Calculation Methodology
Where our rules come from, how we round, how we test, and what our tools do not cover.
Sources
Each rule our tools use comes from the GST Act or from ATO guidance. We never copy figures from other calculator sites. When the Act and the ATO's plain-English guidance seem to differ, we follow the Act.
| Rule | Source | Used in |
|---|---|---|
| GST rate and value of a taxable supply | GST Act s 9-70, s 9-75 | All tools |
| Registration thresholds, taxi and rideshare rule | GST Act, Div 144 | Registration checker |
| Tax invoice requirements | ATO tax invoice rules | Invoice generator |
| Cash rounding | ATO guidance on rounding | GST calculator |
The law itself is the A New Tax System (Goods and Services Tax) Act 1999.
Rounding and Input Policy
Testing
Before any change goes live, each tool must reproduce a fixed set of test cases to the cent. The cases cover threshold edges, half-cent amounts, cash rounding, zero and GST-free inputs, and very large values. A sample:
| Input | Expected GST | What it tests |
|---|---|---|
| Add GST to $10.35 | $1.04 | Floating-point half-cent |
| Remove GST from $82.51 | $7.50 | 1c over the tax invoice threshold |
| Remove GST from $27.37, cash | $2.49 | Total rounded down to $27.35 |
| Remove GST from $9,999,999.99 | $909,090.91 | Cent precision on large values |
We re-check every source and re-run every test at the start of each financial year (1 July) and whenever a GST rule changes. Last full review: October 2026.
What Our Tools Do Not Cover
These situations depend on facts a calculator cannot check. Use a registered tax agent for them:
- Margin scheme sales of property
- Sale of a business as a going concern
- Splitting a purchase between private and business use
- Luxury car tax and wine equalisation tax
- Foreign-currency amounts. Our tools work in AUD only.