What Is BMI? How to Calculate Body Mass Index and Read Your Result
By SEOAnalytics Lab · Updated · 5 min read
Body mass index, or BMI, is one of the most searched health numbers online, and also one of the most misunderstood. Doctors use it as a quick screening tool, health websites show it on every chart, and a free BMI calculator can give you the answer in seconds. But what does the number really mean, and how far should you trust it?
This guide explains what BMI is, how to calculate body mass index in kilograms or pounds, what the BMI categories mean and where BMI falls short. It is general information and not medical advice.
What is BMI?
BMI stands for body mass index. It is a single number that compares your weight with your height. The idea goes back to the Belgian mathematician Adolphe Quetelet in the 19th century, and the name body mass index became common in the 1970s.
Because it needs only two measurements that anyone can take at home, BMI is an easy way to get a rough idea of whether a weight is low, typical or high for a given height. It does not measure body fat directly, which is important to remember when you read your result.
BMI formula: how to calculate body mass index
The calculation is the same in every country. Only the units change.
- Metric formula. BMI equals weight in kilograms divided by height in meters squared. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 divided by 3.0625, which is about 22.9.
- Imperial formula. BMI equals 703 multiplied by weight in pounds, divided by height in inches squared. A person who weighs 154 lb and is 69 inches tall has a BMI of 703 x 154 divided by 4,761, which is about 22.7.
To do the metric calculation by hand, convert your height to meters, multiply it by itself, and divide your weight by that number. A BMI calculator does these steps for you and avoids rounding mistakes.
BMI categories for adults
The World Health Organization groups adult BMI values into these categories:
- Underweight. BMI below 18.5.
- Normal weight. BMI from 18.5 to 24.9.
- Overweight. BMI from 25 to 29.9.
- Obesity. BMI of 30 or more. It is often divided into class I (30 to 34.9), class II (35 to 39.9) and class III (40 or more).
Some health organizations suggest lower cut-off points for people of South Asian and East Asian background, because health risks can rise at a lower BMI. Ask your doctor which ranges are right for you.
Your healthy weight range
You can turn the normal BMI range into a weight range for your height. Multiply your height in meters by itself, then multiply the result by 18.5 for the lower end and by 24.9 for the upper end.
For a height of 1.75 m, the range is about 56.7 kg to 76.3 kg. For a height of 1.60 m, it is about 47.4 kg to 63.7 kg. Our BMI calculator shows this range for you automatically, in kilograms or pounds.
Example BMI results
These examples all use a height of 1.70 m, so you can see how the category changes with weight:
- 48 kg. BMI 16.6, which is in the underweight range.
- 55 kg. BMI 19.0, which is in the normal weight range.
- 75 kg. BMI 26.0, which is in the overweight range.
- 90 kg. BMI 31.1, which is in the obesity range (class I).
The same weight can mean something different at a different height, which is exactly why BMI uses both numbers.
Limits of BMI
BMI is a screening tool, not a diagnosis. It has clear limits, and it is worth knowing them before you draw conclusions from your number.
- Muscle and fat. BMI cannot tell muscle from fat, so a very muscular person can have a high BMI without carrying extra body fat.
- Fat distribution. It does not show where fat is stored. Fat around the waist is linked to higher health risks, which is why doctors may also measure waist size.
- Age and sex. Older adults often lose muscle, and men and women carry different amounts of body fat at the same BMI.
- Background. Healthy ranges can differ between ethnic groups.
- Children and pregnancy. BMI for children and teenagers is compared with growth charts for their age and sex, and it is not the right measure during pregnancy.
What to do with your BMI result
Treat your BMI as a starting point for a conversation, not as a verdict. A result in the normal range does not guarantee good health, and a result outside it does not automatically mean a problem.
If your BMI is outside the normal range, or you are worried about your weight, speak to a doctor or another healthcare professional. They can look at your waist size, blood tests, family history and lifestyle. In general, regular physical activity, a balanced diet, enough sleep and avoiding crash diets support long-term health at any BMI.
Check your BMI now
You can work out your BMI in a few seconds with our free BMI calculator. Choose metric or imperial units, enter your weight and height, and you will see your BMI, your category and the healthy weight range for your height. Your numbers are calculated in your browser and are never sent to a server.
Frequently asked questions
What is a healthy BMI for adults?
For most adults, a BMI from 18.5 to 24.9 is considered the normal weight range. Some organizations use lower cut-offs for people of South Asian and East Asian background.
Is BMI accurate for athletes?
Not always. BMI cannot tell muscle from fat, so athletes with a lot of muscle can have a high BMI without extra body fat.
Can I use a BMI calculator for children?
No. Children and teenagers are assessed with BMI-for-age growth charts, so ask a doctor or nurse.
Is my weight and height saved by the BMI calculator?
No. Our BMI calculator runs in your browser and your numbers are not uploaded or saved.