Australian tax calculators

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

Six calculation worksheets for established facts: ordinary GST, resident basic income tax, CGT losses and discount, ordinary employer FBT, first-year depreciation and quarterly super guarantee. 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. Contribution caps, SMSF tax, trusts, payroll tax, HELP and Medicare calculations remain outside these worksheets.

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.