Singapore GST pack (khata-sg)

KhataOS can keep books for a business in Singapore. It checks GST registration numbers, charges GST at the rate in force on the invoice date (9% from 1 Jan 2024), checks tax invoice numbers and works out GST F5 figures from the books. The GST F5 figures are a preparation aid, not a filing, and nothing here is tax advice. Check them, ideally with a Singapore tax agent, before you file on myTax Portal.

Code: crates/khata-sg (the rules: gst_no.rs, gst.rs, returns.rs), apps/desktop/src-tauri/src/gst_sg.rs (vouchers to GST F5 documents, the gst_f5(from, to) command), and apps/desktop/src/screens/gst_sg.ts (the GstF5Report screen, in Go To (Alt+G) as GST F5 (Singapore GST return) for Singapore companies). Sources and open questions are in docs/research/singapore-compliance.md. Tags: [PRIMARY] read on an official IRAS/ACRA/government page, [SECONDARY] professional sources only, [UNCERTAIN] not confirmed.

Setting up a Singapore company

1. Pick Singapore as the country. The books use SGD (2 decimals, printed "S$") and calendar years (January to December). A company chooses its own financial year end, and the first year may run up to 18 months (§6 [SECONDARY]), so change the books' first month if yours differs. GST periods are usually calendar quarters, whatever the FYE.
2. In Company → Company details (Alt+K), enter the GST Reg No. Singapore has no tax regions, so leave the State/region field empty. Invoices never ask for a region: the country code SG stands in on both sides of every supply.
3. Add the rate list. The suggested classes are below, and also in khata_sg::gst::default_rates:

Class	Rate	From	Meaning
STANDARD	3 / 4 / 5 / 7 / 8 / 9	1994-04-01 / 2003-01-01 / 2004-01-01 / 2007-07-01 / 2023-01-01 / 2024-01-01	Standard rate history (IRAS rate table [PRIMARY])
ZERO	0	1994-04-01	Zero-rated: exports of goods, international services under s21(3)
EXEMPT	0	1994-04-01	Exempt: financial services, sale/lease of residential property, investment precious metals, digital payment tokens
OUT-OF-SCOPE	0	1994-04-01	Out of scope: overseas-to-overseas goods, overseas goods sold in an FTZ / Zero GST Warehouse, private transactions

You only need the rates for the years you keep books for (for example STANDARD 8 from 2023-01-01 and 9 from 2024-01-01). Zero-rated, exempt and out-of-scope are told apart by name only. A class starting with EXEMPT is exempt, one starting with OUT-OF-SCOPE (or OUT OF SCOPE) is out of scope, and any other 0% class is zero-rated. 4. Create Output GST and Input GST ledgers under Duties & Taxes, and set their type of tax to GST.

What is checked and computed

GST registration number (gst_no.rs). After spaces and hyphens are removed and letters are upper-cased, one of:

Shape	Example	Who	Source
nnnnnnnnX (9)	12345678A	UEN of a business registered with ACRA	Annex E UEN format [PRIMARY]
yyyynnnnnX (10)	199012345X	UEN of a local company	Annex E [PRIMARY]
TyyPQnnnnX (10)	T08LL1234A	UEN of any other entity (LLP, foreign company, society…); R/S for 18xx/19xx years	Annex E [PRIMARY]; R/S [SECONDARY]
M + 9 (10)	M2-1234567-K	A number issued by IRAS when it differs from the UEN	IRAS Invoicing customers [PRIMARY example]

No check letter is validated. The UEN format names a "check alphabet", but no algorithm is published. IRAS gives only an example for M numbers, so KhataOS accepts M, a letter or digit, 7 digits and a letter or digit: a lenient superset that won't refuse an older real number [UNCERTAIN]. Entity-type codes (PQ) are not checked against ACRA's list. A UEN is shown as the entity ID; an M number has none. Only the IRAS "GST registered business search" confirms a registration.

GST on an invoice line (gst.rs). One GST component at the class's rate on the invoice date, from the company's rate list (rates are never hard-coded). Each line is rounded to the cent, half away from zero. - IRAS lets you round "the total GST … to the nearest cent" [PRIMARY]. The half-cent direction is not stated [UNCERTAIN]. document_gst rounds once per invoice for e-invoicing; the books round per line, so a long invoice can differ by a few cents. - An export (place of supply abroad) is zero-rated whatever the class. The export evidence and the s21(3) conditions are not checked. Note that the invoice screen does not yet mark supplies abroad, so give export lines the ZERO class. The GST F5 also treats an untaxed sale to a party whose mailing country is not SG as zero-rated. - An OUT-OF-SCOPE line carries no GST component at all. A supply to a special zone marked "without payment" is the user's statement that it is out of scope (FTZ). Any other local supply marked "without payment" is refused.

Invoice numbers. IRAS asks only for "an invoice identifying number" [PRIMARY]. KhataOS keeps the UAE pack's rule: not empty, at most 35 characters of letters, digits, - and /, and unique across all years (no yearly reset). This is a KhataOS choice that keeps numbers safe for InvoiceNow.

GST F5 (returns.rs, desktop gst_f5). From Sales and Credit Notes (outward), and Purchases and Debit Notes (inward), in the period:

Box	Worked out as
1	Standard-rated value, net of credit notes, plus reverse-charge purchases
2	Zero-rated value (0% classes, and untaxed sales to parties abroad)
3	Exempt value
4	1 + 2 + 3 (out-of-scope supplies are in no box; shown for reference)
5	Standard- and zero-rated purchases from suppliers with a GST Reg No, net of purchase returns, plus reverse-charge purchases and imports (no supplier number needed)
6	GST on Box 1, including the reverse-charge GST
7	GST on Box 5, all taken as claimable, including reverse-charge GST and import GST
8	6 − 7. A warning appears when it is under S$5, which IRAS neither collects nor refunds
9–12, 15–17	Not modelled: shown as 0 / "No"
14	"Yes" with the value of reverse-charge purchases (also in Boxes 1 and 5)
13	Estimate: the net value of all Sales and Credit Notes. IRAS accepts a best estimate, but replace it with revenue from your accounts
18–21 (IGDS)	Not shown

Warnings list purchases with GST but no supplier GST number, GST on zero-rated or exempt lines, GST that does not match the rates on file, lines without a class, and journals that post to tax ledgers.

Reverse charge and imports

How a line is marked (no core change): on the tax class, as in the UAE, UK and Saudi packs (khata_sg::gst::Mechanism). The rest of the class is the rate class; the invoice carries no GST for such a line.

• RC:STANDARD on the stock item or service ledger: imported services (from 1 Jan 2020) and low-value goods (from 1 Jan 2023) under the reverse charge. Only an RC business applies it: a GST-registered person not entitled to full input tax credit (it makes exempt or non-business supplies beyond the de minimis rule), or in a GST group with such a member (e-Tax Guide "GST: Reverse Charge", 10th ed., 30 Jan 2026, §§4.1.2-4.1.5 [PRIMARY]). Low-value goods: not dutiable, outside Singapore when sold, delivered by air or post, S$400 or less per item (§4.3.1). The value goes in Boxes 1, 5 and 14, the GST worked out at the rate class in Boxes 6 and 7 (§11.1.1 [PRIMARY]). Box 7 is claimed in full; a warning reminds that the normal recovery rules (apportionment) apply, since an RC business is by definition not fully taxable. Self-account in the books (Dr expense, Dr input GST, Cr supplier, Cr output GST); a warning compares what the voucher posts to output GST. A fully taxable business does not use the reverse charge and need not complete Box 14.
• IMPORT:STANDARD on the stock item: goods imported with GST paid to Singapore Customs. The line's value goes in Box 5 (it should match the import permit's value: CIF plus duty and other charges, IRAS "Importing of goods" [PRIMARY]; whether Box 5 takes the permit value or the CIF value is [UNCERTAIN]). The permit's GST is posted to a GST ledger classed IMPORT on any voucher (the Payment to the forwarder, say): its debits go in Box 7. Neither needs a supplier GST number. A warning shows when imports are in Box 5 with no import GST posted.
• A purchase from a party abroad with no GST and nothing marked gets a reminder.

Pointers only (not modelled; IRAS e-Tax Guides [PRIMARY]): MES / A3PL imports (GST suspended): the permit value in Box 5 and Box 9, no GST in Box 7 (MES guide, 18th ed., App. B). IGDS: the deferred import GST in Box 19 and the value in Box 21 (not Box 6), and, when claimable, the value in Box 5 and the GST in Box 7; monthly filing (IGDS guide, 11th ed., App. D). The labels of Boxes 18 and 20 are [UNCERTAIN]. Reverse-charge refund claims for unpaid overseas suppliers (Box 11) are not modelled.

What is not modelled (check by hand)

• OVR and customer accounting (Boxes 15–17); MES/A3PL (Box 9), IGDS (18–21), as above.
• Input tax disallowance (certain motor car and staff medical costs), private use, partial-exemption apportionment, the Gross Margin Scheme.
• Bad debt relief, tourist refunds, pre-registration claims (Boxes 10–12).
• Net realised exchange gains/losses in Box 3.
• Rate-change transitional rules: the rate is the one on the document date.
• F7 corrections (adjust errors with net GST up to S$3,000 in the next F5 instead [SECONDARY]) and the F8 final return.
• Due dates: one month after the end of each accounting period (30 Apr, 31 Jul, 31 Oct, 31 Jan for calendar quarters) [PRIMARY]. Not tracked yet.

What to verify

• The IGDS box labels, any official UEN check-letter algorithm, and the shapes of older M numbers (research §9).
• The half-cent rounding direction.
• Records must be kept for at least 5 years [PRIMARY], and KhataOS never purges them. Keep exports when you change software.

InvoiceNow (PINT SG e-invoices)

IRAS requires GST-registered businesses to transmit invoice data through InvoiceNow (Singapore's Peppol network) [PRIMARY, IRAS page as of 2026-10-20]: - new voluntary registrants: within 6 months of incorporation from 1 Nov 2025, and all from 1 Apr 2026; - new compulsory registrants, and existing businesses with ≤ S$200k supplies: from 1 Apr 2028; - ≤ S$1m from 1 Apr 2029, ≤ S$4m from 1 Apr 2030, and the rest from 1 Apr 2031.

KhataOS writes the PINT SG Billing UBL (v1.4.1, urn:peppol:pint:billing-1@sg-1) for a Sales or Credit Note voucher (pint_sg_xml, export action exportPintSg), with the PINT SG rules it breaks. GST categories come from the tax class: standard → SR, zero-rated → ZR, exempt → ESN33 (name the class ES33 for regulation 33 supplies), out of scope → OS; a company without a GST number issues NG. The company's UEN (a "UEN: …" line in the company address, or the GST number when it is a UEN) gives its participant id 0195:SGUEN<UEN> (peppol_participant_id). Buyers need a Peppol id or UEN ("Peppol ID: …" / "UEN: …" in their mailing address, or a UEN-shaped GST number), a street and a postal code; the voucher needs a buyer reference or PO ("PO: …" in the narration).

Sending is through an IMDA-accredited Access Point: peppol_ap_send POSTs the UBL to a configured URL with an API key, a generic adapter that does not claim compatibility with any provider's API. Not supported: foreign currency (GST in SGD), customer accounting (SRCA-*), GIRO. Details and gaps: docs/peppol.md. Until checked with your Access Point, treat the output as a draft.