GST billing for small businesses: the complete guide

What does GST billing involve for a small business?

GST billing is the work of turning every sale into a tax invoice that is correct on the day it is issued and correct again when it is reported in a return. It has five parts, and each one depends on the one before it.

1. 1

   Register when you cross the threshold

   Get a GSTIN, which fixes your state and your legal name on every invoice.

2. 2

   Set up products and customers

   Each product carries its HSN or SAC code and tax rate. Each registered customer carries a GSTIN, which gives their state.

3. 3

   Issue tax invoices

   Mandatory fields, one numbering series per year, CGST and SGST or IGST by place of supply.

4. 4

   Correct with notes, never by editing

   A credit note or debit note changes an issued invoice; the original stays as it was.

5. 5

   Report from the registers

   The sales register, purchase register and HSN summary are the books your returns are filed from.

The rules below are stated as of October 2026. Each section links to a deeper guide in this series, and the official sources are listed at the end.

When does a small business need GST registration?

In most states, a business must register for GST once its aggregate turnover in a financial year crosses ₹40 lakh for goods or ₹20 lakh for services. Special-category states have lower limits, so check the figure for your state.

Some businesses must register whatever their turnover, and many register early by choice because registered buyers prefer suppliers who can pass on input tax credit. Your chartered accountant can tell you which case you fall in. Once registered, every taxable sale needs a tax invoice.

What must every GST tax invoice show?

A tax invoice must show the supplier's and the registered buyer's names, addresses and GSTINs, a unique invoice number and date, the place of supply, the HSN or SAC code, description, quantity, taxable value, the rate and amount of each tax, the total, and the signature.

The full list, with an example value for each field, is in the GST invoice format and requirements guide. Two of those fields decide most of what goes wrong later: the tax type and the code on each line.

How do you choose between CGST and SGST or IGST?

Place of supply decides the tax type: a supply that stays within the supplier's state carries CGST and SGST in equal halves, and a supply that crosses a state line carries IGST.

• Goods: the place of supply is generally where the movement of the goods ends.
• Services to a registered business: generally the recipient's location, with exceptions such as immovable property and events.
• State from the GSTIN: the first two digits of a GSTIN are the state code, so 33 means Tamil Nadu and 29 means Karnataka.

Generally, charging the wrong head is not fixed by paying the difference: the tax paid under the wrong head is refunded and the right one paid again. Confirm the route with your chartered accountant. The place of supply guide works through six common cases, including bill-to ship-to and buyers with no GSTIN.

Which HSN or SAC code goes on each line?

HSN codes classify goods and SAC codes classify services, and as of October 2026 a business with aggregate annual turnover up to ₹5 crore shows at least 4 digits, while one above ₹5 crore shows 6.

The code belongs on the product, not on the invoice. Set it once and every line inherits it, which keeps the HSN summary in GSTR-1 table 12 consistent with your invoices. Table 12 now has separate B2B and B2C tabs and takes the code from a dropdown (GSTN advisory, 11 April 2025). The HSN code guide shows how the summary rolls up from invoice lines.

What changed in GST rates in 2025?

GST moved to two main rates, 5% and 18%, with 40% for specified luxury and sin goods, effective 22 September 2025. The 56th GST Council meeting on 3 September 2025 removed the 12% and 28% slabs and discontinued compensation cess.

For billing, this means checking the rate on every product against the CBIC notification for its item, because the slab a product moved to is set item by item. The GST rate changes 2025 guide covers which rate applies to invoices dated either side of the change and what to do with drafts and old-rate credit notes.

What are the rules for numbering GST invoices?

An invoice number can be up to 16 characters, using letters, numerals, hyphens and slashes, and must be unique within a financial year (CGST Rules, rule 46(b)). You may run one series or several, such as one for invoices and another for credit notes.

• Put the financial year in the prefix, so numbers restart cleanly each April without clashing: INV-2026-27-0001
• Count the characters: INV-2026-27-0001 is exactly 16
• Never reuse a number, even for a cancelled invoice; the cancelled number stays in the series
• Keep a separate series for credit notes and debit notes so each document type is easy to trace
• Do not leave gaps you cannot explain; a missing number invites questions

When do you issue a credit note or a debit note?

A credit note reduces the value or tax of an invoice already issued, for example when goods come back or a discount is given after the sale, and a debit note increases it.

As of October 2026, a credit note must be declared by 30 November following the end of the financial year, or by the date the annual return is filed, whichever is earlier (CGST Act, section 34). Notes appear in GSTR-1 in the CDNR table for registered buyers and CDNUR for unregistered ones. The credit note and debit note guide works through a 10-chair return with its journal and stock movement.

Which registers feed GSTR-1, GSTR-3B and the GSTR-2B check?

Your returns are filed from three registers: the sales register, the purchase register and the HSN summary. If those three agree with your invoices and bills, the returns follow.

GSTR-2B is the statement of input tax credit drafted from what your suppliers reported. Many businesses claim credit only for bills that appear in it, so matching it to the purchase register each month matters. The GSTR-2B reconciliation guide shows the four outcomes of a match and what to do with each.

How does one invoice become a GSTR-1 row?

Each field of a B2B invoice maps to a column in GSTR-1. Here is invoice INV-2026-27-0213 from Lakshmi Steel Furnitures, the fictional Coimbatore business on this site, to Ravi Stores, also in Coimbatore: 2 steel racks 4 shelf at ₹4,990 each, at 18%.

The same lines also roll into the HSN summary. If this were the only sale of the month, the table 12 row on the B2B tab would read:

Taxable value and tax are generally reported as invoiced, before round off; only the invoice value carries the rounded total. When the ledger, the sales register and these rows all start from the same posted invoice, they cannot disagree.

When does e-invoicing apply to a small business?

As of October 2026, e-invoicing is mandatory for a business whose aggregate turnover exceeded ₹5 crore in any financial year from 2017-18 onwards, a rule in force since 1 August 2023. B2C invoices are excluded.

E-invoicing means reporting each B2B invoice to the invoice registration portal, which returns an IRN and a signed QR code. Larger taxpayers must report within a 30-day window; check the current limit on the e-invoice portal. Prepare a year before you expect to cross ₹5 crore: clean HSN codes, a GSTIN on every B2B customer and one invoice series. The e-invoicing threshold guide covers the details.

Is the composition scheme right for a small business?

The composition scheme is an option for smaller businesses that pay GST at a flat percentage of turnover and file simpler returns, in exchange for giving up some things a regular taxpayer has. Eligibility depends on turnover and the kind of business, and some activities are excluded.

The trade-off is usually that a composition business cannot charge GST on its bills or claim input tax credit, so registered buyers cannot claim credit on what they buy from it. That suits a shop selling mostly to consumers and rarely suits a business selling to other businesses. Decide with your chartered accountant; this guide assumes regular registration and tax invoices.

What should you check at the end of every month?

A short month-end routine catches most errors while they are still cheap to fix, before they reach a filed return.

• Every sale of the month has a posted invoice; no drafts are left that should have been issued
• The invoice series has no unexplained gaps, and cancelled numbers are accounted for
• Every B2B invoice carries the buyer's GSTIN, and the place of supply matches the state in it
• Inter-state invoices carry IGST and intra-state invoices carry CGST and SGST
• Every line has an HSN or SAC code, and the HSN summary total equals the sales register total
• Returns, discounts and price corrections have credit or debit notes, not edits
• The purchase register has been matched to GSTR-2B and mismatches are listed for follow-up
• Output tax in the sales register less eligible input tax agrees with the GST ledger balances

How does SimpleERP handle GST billing?

In SimpleERP, posting an invoice writes the journal, the stock issue, the receivable and the GST in one step, and the tax type is chosen by place of supply, with the customer's state read from their GSTIN.

• The HSN or SAC code and tax rate are set on the product and copied to each line, where they can be changed.
• One invoice can carry lines at different rates.
• Invoices are numbered by financial year, INV-2026-27-0001, exactly 16 characters.
• Posted invoices cannot be edited. Corrections go through credit and debit notes, and cancellation reverses rather than deletes, with a full audit history.
• Sales and purchase registers and an HSN summary report are ready for GSTR-1 and the GSTR-2B check, with CSV export.
• Tax rates are editable, so a rate change is an update to the product, not a new system.
• Invoices can carry a UPI QR code so customers can pay from the printout.

See it on the GST billing software page. SimpleERP does not calculate cess.