Australia GST Registration Decision Tool methodology

Pᵢ = Pₑ × (1 + tₒ) when registered and tax-inclusive; otherwise Pᵢ = Pₑ

Pₑ = Cₑ ÷ (1 − m)

T = Pᵢ − Pₑ

Recoverable input tax: Cₑ = C ÷ (1 + tᵢ) when C is tax-inclusive, otherwise Cₑ = C. Non-recoverable input tax: Cₑ = C when C is tax-inclusive, otherwise Cₑ = C × (1 + tᵢ)

G = Pₑ − Cₑ

M = G ÷ Pₑ

ΔP = Pᵢ − P₀

tₒ = tⱼ

R = 2 when a reviewed trigger applies; R = 1 at an equality boundary; otherwise R = 0

L = Lᵣ

T₁ = ΣT₁ᵢ

T₂ = ΣT₂ᵢ

Tₕ = max(T₁, T₂)

H = max(0, L − Tₕ)

Money inputs and outputs use AUD. Rates, margins, utilisation and buffers are entered as percentages and converted to decimal values for calculation.

• Turnover values must be finite, non-negative and use the official turnover scope.
• Cost, target margin, registration, jurisdiction/place-of-supply, supply/customer class and effective-date inputs must remain inside the supported source-versioned policy branch.
• Tax-inclusive and tax-exclusive price identities reconcile through the selected official rate; retained margin is measured on revenue excluding tax.
• The current and projected GST-turnover records each contain exactly 12 monthly values; incomplete records fail closed.
• Each supported period is compared independently; the engine never adds unlike periods together.
• Equality does trigger the selected rule, matching the reviewed authority wording.
• Unsupported business classes and unconfirmed turnover scope fail closed.
• The dependency version is returned by the policy contract and acknowledged in canonical tool history.

• The selected class is inside the bounded scope described by ato-gst-registration-threshold-au.
• The user has excluded amounts that the authority excludes from the relevant turnover definition.
• The prospective value is a user-owned scenario, not a Margin101 forecast.
• The customer/transaction, jurisdiction and effective-date confirmations are user-owned classifications and are not inferred from names, addresses or dates.
• No below-threshold result overrides another statutory trigger or special rule.

Official dependencies used by this decision tool are captured in the Margin101 evidence register and version-acknowledged in this page history. User-entered commercial assumptions remain separate from those official inputs.

• The tool does not decide registration, effective dates, deregistration, voluntary registration, taxability, exemptions, groups, cross-border treatment, place of supply or filing.
• The price result does not replace an invoice, accounting or tax-system calculation and supports only the displayed rate, supply, customer and jurisdiction branches.
• Only the displayed business classes and turnover tests are supported.
• The tool does not replace the authority guidance or professional advice.

This is educational decision support, not tax, accounting, legal or financial advice. Check the treatment of your actual transactions under the rules that apply in Australia and seek qualified advice where appropriate.

Registered family-level suites test formula invariants and example reconciliation; the release ledger records that coverage without claiming a separate oracle for every line of public copy. Registered official sources block dependent public workflows when their evidence or version acknowledgement is unavailable or invalid. Commercial inputs remain user-supplied because they vary by business and contract.

1. 2026-07-31: Initial public release of the planner and methodology after pre-launch calculation, content, source and interaction review.