AUD amount modelling
Keep the rounding rule with the people who must explain it
A money-calculation handover needs worked examples, policy ownership and a safe correction path. A list of numeric types is not enough for the next team.
In this article
Preserve the policy as an understandable record
Document the operation order, precision boundaries, tie behaviour and residual allocation rule. Include a small set of independently approved examples with exact inputs and outputs.
Name the business owner who can approve a policy change. Engineering can implement and test the rule, but should not infer its commercial or accounting meaning from the current code.
Version significant changes and state their effective scope. A new policy may apply to new quotations while existing accepted quotes retain their original terms.
Map the technical boundaries
Record which fields contain rates, decimal amounts and integer minor units. Include the database driver's behaviour and the conversion contract for each external provider.
Show one transaction from input through calculation, persistence, external request and reconciliation. The incoming team should be able to locate the original evidence without reverse engineering several services.
Keep formatting rules separate in the handover. A change to currency display should not require changing calculation logic or stored values.
Rehearse a discrepancy
Give the receiving team a synthetic allocation with an extra cent assigned under the residual rule. Ask them to explain the recipient, reverse the allocation through the supported path and verify the final total.
Then introduce a mismatch caused by a fee or partial settlement. They should distinguish that from a rounding defect using provider evidence and transaction state.
Document who approves corrections, which actions are reversible and how an adjustment links to the original transaction. Direct database edits should not become the default operating procedure.
Maintain examples with integrations
Provider amount contracts, supported currencies and application limits can change. Assign an owner to review those changes and update the adapter tests when necessary.
Keep historical fixtures when policy changes. They prove that old transactions remain explainable while new work uses the intended rule.
A successful handover leaves the next team able to answer a customer's amount question with exact evidence. It should not require finding the developer who remembers why a particular service rounded a line before summing it.
Primary sources
Stripe: currency and amount representationMDN: Intl.NumberFormatReferences checked 11 September 2026.