By the Tooliday Team. Last updated 14 November 2026.
Body mass index, or BMI, is one of the most quoted health numbers and also one of the most misunderstood. It takes two measurements, your weight and your height, and turns them into a single figure. That is useful as a quick screening tool. It is not a verdict on your health.
This guide shows how the calculation works, walks through an example, and explains where BMI helps and where it falls short. If you would rather skip the arithmetic, our BMI Calculator does it in a second.
The BMI formula
In metric units the formula is:
BMI = weight in kilograms ÷ (height in metres × height in metres)
In pounds and inches it is:
BMI = 703 × weight in pounds ÷ (height in inches × height in inches)
The number 703 simply converts the imperial units so both formulas give the same answer for the same person.
A worked example
Take someone who weighs 70 kg and is 170 cm tall.
- Convert the height to metres: 170 cm = 1.70 m.
- Square the height: 1.70 × 1.70 = 2.89.
- Divide the weight by that number: 70 ÷ 2.89 = 24.2.
So the BMI is about 24.2. Check it the imperial way: 70 kg is roughly 154 lb and 170 cm is roughly 67 in. Then 703 × 154 ÷ (67 × 67) comes to about 24.1, which matches once you allow for rounding.
A common slip is using centimetres instead of metres. If you divide 70 by 170 × 170 you get a tiny number that makes no sense. Always convert to metres first.
What the categories mean
The World Health Organization groups adult BMI like this:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal range |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
You can read the WHO’s own explanation on its overweight and obesity fact sheet.
Lower cut-offs are often used in India and Asia
Research has shown that, at the same BMI, many people of South Asian and other Asian backgrounds carry more body fat and have a higher risk of conditions such as type 2 diabetes. Because of this, health bodies in India and across Asia commonly use lower cut-offs, with overweight starting at 23 and obesity at 25. If you live in India, it is worth asking your doctor which scale they use.
What BMI cannot tell you
BMI only uses height and weight. It does not know what your weight is made of or where it sits on your body. That leads to some well-known blind spots:
- Muscle: A person with a lot of muscle can have a high BMI without carrying extra fat.
- Age: Older adults can have a normal BMI and still have low muscle mass.
- Fat distribution: Fat around the waist carries more health risk than fat on the hips or thighs, and BMI cannot see the difference.
- Pregnancy: BMI is not a useful measure during pregnancy.
- Children and teenagers: They are assessed on growth charts by age and sex, not with the adult table above. Our calculator is meant for adults.
For these reasons, doctors often look at BMI together with waist measurement, blood pressure, blood sugar, cholesterol and family history.
How to use your result sensibly
- Treat it as a starting point for a conversation, not a diagnosis.
- Measure your height without shoes and weigh yourself at a similar time of day, for example in the morning, so the numbers are comparable over time.
- If your result is outside the normal range, do not panic. A single number does not describe your health. A doctor or registered dietitian can look at the whole picture.
- Avoid crash diets or extreme exercise based on a calculator result.
Quick answers
Is BMI the same for men and women? For adults the same formula and the same categories are used, although body fat differs between men and women at the same BMI.
How often should I check it? Once every few months is plenty. Weight naturally moves up and down from day to day.
What weight range matches a normal BMI for my height? Rearrange the formula: multiply 18.5 and 24.9 by your height in metres squared. For 1.70 m that gives roughly 53.5 kg to 72 kg.
Try it yourself
Enter your own height and weight in our BMI Calculator. It works in metric and imperial units, and it runs in your browser, so the numbers you type are not sent to us.
This article is general information, not medical advice. If you have concerns about your weight or health, please speak to a qualified healthcare professional.