Australian tax calculators

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.

Nine 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 and account-based pension minimums. 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 3.10 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 293 and 296 tax, the transfer balance cap, trusts, payroll tax, HELP and Medicare calculations 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.

Worksheet discovery

The development source adds austaxcalc.calculations.worksheet_catalogue(). It 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. This addition is published in the 0.1.4 wheel.

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.