Why GST touches every part of your ERP
For a GST-registered Singapore manufacturer, tax is not a separate module — it is a field on every transaction. Purchase orders need to capture input tax. Sales invoices need to calculate and display output tax. Stock valuation needs to exclude input tax (for GST-registered businesses, input tax is a recoverable cost, not part of inventory value). Job costing needs to handle GST on subcon purchases correctly. And every year (or quarter), the business needs to produce an accurate GST F5 return.
An ERP that does not handle this correctly — or that requires manual GST adjustment at every step — creates both compliance risk and operational drag. The checklist below covers what a GST-registered manufacturer's ERP must handle.
The GST ERP checklist
Purchase invoice GST: the ERP must support Standard-Rated (9%), Zero-Rated (0%), and Out-of-Scope supply types on purchase invoices. Input tax must be separated from the cost for inventory and job costing purposes. Input tax claimed must be recorded per invoice for F5 return preparation.
Sales invoice GST: output tax must be calculated on the taxable value, not on a GST-inclusive price, and displayed separately on the invoice. The ERP must support GST-inclusive pricing entry (customer quotes in GST-inclusive price) with automatic back-calculation of the GST component.
Import GST: for imported raw materials and components, import GST (paid at point of import or deferred under MES) must be tracked separately. The ERP must record import permit numbers and distinguish between standard import GST and MES-suspended GST.
Customer credit notes and returns: credit notes must reverse the original GST treatment. A credit note issued for a Standard-Rated invoice must reduce output tax by the correct GST amount. The ERP must prevent incorrect GST treatment on credit notes (a common error in manually-configured systems).
GST reporting: the ERP must be able to produce a transaction-level GST report for any period, showing: invoice number, date, customer/supplier, taxable amount, GST amount, and GST treatment code. This report is the audit trail for IRAS inspection.
F5 return preparation: the ERP should produce a summary by GST period showing Box 1-14 values. This does not replace the accountant's review, but it should eliminate the manual tallying of transaction totals.
Common GST gaps in standard ERPs
The most common gap in off-the-shelf ERPs configured for Singapore is incorrect handling of partial-period adjustments — when a GST return period does not align with the invoice date and the ERP counts invoices by date-entered rather than invoice date. A second common gap is the absence of an Out-of-Scope treatment code for financial transactions (loans, insurance proceeds) that a manufacturer may occasionally record.
For manufacturers using MES, the most common gap is that the ERP has no field for import permit numbers on purchase invoices, making reconciliation manual.
What a custom build includes
A custom ERP built for Singapore manufacturers includes GST handling as a core feature: Standard/Zero/Out-of-Scope treatment codes on all transactions, import GST with MES flag, GST-inclusive price entry with back-calculation, F5 report generation, and InvoiceNow output that includes GST amounts in the correct PEPPOL SG format. These are not add-ons — they are part of the baseline build scope.