Methodology · AU-BIZ-001
How the GST Calculator works
This page documents how the GST Calculator produces its figures: the engines and formulas it runs, the rule packs it reads statutory values from, the official documents those packs cite, and how the engines are verified. Official sources were last retrieved on .
Engines and formulas
- E20 · Business arithmetic engine
GST arithmetic per §13.18 with explicit rounding levels and largest-remainder allocation so invoice lines always reconcile to totals.
Registered formulas: F-GST-001 · F-GST-002
Rule packs
Statutory values come only from versioned, hash-verified rule packs citing official sources. Current status of the packs this calculator can use:
- au-gst-standardactive · v0.1.0
- How GST worksAustralian Taxation Office · retrieved 2026-08-20
Verification
Engines carry boundary tests at every bracket edge, property-based tests, reconciliation invariants and, where official worked examples exist, tests that reproduce them exactly. The full source register is on the sources page.