How the BMI Calculator Works (With a Worked Example)
Calculates body mass index for adults using metric units. This guide walks through what each input means, the formula behind the answer and a worked example using typical Australian figures. You can follow along in the free BMI Calculator.
Why it matters
Australian nutrition labels use kilojoules, alcohol is measured in standard drinks of 10 grams, and health guidance comes from sources such as the NHMRC and Healthdirect. These calculators use common, published formulas as a starting point.
What you need to enter
The calculator asks for 2 inputs. The table shows each one with the example value used later in this guide.
| Input | Example value |
|---|---|
| Weight (kg) | 75 |
| Height (cm) | 175 |
The formula
BMI = weight (kg) ÷ height (m)².
Worked example
Entering the example values above gives these results:
| Result | Value |
|---|---|
| BMI | 24.5 |
| Category | Healthy weight |
| Healthy weight range | 56.7 to 76.3 kg |
The headline figure is 24.5 (BMI). Change any input in the calculator and every result updates instantly, so you can test different scenarios side by side.
How the BMI changes with weight
The table below keeps every other input at the example value and moves only “Weight (kg)” up and down by 10% and 20%. It shows how sensitive the BMI is to that one figure.
| Weight (kg) | BMI |
|---|---|
| 60 | 19.6 |
| 68 | 22.2 |
| 75 | 24.5 |
| 82 | 26.8 |
| 90 | 29.4 |
At 60 the result is 19.6; at 90 it is 29.4. If a small change in this input moves the answer a lot, it is worth double-checking that figure before you rely on the result.
A practical tip
BMI doesn’t measure body fat or muscle. Waist measurement is another useful check.
Keep going
Before relying on the number, read Getting the Most From a BMI Calculator: Tips and Common Errors. You might also find these guides useful:
- BMR Calculator: Formula, Example and What the Result Means
- Using a Daily Kilojoule Needs Calculator in Australia: A Step-by-Step Guide
- Body Fat Calculator Explained for Australians
Frequently asked questions
- Are these calculators medical advice?
- No. They provide general information. See your GP or visit healthdirect.gov.au for personal advice.
- Do they work for children?
- No. Adult formulas are used throughout. Children’s growth is assessed differently.
- Where do the formulas come from?
- Each calculator page names the formula used, such as Mifflin-St Jeor for BMR or the US Navy method for body fat.
Run your own numbers in the BMI Calculator.
Open the calculatorGeneral information only. Rates and thresholds change, usually on 1 July. Confirm current figures with the ATO or the relevant authority, and get personal advice for decisions about your situation.