Duplicate INV-2026-001. Skipped 004. A “final” invoice numbered “3b”. I have seen all three — and each one triggered an accounts query that cost days. Invoice numbering looks trivial until money depends on it. Here is the GST-compliant way to format numbers, plus HSN placement and tax-field rules that keep Indian, US and UK bills clean.
Part of the free invoice generator guide. Build numbered bills in the free invoice generator (general) or freelance GST invoice generator (auto tax split). Convert totals to words with the number to words converter.
Numbering formats that pass audits (pick one, never change mid-year)
| Format | Example | Best for |
|---|---|---|
| Year-prefix sequential | INV-2026-001 → 002 → 003 | Freelancers, studios (my default) |
| GST series | INV-GST-011, INV-GST-012 | Registered sellers, separate tax series |
| Client-prefix | ACME-2026-014 | Retainers, 5+ bills per client |
| Credit notes separate | CN-001, CN-002 | Refunds and corrections (never reuse INV) |
- Unique + sequential, no gaps. I skipped 004 in Aug 2026 and spent 3 days answering my CA — auditors read gaps as missing income. Leave voids logged (“void — draft error”), never backfill.
- One series per financial year (India: April–March reset is common). Note the reset in your sheet.
- Never reuse. Correct with a credit note (CN-001 reverses INV-2026-001 line 3), not by editing history.
- Keep GST and non-GST apart if you issue both — INV-GST vs INV series prevents split confusion.
GSTIN, HSN and tax fields: exact placement
- GSTIN: supplier top-right (27ABCDE1234F1Z5), buyer GSTIN in bill-to for B2B. Copy-paste from the portal — one wrong digit bounces.
- HSN/SAC: per line next to description (SAC 9983 design, 9984 IT). 4 digits under ₹5 cr turnover, 6 above.
- Tax split: CGST 9% + SGST 9% same-state; IGST 18% inter-state. Place-of-supply state code decides — see GST format tutorial.
- Total in words: “Rupees twenty-three thousand six hundred only” — use the number to words tool to avoid typos.
- US/UK: sales-tax jurisdiction + rate (US), VAT number + 20% split (UK). Keep 3–7 years (IRS) / 6 years (HMRC).
The 10-minute monthly check I do
Last Friday of the month: export/list numbers 001→0NN, tick off paid (receipt RCP-0231 issued), nudge unpaid per payment terms guide, and confirm no duplicates. I caught a double-billed ₹12,000 retainer this way — client thanked me, trust went up.
US and UK numbering and tax-field equivalents
Numbering discipline is universal — only the tax rows change. In the US, invoice numbers work identically (INV-2026-001 sequential), but sales tax appears only where you have nexus: show rate + jurisdiction (“8.5% NYC sales tax”) or write “No sales tax collected — no nexus”. Keep invoices 3–7 years for the IRS. In the UK, VAT-registered sellers show VAT number + net + VAT (usually 20%) + gross on every VAT invoice, sequential numbering included; non-registered sellers omit VAT entirely and note it. HMRC wants 6 years of records. I keep a separate yearly folder per country-client — Sept 2026 audit took 20 minutes because of it.
- Multi-currency: one currency per invoice, stated next to the total ($550 USD, not just $550). Mixed-currency lines confuse AP software.
- Cross-check: run US totals through the US sales tax calculator and Indian ones through the GST calculator before sending.
- Not advice: thresholds and nexus rules shift — confirm with your accountant. This page covers document format only.
Fixes for the 4 most common numbering messes
- Duplicate number sent: do not edit. Issue credit note CN-00X, re-issue with next number, one-line apology.
- Skipped number: leave the gap, note “void — draft error” in your sheet. Auditors prefer honesty over backfilling.
- Two series collided: freeze one, rename forward (INV-GST-011 onward), log the change date.
- Wrong tax on numbered bill: credit-note the line, re-issue corrected split. Cross-check with the GST calculator.