BMI Calculator
Calculate your Body Mass Index using metric or imperial units.
What Is a BMI Calculator?
A BMI calculator works out your Body Mass Index, a single number based on your height and weight. The calculation divides your weight in kilograms by the square of your height in metres. The result is placed into a general category: underweight, normal weight, overweight, or obese. These categories give a quick, standardised way to describe where a result falls on a commonly used scale, without needing any special equipment beyond a height and weight measurement.
BMI is best understood as a rough screening measure, not a complete picture of health. It does not distinguish muscle from fat, and it does not measure body composition, fitness, or where weight is carried. A muscular person and someone with more body fat can share the same BMI. For that reason, many people use BMI only as a starting point for a broader conversation about health, rather than as a final judgement on its own.
How to Use the BMI Calculator
- Choose your units using the toggle at the top — Metric (cm / kg) or Imperial (ft/in / lb).
- Enter your height and weight in the fields shown for that unit system.
- Click the Calculate BMI button.
- Read your BMI number and category in the result box, alongside the general category ranges.
Who Uses a BMI Calculator?
Adults who want a quick sense of how their height and weight relate to a standard scale often use this tool before a routine appointment, when starting a general fitness routine, or when tracking change over time. Students learning about health measurements and people comparing metric and imperial figures also find it handy. It suits anyone who wants a fast, private estimate rather than a detailed body composition assessment, which requires different methods and professional equipment.
Tips for Accurate Results
- Measure your height without shoes and enter it carefully — a small height error changes the result more than you might expect, because height is squared in the formula.
- Weigh yourself in similar conditions each time you check, such as at the same time of day, if you plan to compare results over time.
- Double-check that you selected the correct unit system before calculating, as mixing up centimetres and inches is a common mistake.
Frequently Asked Questions
Is this BMI calculator free to use?
Yes. It is completely free with no signup required. Your height and weight are calculated in your browser and are not sent to or stored on a server.
What do the BMI categories mean?
As a general guide, a BMI below 18.5 is classed as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obese. These are general educational ranges for adults.
Why might BMI be misleading for athletes?
Muscle weighs more than fat by volume, and BMI cannot tell the two apart. Someone with a lot of muscle may get a higher BMI even with low body fat, which is one of its well-known limitations.
Should I use this calculator for children?
This tool uses the adult categories shown in the result. BMI is interpreted differently for children and teenagers, usually with age- and sex-specific percentiles, so a different approach is normally used for younger people.