The GST F5 return: what each box wants, and where errors hide

Once registered, the quarterly F5 return becomes the rhythm of your GST life: due one month after each quarter ends, filed even when nil, settled by GIRO. The form is short; the discipline is in where its numbers come from.

The boxes, in plain terms

Per IRAS's completion guide:

The errors IRAS actually finds

A handful of patterns account for most GST assessments on small companies:

1. GST-inclusive figures in Boxes 1–5. Supplies and purchases are reported excluding GST; entering gross figures overstates everything and misaligns the boxes.
2. Deriving instead of tracking. Box 6 computed as Box 1 × 9%, or Box 5 reverse-engineered from Box 7 ÷ 9% — rounding and mixed-rate items make the derived figure wrong, and the mismatch is exactly what IRAS's checks flag. Track output and input tax as ledger balances in their own right; decent software does this natively.
3. Input tax without adequate documentation. A claim needs a valid tax invoice from a GST-registered supplier — or, where the total payable including GST is S$1,000 or less, a simplified tax invoice, which many till receipts satisfy if they show the supplier's name, address, GST registration number, date, a description of the supply and the total payable including GST. A bare receipt without those details does not support a claim, quotes and pro-formas never do, and blocked items (private car expenses among them) are never claimable however good the paperwork.
4. Zero-rating without meeting the conditions. Exports and international services zero-rate only with the prescribed evidence; "the customer is overseas" alone is not the test.
5. Missed reverse-charge on imported services — businesses buying services from overseas suppliers may need to account for GST on them under the reverse-charge rules, a separate mechanism from the income-tax withholding that can apply to the same payment. A growing IRAS focus area.

Building a return that files itself

The quarterly close that never produces surprises: reconcile the GST control accounts to the ledger, tie Box 1 to revenue and Box 5 to purchases with a short bridge for exempt and out-of-scope items, spot-check the five error patterns above, and file the source documents — GST records carry the same five-year retention as everything else. From the phase-in dates onward, InvoiceNow transmission supplies much of this data to IRAS in structured form as you invoice, which reduces re-keying at quarter end. It does not, however, mean IRAS has reviewed or agreed anything — the F5 remains your return to prepare, check and stand behind.

If a past return is wrong, correct it before IRAS asks — small errors can be adjusted in a subsequent F5 within limits, larger ones via the F7 disclosure form, and voluntary disclosure is dramatically cheaper than discovery. Quarterly GST is also, frankly, the workflow small companies most often hand to their accountant — at S$150–400 a quarter, error-free filings are rarely the place to economise.