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About & sources
Where each number comes from, when it was checked, and how the calculators are tested. If a figure on this site is wrong, this page is where it should be traceable to.
Sources, and when they were checked
Every figure used by a calculator is stored with its source and the date it was checked. Rates were last verified against the official pages below on 11 October 2026.
| Figure | Value used | Source | Page last updated |
|---|---|---|---|
| GST rate | 10% | ATO — How GST works; GST Act 1999 s 9-70 | 14 Sep 2026 |
| GST registration threshold | $75,000 | ATO — Registering for GST | 14 Sep 2026 |
| Super guarantee rate | 12% | ATO — Super guarantee | 17 Apr 2026 |
| Maximum contribution base (annual, 2026–27) | $270,830 | ATO — Super guarantee, Table 23 | 17 Apr 2026 |
| Maximum contribution base (quarterly, to 30 Jun 2026) | $62,500 (2025–26) | ATO — Super guarantee, Table 24 | 17 Apr 2026 |
| Quarterly BAS due dates | 28 Oct / 28 Feb / 28 Apr / 28 Jul | ATO — Due dates for lodging and paying your BAS | 17 Sep 2026 |
| Monthly BAS due date | 21st of the following month | ATO — Due dates for lodging and paying your BAS | 17 Sep 2026 |
| Annual GST return due date | 31 October (28 Feb if no tax return required) | ATO — Due dates for lodging and paying your BAS | 17 Sep 2026 |
| Weekend/public holiday rule | Next business day | ATO — Due dates for lodging and paying your BAS | 17 Sep 2026 |
| Two-week online lodgment concession | Quarters 1, 3 and 4 only | ATO — Two week lodgment concession | 18 Aug 2025 |
| Registered agent concession dates 2026–27 | 25 Nov 2026 / 26 May 2027 / 25 Aug 2027 | ATO — BAS agent lodgment program 2026–27 | 1 Jul 2026 |
| ABN check-digit algorithm | modulus 89, weights 10–19 | ABN Lookup — Format of the ABN | — |
| Australian public holidays, 2025 | by state and territory | data.gov.au — Australian Public Holidays Machine Readable Dataset (CC BY 3.0 AU) — now marked inactive and stops at 2025 | 11 Oct 2026 |
| Australian public holidays, 2026 and 2027 | by state and territory | Fair Work Ombudsman — 2026 and 2027 public holidays (Australian Government, compiled from the state and territory lists) | 11 Oct 2026 |
| First marginal tax rate (2026–27) | 15% above $18,200 | ATO — Tax rates, Australian resident | 13 Aug 2026 |
| Medicare levy | 2% | ATO — What is the Medicare levy? | 30 Apr 2026 |
Two changes that catch people out. The first marginal tax rate above the tax-free threshold is 15% in 2026–27 — it was 16% for 2024–25 and 2025–26 and falls to 14% on 1 July 2027. And from 1 July 2026, employers must pay super guarantee each payday (Payday Super), calculated on qualifying earnings, with the maximum contribution base expressed as an annual $270,830 rather than a quarterly $62,500.
How the maths is tested
The calculators are not just clicked through by hand. The same JavaScript file that runs in your browser is loaded by a Node test script, test/run.js, which checks it against known cases:
- GST conversions round-trip, including awkward amounts such as $9.99 and $0.05.
- The ABN checker is verified against the worked example on the ABN Lookup page — 51 824 753 556, weighted sum 534.
- BAS quarter mapping, the 28 February December-quarter date, the next-business-day rule and both concession paths.
- Super at 12%, with the annual cap converted correctly to a per-pay cap, and the pre-1 July 2026 quarterly cap for past periods.
- Pay cycle conversions round-trip between weekly, fortnightly, twice-monthly and monthly.
- Markup and margin convert in both directions, including the 100%-margin edge case.
- The generated invoice PDF is parsed back: the cross-reference table, byte offsets and trailer are checked, so a corrupt file cannot ship.
- Every public holiday date is a real date, in order, and attributed to at least one state.
Run them yourself: node test/run.js.
Privacy
Short version: nothing you type leaves your browser.
- There is no server-side code on this site. It is static files.
- No analytics, no tracking pixels, no cookies, no local storage of your figures, no fingerprinting.
- No third-party scripts are loaded at all. The stylesheet, the fonts (system fonts) and the JavaScript are all served from this domain.
- The invoice generator builds the PDF in memory in your browser. Your client's name, your bank details and your prices are never transmitted anywhere.
- The only record of what you typed is in the page's own address bar where a tool supports shareable links, and that stays on your machine unless you copy it.
How the site is paid for
Running a site costs money, so it is right to be clear about where that comes from. This site is funded by:
- Our own products — spreadsheet templates and similar tools we make ourselves, linked from clearly-labelled slots.
- Advertising — currently reserved, not live. Ad slots are marked, sized and empty; if advertising is switched on it will be disclosed here and the layout will not shift when it loads.
What we will not do: accept payment to change a rate or a due date, hide a source behind an affiliate link, or put a calculator's answer behind an email signup.
Corrections
If a rate or date here is wrong, the source table above is what it is wrong against. Rates change; the fastest fix is that a reader notices. Every rate lives in one file, data/rates.json, which records the value, the source URL and the date it was checked, so a correction is a one-line change plus a rebuild.
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