Getting the Most From a BMI Calculator: Tips and Common Errors

Updated 2 September 2026 · Health and fitness

A BMI Calculator gives a fast answer, but the answer is only as good as what goes in. These are the mistakes we see most often with BMI calculations in Australia, and how to avoid them.

1. Treating estimates as diagnoses

Formulas describe populations, not individuals. They can’t replace advice from a health professional.

2. Mixing kilojoules and calories

1 calorie equals about 4.2 kilojoules. Australian labels use kJ.

3. Relying on one measure

BMI, waist size and body fat each tell part of the story. Look at them together.

4. Underestimating drink sizes

Many pours and cans are more than one standard drink.

5. Overlooking the detail that matters most here

BMI doesn’t measure body fat or muscle. Waist measurement is another useful check.

How the BMI changes with height

A common mistake is getting one input slightly wrong. Here is what happens to the BMI when “Height (cm)” is off by up to 20% in either direction, with everything else held steady.

Height (cm)	BMI
140	38.3
158	30
175	24.5
193	20.1
210	17

At 140 the result is 38.3; at 210 it is 17. 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 quick sense check

BMI = weight (kg) ÷ height (m)². With typical inputs the calculator returns BMI of 24.5. If your own result looks wildly different, check that each figure is in the right unit and period (weekly, monthly or yearly) before drawing conclusions.