Australian tax calculators

Release australian-tax-calculators==0.2.0 contains fourteen worksheets, including Schedule 8 withholding, annual Medicare levy, LITO and Division 293. Earlier releases through 0.1.9 contain ten. The release handoff keeps the engine release and MCP integration separate.

Ryan Duguid is not a registered tax agent or BAS agent. Project support is limited to software issues reproduced with fabricated data. Do not send taxpayer information or request advice, return preparation, tax-treatment confirmation, or lodgement.

Distribution australian-tax-calculators, import package austaxcalc. Library only: this component ships no command.

Fourteen calculation worksheets for established facts: ordinary GST, resident basic income tax, CGT losses and discount, ordinary employer FBT, first-year depreciation, quarterly super guarantee, Schedule 1 PAYG withholding, super contribution caps, account-based pension minimums, study and training loan repayments, Schedule 8 study loan withholding, a single person's Medicare levy, the low income tax offset and Division 293 tax. These are experimental review aids, not advice or return preparation. Callers must establish every scope condition before calculating.

The Python functions in austaxcalc.calculations accept Decimal amounts and an explicit period. Results include amounts, scope exclusions, official source URLs, the source-check date and engine version. They perform no network calls or writes. Amounts must be non-negative, finite, at most 2 decimal places and no more than AUD 1 trillion. Rounding uses half-up to cents at output; individual worksheets state additional conventions.

Install

Python 3.14 or later. No runtime dependencies.

pip install australian-tax-calculators

Use

Every worksheet takes Decimal amounts, an explicit period, and a scope_confirmed flag the caller sets only once it has established the scope conditions the worksheet states. Splitting $1,100.00 GST-inclusive into its exclusive amount and its GST:

from decimal import Decimal

from austaxcalc.calculations import gst

result = gst(Decimal("1100.00"), gst_inclusive=True, scope_confirmed=True, year="2025-26")
print(result["amounts"], result["source_checked"])

{'gst': '100.00', 'exclusive': '1000.00', 'inclusive': '1100.00'} 2026-09-10

Amounts come back as quoted decimal strings rather than numbers, so an amount that leaves a worksheet cannot arrive at a JSON reader as a float.

The full result also carries scope, the official sources the figures came from, the engine version and the warnings that travel with every worksheet:

>>> result["scope"]
'One ordinary taxable supply at 10%, already classified by the operator. Excludes mixed supplies, exemptions, margin schemes, adjustments, tax invoice rounding across line items, input-credit entitlement and BAS preparation.'
>>> result["warnings"]
['Operator-confirmed facts and scope; eligibility is not independently verified.', 'Worksheet only, not advice, an assessment or a lodgment. Obtain human review.']

Full boundary statement: DISCLAIMER.md.

Supported periods

Unsupported periods fail. The SG worksheet does not implement post-June 2026 Payday entitlement rules. The separate Payday engine reviews timing on a supplied liability. SMSF fund tax, Division 296 tax, the transfer balance cap, trusts, payroll tax, the Medicare levy surcharge and Medicare levy reductions other than the single low-income reduction remain outside these worksheets.

PAYG withholding

payg_withholding applies the Schedule 1 (NAT 1004) formulas to one regular weekly, fortnightly or monthly pay made from 1 July 2026, on scale 1, 2, 3, 5 or 6 as the operator established from the payee's declarations. Earnings include allowances subject to withholding. The result gives the weekly earnings used, the weekly amount and the amount for the pay period, with the coefficients applied. Scale 4, tax offsets, Medicare levy adjustments, study and training support loans, extra amounts for 53 or 27 pays, quarterly pays and the other schedules are outside it. The tests reproduce every amount in the ATO's published sample data; see calculation evidence. This worksheet is published in the 0.1.5 wheel.

Super contribution caps and pension minimums

contribution_caps tests one year's classified contributions against the caps. Unused concessional cap carries forward only when the total super balance at the previous 30 June is below $500,000. The non-concessional cap is 4 times the concessional cap, nil at or above the general transfer balance cap, and otherwise 1, 2 or 3 times the annual cap by the balance bands in ITAA 1997 s 292-85. Bring-forward needs the person to be under 75 at any time in the year. The result gives room and excess under each cap, the multiple of the annual cap available and the bring-forward period the contributions started. A period started in an earlier year is outside the worksheet.

pension_minimum applies SISR Schedule 7 to one account-based pension: the age factor on the balance at 1 July, pro-rated by days in the first year, nil when the pension starts on or after 1 June, and rounded to the nearest $10 with an exact $5 rounding up. The tests check every band boundary against the ATO's published tables; see calculation evidence. These worksheets are published in the 0.1.6 wheel.

Study and training loan repayment

study_loan_repayment applies the marginal repayment rates that apply from 2025-26 to one individual's established whole-dollar repayment income: nil at or below the minimum repayment income, 15% of the excess up to the top of the first band, then the 15% band in full plus 17% of the excess, and 10% of the whole repayment income once that is the smaller amount. The result gives the marginal amount, 10% of income, the compulsory repayment and the rate that applied, with the thresholds. Repayment income (taxable income excluding assessable FHSS released amounts, plus reportable fringe benefits, total net investment loss, reportable super contributions and exempt foreign employment income) is the operator's established fact. The formula amount is not limited to the loan balance, and whether a debt exists, voluntary repayments, the overseas levy, allocation between loan types, indexation and the Schedule 8 withholding component are outside it. The tests hold the worksheet to the ATO's worked examples, the Gazette that notified the 2026-27 thresholds and every band boundary; see calculation evidence.

Study loan withholding

study_loan_withholding adds the Schedule 8 (NAT 3539) study and training support loan component to the Schedule 1 amount for one regular weekly, fortnightly or monthly pay made from 1 July 2026 to a payee who declared a study and training support loan debt. Scale 1 takes the component for a payee who did not claim the tax-free threshold; scales 2, 3, 5 and 6 take the one for a payee who claimed it or is a foreign resident. The result gives the Schedule 1 amount, the component and their total. The ATO also publishes combined coefficients and accepts either method; the tests hold the worksheet within $1 of them for scales 1 and 2, and to the schedule's three worked examples. See calculation evidence.

Medicare levy and low income tax offset

medicare_levy applies the 2% rate and the low-income reduction in s 7 of the Medicare Levy Act 1986 to a full-year resident single who is not entitled to the seniors and pensioners tax offset and has no exemption: nil at or below the threshold amount, and no more than 10% of the excess up to the phase-in limit. The caller states whether s 9A (lump sum payments in arrears) applies, and the worksheet refuses when it does. It covers 2024-25 and 2025-26; 2026-27 is refused until its thresholds are published. low_income_tax_offset gives the ITAA 1997 s 61-115 offset from established taxable income for 2024-25 to 2026-27. Neither applies the result against tax payable. See calculation evidence.

Division 293

division_293 applies ITAA 1997 s 293-20 to one individual's established Division 293 income and low tax contributions: when their sum exceeds $250,000, taxable contributions are the lesser of the contributions and the excess, taxed at 15%. It covers 2024-25 to 2026-27 and holds the ATO's worked example. Working out either input, defined benefit interests and deferred debt accounts are outside it. See calculation evidence.

These four worksheets are included from release 0.2.0.

Worksheet discovery

austaxcalc.calculations.worksheet_catalogue(), published from the 0.1.4 wheel, returns supported periods with inclusive dates, required inputs and units, available methods, scope exclusions and a fabricated example for each worksheet. Money in the catalogue uses decimal strings. Python callers convert these to Decimal; MCP clients can send example.facts to calculate_tax_worksheet.

The engine owns the supported periods for both discovery and calculation. Returned dictionaries can be edited without changing later calls or engine rules.

Calculation evidence

Calculation evidence records Library paragraphs, document review dates, independent arithmetic and the limits of each example. example_evidence in the catalogue points to these records. It requires no local Library installation or network access at runtime.

Source URLs and exact exclusions remain in austaxcalc/calculations.py and every result. The existing 10 September 2026 source-check baseline is now stored per worksheet. A later check of one worksheet cannot refresh another's date. The 15 September Library example check does not extend that baseline to other periods or establish a taxpayer's circumstances.