GST filing intimidates a lot of small business owners not because it's conceptually hard, but because the terminology is dense and scattered across dozens of forms, thresholds, and deadlines. This guide puts all of it in one place, in plain language, so you understand not just what to do, but why each piece exists.
What GST filing actually is
GST is a tax on the value you add when you sell something. You collect GST from customers (output tax) and pay GST to your suppliers on business purchases (input tax). Filing is reporting both numbers to the government each period. The gap between what you collected and what you already paid — after adjusting for Input Tax Credit (ITC) — is roughly what you owe. ITC is the mechanism that stops tax from stacking on top of itself at every stage of a supply chain.
Do you need to register?
Registration becomes mandatory once your aggregate turnover crosses a threshold:
- ₹40 lakh for goods, in most states
- ₹20 lakh for services, in most states
- ₹20 lakh (goods) and ₹10 lakh (services) in special category states (mostly North-Eastern and hill states)
Aggregate turnover is calculated across your entire business under one PAN, nationwide, and includes taxable, exempt, and export supplies. Certain activities force registration regardless of turnover — selling through an e-commerce platform like Amazon or Flipkart, or making inter-state supplies, being the two most common for small businesses. If either applies to you, register before you hit any threshold.
Every return you might actually encounter
This is usually where guides get thin. Here's the full set:
GSTR-1 — statement of outward supplies (sales). Every invoice you issued gets reported: B2B, B2C, exports, credit/debit notes. Can't be revised once filed. Monthly filers (turnover above ₹5 crore) file by the 11th of the following month; QRMP filers file quarterly by the 13th of the month after the quarter.
GSTR-1A — the correction window. If you (or your buyer, via their reconciliation) spot an error in your filed GSTR-1 before you file GSTR-3B for the same period, GSTR-1A is where you fix it. It exists specifically to stop small errors from locking themselves into your final numbers.
What happened to "GSTR-2"? If you've seen an old guide mention GSTR-2 as a return you're supposed to file, that's outdated — GSTR-2 (your own detailed inward-supply return) was suspended early in GST's rollout and never revived. In its place, two auto-generated statements now do that job:
- GSTR-2A — a dynamic, real-time statement that updates continuously as your suppliers file their GSTR-1s. Useful for a running view, but not the number you rely on for actually claiming credit.
- GSTR-2B — a static, once-a-month snapshot generated on a fixed date, listing exactly which purchase invoices are eligible for ITC in that period. This is the one that matters: you can only claim input tax credit for invoices that appear in your GSTR-2B, full stop. If your supplier filed late or filed incorrectly, their invoice won't show up here, and the credit isn't yours to claim yet — no matter how legitimate the purchase was.
GSTR-3B — the summary and payment return. You declare total sales, total ITC (matched against GSTR-2B), and pay the net tax due. Filed monthly by the 20th, or quarterly by the 22nd/24th under QRMP depending on your state. Required even for a nil period — skipping it still triggers a late fee.
CMP-08 and GSTR-4 — composition scheme returns. Covered below.
GSTR-9 and GSTR-9C — annual return and reconciliation statement. GSTR-9 is a yearly summary of everything you filed across the year, due by 31 December following the financial year. It's mandatory for regular taxpayers with turnover above ₹2 crore (optional below that). GSTR-9C, a reconciliation statement matching your books against your returns, kicks in above ₹5 crore turnover and generally needs a professional's sign-off.
GSTR-10 — the final return. Only relevant if you ever cancel your GST registration. It has to be filed within three months of cancellation, and unlike other returns, its late fee has no upper cap — it keeps accruing.
A simple way to hold all this in your head: GSTR-1 says what you sold. GSTR-2B says what you're allowed to claim credit for. GSTR-3B is where you settle the difference. GSTR-9 is the once-a-year summary of the whole year's story.
E-invoicing: does it apply to you?
E-invoicing means uploading B2B invoices to a government portal (the IRP) in real time, which issues a unique Invoice Reference Number (IRN) and QR code — without which, technically, the invoice isn't valid and your buyer can't claim ITC on it.
It's mandatory once your turnover has crossed ₹5 crore in any financial year since 2017–18 — and once you cross it, it applies permanently, even if your turnover later drops back below the threshold. It covers B2B invoices, exports, and SEZ supplies, not ordinary B2C retail sales. If your turnover is above ₹10 crore, there's an additional rule: invoices must be reported to the IRP within 30 days of issue, or they're rejected outright.
Most small businesses aren't there yet, but it's worth knowing before you cross the line, not after.
HSN/SAC codes: the detail everyone gets slightly wrong
Every invoice needs the right HSN code (goods) or SAC code (services) — it determines the GST rate that applies, and a wrong code can mean ITC denial for your buyer and a penalty for you. How many digits you need depends on your turnover:
- Below ₹1.5 crore — HSN codes aren't required on the invoice at all
- ₹1.5 crore to ₹5 crore — minimum 2-digit HSN
- Above ₹5 crore — minimum 6-digit HSN, on both B2B and B2C invoices
Even below ₹5 crore, HSN reporting in the GSTR-1 HSN summary table is often still required, separately from what appears on the invoice itself — a distinction that trips a lot of businesses up.
Reverse charge: when your buyer pays your tax
Normally, the seller collects and pays GST. Under the reverse charge mechanism, that flips — the buyer pays GST directly to the government instead of to the seller. This applies to a specific list of situations, most commonly when you buy from an unregistered supplier, use certain notified services (like goods transport agencies), or import services. If reverse charge applies to a purchase you made, you report and pay that tax yourself in GSTR-3B — it's easy to miss because there's no invoice prompting you to notice it.
TCS for e-commerce sellers
If you sell through Amazon, Flipkart, or a similar platform, the platform itself is required to collect a small percentage of your sale value as Tax Collected at Source (TCS) and deposit it against your GSTIN. You then claim that TCS as a credit against your own liability when you file. If your reconciliation doesn't include checking this credit shows up correctly, you can end up paying tax twice on the same sale without noticing.
QRMP and the composition scheme
QRMP (Quarterly Return Monthly Payment) — if turnover is at or below ₹5 crore, you can file GSTR-1 and GSTR-3B quarterly instead of monthly, while still paying tax every month via a simple challan (PMT-06, due the 25th). Fewer filing events, same payment discipline.
Composition scheme — if turnover is under ₹1.5 crore (₹75 lakh in special category states) and you're a trader, retailer, or manufacturer, you can pay a flat 1% of turnover quarterly (CMP-08) and file one annual return (GSTR-4), instead of monthly GSTR-1/GSTR-3B. Service providers have a separate composition option capped at ₹50 lakh, taxed at 6%. The trade-off: no collecting GST from customers, and no ITC on your purchases. Worth it if most of your suppliers are unregistered anyway; usually not worth it if you have significant GST-paid inputs. Talk to an accountant before choosing.
What happens if you're late
- Late fee: ₹50/day (₹25 CGST + ₹25 SGST) for a return with tax due, ₹20/day for nil returns — capped per return, but higher than most owners expect.
- Interest: 18% per annum on unpaid tax, from the day after the due date until actual payment. If you claimed and used ITC you weren't entitled to, that jumps to 24%.
- Downstream effects: your buyers can't claim ITC on your invoices until you file, your own GSTR-3B can get blocked pending GSTR-1 reconciliation, and two consecutive missed GSTR-3B filings disable e-way bill generation — which can stop goods movement outright for a goods business.
A worked example
A small retail shop has ₹12,00,000 in taxable sales for the month, and ₹1,80,000 in ITC correctly reflected in its GSTR-2B. Net tax payable is roughly ₹36,000, reported and paid via GSTR-3B by the 20th, after GSTR-1 for the same period went in by the 11th. Paid thirty days late, that ₹36,000 accrues roughly ₹530 in interest at 18% per annum — on top of the late fee on the return itself. A routine, on-time filing avoids both entirely.
Where mistakes actually happen
Invoices that don't match records. Gaps in numbering or inconsistent recording turn reconciliation into a hunt.
Claiming ITC not in GSTR-2B. A legitimate purchase invoice that your supplier didn't file correctly isn't claimable yet — the credit is only as reliable as your supplier's own compliance.
Missing reverse charge and TCS entries. Both are easy to overlook because neither shows up as a normal invoice prompting action.
Reconstructing sales at the deadline. The businesses that file smoothly are the ones recording every sale as a proper invoice the moment it happens, not from memory during the last week of the month.
The habit that fixes most of this
The single highest-leverage change is treating every sale as a structured invoice immediately, not reconstructing it later. When every invoice already carries the customer, amount, tax, and HSN/SAC code correctly, filing becomes an export rather than a research project. Billing tools — including Vylot — can generate a GST-ready export directly from your existing invoice records, worth exploring if you're still assembling this by hand each month.
The bottom line
None of this is inherently difficult — it's bookkeeping discipline applied consistently through the month, not reconstructed under deadline pressure. Understand which return actually matters for your situation, which scheme fits your business, and where credit genuinely comes from, and GST filing stops being something you dread.
GST rules, thresholds, and rates are updated periodically by the GST Council and CBIC notifications. This guide reflects rules current as of 2026 — always confirm current figures with a GST practitioner or the official GST portal before filing.