into classes starting at 30, 35 and 40.
These categories are not meant for children and teenagers. Because young people are still growing, their BMI is compared with age- and sex-specific growth charts and reported as a percentile instead. For a fuller explanation of what each band means, read our guide on what BMI is and what your number means.
Your healthy weight range
The healthy weight range shown is simply the BMI formula run backwards: the weights that give a BMI between 18.5 and 24.9 at your height. For the 180 cm example above that is roughly 59.9 to 80.7 kg, and for 5 ft 9 in it is about 125.3 to 168.6 lb.
Treat this as a broad guide rather than a target. A muscular person may sit above the range while being very fit, and an older adult may carry less muscle than the number suggests. If you are working toward a weight within the range, our calorie calculator can estimate how much to eat each day.
Limitations of BMI
BMI cannot tell fat from muscle or bone, and it says nothing about where fat is stored. Athletes with a lot of muscle are often classed as overweight, while people with little muscle can have a normal BMI but a high proportion of body fat. Waist size and body composition add useful context, and our body fat calculator estimates body fat from a few tape measurements.
Risk at a given BMI also differs between populations; some health bodies use lower thresholds for people of Asian descent, for example. BMI is a quick screening tool, not a diagnosis, so discuss any concerns about your weight with a doctor.
Learn more
- What Is BMI and What Does Your Number Mean?What Body Mass Index measures, how it is calculated, what the categories mean and why BMI is only one part of the picture.
- Metric vs Imperial Units: A Practical Conversion GuideHow the metric and imperial systems differ, the exact conversion factors worth remembering and quick mental-math shortcuts.
Frequently asked questions
What is a healthy BMI for adults?
For adults aged 18 and over, a BMI from 18.5 to 24.9 is considered a healthy weight by the World Health Organization. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or higher is classed as obesity.
Can I use this BMI calculator for children?
No. This calculator applies adult categories. Children and teenagers should be assessed with BMI-for-age percentile charts, which account for normal changes in body shape as they grow.
Is BMI different for men and women?
The formula and the adult categories are the same for both sexes. Women typically carry more body fat than men at the same BMI, which is one reason BMI is best read alongside other measures.
Why does my BMI say overweight when I am muscular?
BMI only uses height and weight, so it cannot distinguish muscle from fat. Muscle is dense, so very muscular people often have a high BMI without excess fat. A body fat estimate gives a better picture in that case.
How do I convert my height and weight for the calculator?
You do not need to; just switch the Units toggle. If you need to convert measurements for other purposes, the weight converter handles kilograms, pounds and stone.