How to Calculate BMI: Free BMI Calculator Online
You step on the scale, but the number alone tells you little — what matters is your weight relative to your height. That is exactly what Body Mass Index measures. In this guide you will learn the BMI formula, how to read your result against the official WHO categories, and where BMI helps and where it falls short. You can crunch the numbers by hand or skip straight to the free BMI Calculator, which runs entirely in your browser and gives an instant, private result in metric or imperial units.
What BMI is and the formula behind it
Body Mass Index (BMI) is a single number that estimates whether your weight is in a healthy range for your height. It does not measure body fat directly, but it correlates well enough with it to be a useful screening tool.
The metric formula is simple:
BMI = weight (kg) / height (m) squared
If you use imperial units, the formula is:
BMI = 703 x weight (lb) / height (in) squared
The 703 factor just converts pounds and inches into the metric result, so both formulas give the same number for the same person.
How to use the BMI Calculator
You do not need to memorise the formula. Here is how to get your result in seconds:
- Open the BMI Calculator in your browser.
- Choose metric (kg and cm) or imperial (lb and ft/in).
- Enter your height.
- Enter your weight.
- Read your BMI and the WHO category it falls into, shown instantly.
There is nothing to install and no sign-up. Change a value and the result updates immediately, so you can see how a few kilograms would shift your number.
A worked example
Say you are 1.75 m tall and weigh 70 kg.
BMI = 70 / (1.75 x 1.75) = 70 / 3.0625 = 22.9
A BMI of 22.9 sits comfortably in the healthy weight range. Now the imperial version: a person who is 5 ft 9 in (69 in) and weighs 154 lb gets:
BMI = 703 x 154 / (69 x 69) = 108,262 / 4,761 = 22.7
The small difference is just rounding — both describe the same healthy range.
BMI categories (WHO)
The World Health Organization groups adult BMI into these standard categories:
| Category | BMI range |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obesity (class 1) | 30.0 to 34.9 |
| Obesity (class 2) | 35.0 to 39.9 |
| Obesity (class 3) | 40.0 and above |
These ranges apply to most adults aged 20 and over. They are screening bands, not a diagnosis.
When BMI is useful and when it is not
BMI is a quick, free way to flag whether your weight may carry health risks, and it works well for population studies and for tracking your own trend over time.
It has real limits, though. BMI does not distinguish muscle from fat, so a very muscular athlete can read as overweight while carrying little fat. It can also under-estimate body fat in older adults who have lost muscle. It is not designed for children, pregnant people, or competitive athletes without extra context.
For a fuller picture, pair it with the Body Fat Calculator or check your target with the Ideal Weight Calculator.
Tips and common mistakes
A few things to watch:
- Measure height without shoes and weigh yourself at a consistent time of day.
- Do not mix units — entering height in cm but weight in pounds gives nonsense.
- Treat BMI as one signal among many, alongside waist measurement, activity and how you feel.
- Track the trend, not a single reading; daily weight naturally fluctuates by a kilo or more.
Is the calculator private?
Yes. The BMI Calculator does all the maths in your browser on your own device. Your height and weight are never uploaded to a server, so the result is completely private and instant. Explore more free calculators or browse all tools for health, finance and everyday maths.
Frequently asked questions
How do I calculate BMI?
Divide your weight in kilograms by your height in metres squared (BMI = kg / m^2). In imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared. The free BMI Calculator does this for you instantly.
Is the BMI Calculator free?
Yes, the BMI Calculator is completely free with no sign-up and no limits. It runs in your browser, so you can use it as many times as you like.
What is a healthy BMI?
For most adults, a healthy BMI is between 18.5 and 24.9 according to the World Health Organization. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is in the obesity range.
Is my data private?
Yes. All calculations happen locally in your browser. Your height and weight are never sent to or stored on any server.
Is BMI accurate for athletes?
Not always. BMI cannot tell muscle from fat, so very muscular people may read as overweight despite low body fat. Athletes should pair BMI with a body fat measurement for a clearer picture.
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