Health & Fitness
Free BMI Calculator
Work out your body mass index in kilograms or pounds, and see what the number does and does not tell you.
Free to useNo sign-up requiredNo watermarkRuns in your browser
Last reviewed: 30 September 2026 by Vishal Senthilkumar
This free BMI calculator works in either metric or imperial units. Enter your height and weight in kilograms and centimetres, or in pounds, feet and inches, and it returns your body mass index, the standard category that number falls into, and the weight range considered healthy for your height.
BMI is a screening number, not a diagnosis. It was designed to describe populations rather than individuals, and it cannot tell muscle from fat. The explanation below the calculator covers where it is useful and where it misleads, so you can read your result sensibly.
How this tool works
Pick metric or imperial
Metric takes centimetres and kilograms. Imperial takes feet and inches for height and pounds for weight. Switching converts nothing - just enter the numbers in whichever units you know.
Enter your height and weight
Use your current weight and measure your height without shoes. Decimals are accepted, so 172.5 cm is fine.
Read your BMI and category
You get the index to one decimal place plus the category it falls in: underweight, healthy, overweight or obese, using the standard adult thresholds.
Check the healthy weight range
The calculator also shows the weight range that would put you in the healthy band for your height, which is usually more useful than the index itself.
How it works
Body mass index divides your weight by the square of your height. In metric that is kilograms divided by metres squared. The imperial version multiplies pounds by 703 and divides by inches squared, which is the same calculation with the unit conversion folded into the constant.
Squaring the height is what makes BMI a rough tool. It assumes body mass scales with the square of height, when in reality it scales closer to the cube. That is why BMI tends to read slightly high for tall people and slightly low for short people.
The category thresholds - 18.5, 25 and 30 - come from the World Health Organization and apply to adults. They are the same for men and women, which is itself a simplification, since body composition differs on average between the sexes at the same index.
The healthy weight range is calculated by working the formula backwards: for your height, what weights produce a BMI between 18.5 and 25. That range is often the most actionable part of the result.
BMI categories for adults
These are the standard World Health Organization bands for adults aged 20 and over.
| Below 18.5 | Underweight |
|---|---|
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obese |
Where BMI is misleading
BMI knows your height and your weight and nothing else. It cannot tell where that weight is or what it is made of, which is why a single number should never be the whole picture.
- Muscle is denser than fat, so athletes and heavily trained people often read as overweight or obese while carrying very little fat.
- Two people with the same BMI can carry weight very differently. Fat around the abdomen carries more health risk than the same weight on hips and thighs, and BMI cannot see the difference.
- The adult thresholds do not apply to children and teenagers, who are assessed against age and sex percentile charts instead.
- Average body composition differs between ethnic groups, and some health bodies use lower thresholds for people of South Asian descent.
- It is not meaningful during pregnancy, and it is unreliable for older adults, who tend to lose muscle while retaining fat.
- Waist measurement and waist-to-height ratio are often better single indicators, and cost nothing more than a tape measure.
Formula
Metric BMI
BMI = weight (kg) ÷ height (m)²
Height in metres, so 175 cm becomes 1.75.
Imperial BMI
BMI = (weight (lb) × 703) ÷ height (in)²
The 703 converts pounds-per-square-inch into the metric scale.
Healthy weight range
weight range = 18.5 × height (m)² to 24.9 × height (m)²
Worked examples
70 kg at 175 cm
1.75 squared is 3.0625. 70 divided by that is a BMI of 22.9, which sits in the healthy band.
154 lb at 5 ft 9 in
5 ft 9 in is 69 inches. 154 × 703 ÷ 4,761 gives 22.7 - the same person as above, give or take rounding.
A trained rugby player
100 kg at 180 cm gives a BMI of 30.9, nominally obese. With low body fat and high muscle mass, that number describes the formula rather than the person.
Working out a target range
At 165 cm, a healthy BMI corresponds to roughly 50 kg to 68 kg. That range is more useful for planning than a single index value.
Frequently asked questions
Is this BMI calculator free to use?
Yes, and there is no sign-up. Nothing you enter is stored or sent anywhere - the calculation happens in your browser.
What is a healthy BMI?
For adults, 18.5 to 24.9 is the standard healthy range. Below 18.5 is classed as underweight, 25 to 29.9 as overweight and 30 or above as obese.
Does this BMI calculator work in kg and cm?
Yes. Metric mode takes kilograms and centimetres directly. Imperial mode takes pounds with feet and inches, and gives the same result for the same body.
Is BMI accurate for muscular people?
No. BMI cannot distinguish muscle from fat, so people with a lot of muscle often score as overweight or obese despite being lean. Body fat percentage or waist measurement is more informative in that case.
Can I use this for children?
Not meaningfully. Children and teenagers are assessed with age and sex percentile charts, because a growing body changes shape constantly. Ask a doctor or use a paediatric growth chart.
Is BMI the same for men and women?
The thresholds are identical, though average body composition differs between the sexes. That shared scale is one of the known simplifications of the measure.