GSTR-1 vs GSTR-3B: difference, reconciliation, and how to draft a clean GSTR-1

GSTR-1 and GSTR-3B answer two different questions. GSTR-1 answers 'what did I sell, to whom, and how much tax did I charge?' — it captures every B2B invoice (with the customer's GSTIN), every B2CL invoice (large unregistered sale, currently >₹2.5 lakh inter-state), the aggregated B2CS bucket (small unregistered sales), every credit and debit note (CDNR, CDNUR), the HSN-wise summary, and a count of documents issued.

GSTR-3B answers 'what tax do I net-owe this month?' — outward supplies grouped by tax head (CGST + SGST + IGST + cess), inter-state supplies to unregistered persons, ITC claimed, ITC reversed, and the final cash payable after offsetting ITC.

Both are filed monthly by regular taxpayers (QRMP filers do GSTR-1 quarterly plus the optional Invoice Furnishing Facility, and GSTR-3B monthly with quarterly tax payment). They share a tax period but report at different granularities — and that asymmetry is the root of most reconciliation problems.

GSTR-1 is logically split into sections that mirror the kinds of outward supplies an Indian business actually makes:

• B2B (Table 4) — invoices to GST-registered customers. Each row has the customer's GSTIN, invoice number, date, place of supply, taxable value, and tax split (CGST/SGST or IGST).
• B2CL (Table 5) — inter-state invoices to unregistered persons above ₹2.5 lakh. Same shape as B2B minus the GSTIN.
• B2CS (Table 7) — aggregated intra-state and small inter-state sales to unregistered persons, summarised by tax rate and place of supply.
• CDNR (Table 9B) — credit and debit notes against B2B invoices.
• CDNUR (Table 9B) — credit and debit notes against B2CL or export invoices.
• HSN summary (Table 12) — per-HSN totals of taxable value, quantity, UoM (UQC), and tax.
• Documents issued (Table 13) — count of invoice, credit-note, debit-note, and delivery-challan numbers used and cancelled in the period.

If you've ever stared at a 17-tab spreadsheet your CA sent back, those tabs map almost one-to-one onto these sections.

GSTR-3B is shorter and lives at a higher altitude:

• 3.1 — Outward supplies and inward supplies liable to reverse charge. Five rows: taxable outward (other than zero-rated / nil-rated / exempt), zero-rated outward, nil-rated / exempt outward, inward supplies liable to RCM, and non-GST outward. Each row reports total taxable value plus CGST / SGST / IGST / cess.
• 3.2 — Inter-state supplies to unregistered persons, composition dealers, and UIN holders. Place-of-supply-wise breakdown.
• 4 — ITC. Eligible ITC available, ITC reversed (rules 42 and 43, etc.), and net ITC available — broken out by IGST, CGST, SGST, and cess.
• 5 — Nil-rated, exempt, and non-GST inward supplies. Reports inward purchases that didn't carry tax.
• 6.1 — Payment of tax. The final ledger entry — how much of the liability was offset by ITC vs paid in cash.

GSTR-3B is a self-assessment summary. Since the GSTR-2B / 3B auto-population rolled out, the outward portion is partially prefilled from your GSTR-1 / IFF data — you still verify and post, but the seed is there.

The GSTN portal compares the two returns once they're both filed. If your GSTR-3B Section 3.1 outward-supply totals don't match the sum of B2B + B2CL + B2CS + exports + credit notes in GSTR-1, you'll see:

• A mismatch flag in the GSTN portal that follow-up notices (and your CA's stress level) cite.
• GSTR-2B-based ITC restrictions for your buyers. If a B2B invoice is in your books but missing or mistyped in GSTR-1, the buyer's ITC can be blocked — which damages the customer relationship faster than any pricing dispute.
• Refund delays if you're an exporter. IGST refunds are reconciled against GSTR-1 and GSTR-3B; mismatches stop the bank wire.

Keeping the two in sync is therefore not paperwork hygiene. It is a cash and relationship risk.

Across hundreds of GSTR-1 drafts that come out of small-business billing systems, four mistakes account for the majority of mismatches:

• Place of supply (PoS) missing or wrong. PoS determines whether tax is intra-state (CGST + SGST) or inter-state (IGST). A B2B invoice issued from Tamil Nadu to a Karnataka customer where PoS is left blank or defaulted to your own state means the line is taxed as CGST + SGST instead of IGST. The moment it hits GSTR-1, the math against GSTR-3B Section 3.2 (inter-state supplies) breaks.
• UoM mismatched with the HSN summary. Table 12 wants a specific Unit Quantity Code (UQC) per HSN. If your invoices use 'pcs' but Table 12 wants 'PCS-PIECES' for that HSN, the JSON validator rejects the upload. Many SMBs fix this by editing the export file by hand — until they forget once.
• Malformed GSTIN. A B2B invoice with a typo in the customer's 15-character GSTIN gets flagged at upload (the GSTIN must pass a checksum), and the entire B2B section may fail validation rather than just that one row.
• Late or missing credit notes. Credit notes raised in a later month against invoices from an earlier month belong in CDNR / CDNUR for the month the note was raised, not retroactively in the original month's GSTR-1. Skipping them inflates outward supplies in GSTR-1 relative to the net liability you actually report in GSTR-3B.

Tiram exports GSTR-1 as portal-ready JSON and as XLSX for offline review. Before either file is generated, the export pipeline runs the same four checks the GSTN validator runs — but locally, so the user catches them in their own dashboard, not after the upload fails:

• PoS presence and validity. Every invoice destined for B2B, B2CL, or B2CS rolls up only if it has a place of supply, and the engine cross-checks it against the customer's state derived from the GSTIN. Intra-state CGST / SGST vs inter-state IGST is computed at invoice time, not bolted on at export, so the line tax matches the PoS.
• UQC mapping. The HSN summary uses the GSTN-approved unit-quantity codes; the engine maps your stored UoM (`PCS`, `KG`, `LTR`, etc.) into the right `UQC` field per HSN.
• GSTIN checksum. Every B2B customer's GSTIN is validated at customer-creation time, not at export time — wrong characters never get into the database in the first place.
• Credit notes in the correct month. Credit and debit notes are raised against an invoice but recorded in the month they were issued, and the GSTR-1 builder lifts them into CDNR or CDNUR based on whether the original invoice was B2B or unregistered.

The XLSX export is the same data your CA can pivot on offline; the JSON is the version you upload to the GSTN portal. Neither requires hand-editing.

Even a perfectly-drafted GSTR-1 doesn't help if you miss the 11th-of-the-month deadline. Tiram's What's Up priority engine includes a GSTR-1 due-date detector and a separate GSTR-3B due-date detector that surface as cockpit cards three to five days before each filing window closes. The detector knows your filing periodicity (monthly vs QRMP) and your state's GSTR-3B group (the 20th, 22nd, or 24th).

Read more about the cockpit and its 13 detectors in the companion article: The daily business cockpit: 13 detectors that tell Indian shop owners what to fix first.

To stay honest about scope:

• Tiram exports GSTR-1; it does not draft GSTR-3B. 3B is a self-assessment summary that depends on data outside billing (vendor invoices for ITC, reverse-charge purchases). Your CA or accounting software computes 3B; Tiram makes sure the GSTR-1 half reconciles cleanly.
• Tiram does not generate IRN. E-invoicing via the IRP (Invoice Registration Portal) is a separate workflow. The data model captures everything IRN needs (GSTIN, HSN, PoS, line-level tax) and the integration is on the active roadmap — but it isn't shipped today.
• Tiram does not submit to the GSTN portal via API. The portal-ready JSON downloads to your machine; you upload it to the GSTN portal yourself or hand it to your CA. We deliberately don't sit in the middle of that handoff.
• No GSTR-2A / 2B reconciliation. Matching your purchase invoices against vendor-uploaded GSTR-1 is a separate, harder problem that lives outside billing.

Most Indian SMBs draft GSTR-1 in a spreadsheet that's been edited by three different people over four years. The pattern is usually the same:

• A Sales tab pulled from billing software (or hand-typed from receipts).
• A B2B tab that filters to GSTIN-bearing customers.
• A B2CS tab that sums up unregistered counter sales by tax rate.
• A Manual fixes tab that nobody dares delete.

The problem isn't the spreadsheet — it's that the manual-fixes tab is a memory of past errors that the next month's draft inherits. Moving to a GSTR-1 export that runs the four validation checks on the source data fixes the cause, not the symptom. The migration path is the same as moving from any spreadsheet to billing software: bulk-import customers (with their GSTINs) and products (with their HSNs and UQCs), then start raising invoices in the system. The first month's GSTR-1 export will surface the missing data fields. Fix once; reconcile forever after.