BMI Explained: How to Calculate It and What Your Number Means
By ToolNimba Editorial Team June 20, 2026 5 min read
Body mass index, or BMI, is a single number that compares your weight to your height. It was designed as a quick population-level screen, an easy way to flag whether someone may be underweight, a healthy weight, overweight or obese. It is not a body fat measurement and it is not a diagnosis, but it is a useful starting point that doctors, insurers and researchers have used for decades because it needs only two numbers anyone can measure at home.
Quick answer
BMI equals your weight divided by the square of your height. In metric that is kilograms divided by metres squared. In imperial it is 703 times pounds divided by inches squared. For most adults, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above is obese.
This guide is general information
BMI is a screening tool, not medical advice. For decisions about your health, talk to a qualified healthcare professional.
The BMI formula
BMI is your weight divided by the square of your height. The metric version is the cleanest, and the imperial version adds a conversion factor of 703 so that pounds and inches produce the same scale as kilograms and metres. The unit for BMI is always kg per metre squared, even when you start from pounds and inches.
| System | Formula | Example |
|---|---|---|
| Metric | weight (kg) divided by height (m) squared | 70 kg, 1.75 m gives 22.9 |
| Imperial | 703 times weight (lb) divided by height (in) squared | 154 lb, 69 in gives 22.7 |
For the metric example, 1.75 metres squared is 3.0625, and 70 divided by 3.0625 is about 22.9. That sits comfortably in the healthy range. Notice that the metric and imperial examples are the same person and land within a tenth of each other, which shows the 703 factor is doing its job.
A worked example, step by step
Say you weigh 82 kilograms and stand 1.78 metres tall. Here is the full calculation so you can repeat it with your own numbers.
- Write down your height in metres. Here it is 1.78 m.
- Square the height by multiplying it by itself. 1.78 times 1.78 is 3.1684.
- Take your weight in kilograms. Here it is 82 kg.
- Divide the weight by the squared height. 82 divided by 3.1684 is about 25.9.
- Match the result to the category table. A BMI of 25.9 falls in the overweight band, just above the healthy cutoff of 25.
If you prefer imperial, that same person is roughly 181 pounds and 70 inches tall. Multiply 703 by 181 to get 127,243, then divide by 70 squared (4,900). The answer is about 26.0, which matches the metric result. If you would rather skip the arithmetic, the BMI calculator does both versions and labels your category instantly.
The BMI categories
For most adults, the World Health Organization uses these standard bands. They apply to adults aged 20 and over, not children, who use age and sex specific percentile charts instead of fixed cutoffs.
Adult BMI categories
| BMI | Category |
|---|---|
| Below 16.0 | Severe thinness |
| 16.0 to 18.4 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obese class 1 |
| 35.0 to 39.9 | Obese class 2 |
| 40.0 and above | Obese class 3 (severe) |
The broad bands of underweight, healthy, overweight and obese are the ones most people know, but the finer obesity classes matter to clinicians because health risk rises across them. A BMI of 41 carries a very different risk profile from a BMI of 31, even though both are labelled obese.
Why the cutoffs change for some populations
The standard bands were drawn mainly from European and North American data, and research has shown they do not fit every group equally. People of Asian and South Asian descent tend to carry more body fat at a given BMI and develop conditions such as type 2 diabetes at lower numbers, so several health bodies use lower thresholds for these populations.
Example adjusted thresholds
| Population | Overweight from | Obese from |
|---|---|---|
| WHO general | 25.0 | 30.0 |
| Asian and South Asian | 23.0 | 27.5 |
| Japan (national definition) | 23.0 | 25.0 |
The point is not that one set of numbers is correct and the rest are wrong. It is that BMI is a rough screen, and the same number can mean different things depending on who you are and where your body stores fat.
What BMI does not tell you
BMI uses only height and weight, so it cannot tell muscle from fat or show where fat sits. That is why it can mislabel some people, and why it should never be the only number you look at.
- Muscular athletes can read as overweight despite very low body fat.
- It does not capture waist size, which is a strong signal of health risk on its own.
- It can read differently across ages, between sexes, and across some ethnic groups.
- It does not distinguish visceral fat around the organs from fat under the skin.
- It is not validated as a personal diagnosis, only as a population screen.
Treat BMI as one data point. Waist circumference, the waist to height ratio, body fat percentage, blood markers and how you feel and move all add context that a single number cannot. For a deeper look at the maths behind these ratios, our guide on how to calculate percentages breaks down the kind of arithmetic these health metrics rely on.
Better measures to use alongside BMI
- Waist circumference: a waist over 102 cm (40 in) for men or 88 cm (35 in) for women signals higher risk, with lower cutoffs for Asian populations.
- Waist to height ratio: keeping your waist under half your height is a simple, quick rule of thumb.
- Body fat percentage: estimates the share of your weight that is fat, which BMI cannot see.
- Trend over time: a rising or falling BMI tells you more than a single reading on one day.
Calculate your BMI
The tool below does the metric or imperial maths for you and shows your category instantly. Nothing you enter leaves your device.
⚖️ Try the free tool BMI Calculator Free BMI calculator for adults. Enter height and weight in metric or imperial to find your Body Mass Index, weight category, healthy range and BMI Prime fast.To go further, the body fat and daily calorie tools give a fuller picture than weight alone. If you are planning changes to your diet, knowing your maintenance calories from the compound effect of small daily habits mindset can help you set realistic goals over weeks rather than days.
Frequently asked questions
How do I calculate BMI by hand?
In metric, divide your weight in kilograms by your height in metres squared. For example 70 kg divided by 1.75 m squared (3.0625) is about 22.9. In imperial, multiply 703 by your weight in pounds, then divide by your height in inches squared. Both methods give the same result for the same person.
What is a healthy BMI range?
For most adults a BMI from 18.5 to 24.9 is classed as a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese. People of Asian descent are sometimes assessed against lower cutoffs because health risk rises at a lower BMI for them.
Is BMI accurate?
BMI is a useful population screen but not a personal diagnosis. Because it uses only height and weight, it can misclassify very muscular people and does not show where fat is stored. It is most reliable across large groups and least reliable for individuals with unusual body composition. Use it alongside other measures.
Why is my BMI high even though I am fit?
Muscle is denser than fat, so people who train heavily can weigh more and read as overweight on BMI while carrying very little body fat. BMI cannot tell muscle from fat. If you are strong and active, a body fat measurement or waist reading will describe your health far better than BMI alone.
Does BMI work the same for men and women?
The formula and the standard cutoffs are identical for men and women, even though women naturally carry a higher proportion of body fat at the same BMI. Because of this, two people with the same BMI can have different amounts of fat, which is one reason BMI is treated as a screen rather than a precise body fat figure.
What BMI is considered obese?
A BMI of 30 or above is classed as obese under the standard WHO bands, split into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 and above). Some countries use a lower threshold for specific populations, for example a BMI of 25 or more in Japan and 27.5 for Asian groups.
Is BMI used for children?
No, not in the same way. Children and teenagers grow at different rates, so their BMI is compared against age and sex specific percentile charts rather than the fixed adult cutoffs. A child in the 95th percentile or above is generally classed as obese for their age. Always use a paediatric chart for anyone under 20.