GST has been the law of the land for years, but every quarter we meet small business owners who still aren’t fully confident their bills are GST-compliant. This guide is a plain-English walkthrough of what GST billing means in practice, what fields a valid tax invoice needs, and how good billing software takes most of the complexity off your plate.
Nothing here is legal advice — for filings and edge cases, talk to your CA. But the basics are clear enough that no shop owner needs to be in the dark.
What “GST-compliant” actually means
A bill is GST-compliant when it is structured the way the law expects, with all mandatory fields visible, and the tax math reconciles with what you report at filing time. For a registered small business, that means:
- You issue a tax invoice for B2B sales and (above the threshold) for B2C sales
- Each item has an HSN code (for goods) or SAC code (for services)
- Tax is split into CGST + SGST for intra-state sales and IGST for inter-state sales
- The invoice has a unique, sequential number
- Both your GSTIN and (for B2B) the buyer’s GSTIN are printed
- The place of supply is shown
Get those right, every time, and you’re largely covered.
The mandatory fields on a tax invoice
Here’s what every tax invoice should have:
- Supplier’s name, address and GSTIN
- A consecutive, unique invoice number (up to 16 characters)
- Date of issue
- Recipient’s name, address and GSTIN (for B2B)
- HSN/SAC code for each item
- Description, quantity and unit of each item
- Total value, and taxable value (after discounts) per item
- Rate of tax (CGST, SGST, IGST, UTGST, cess as applicable)
- Amount of tax charged in each category
- Place of supply (state name & code)
- Whether tax is payable on reverse charge
- Signature (digital or physical)
It looks like a lot. In practice, good software fills 90% of these automatically from your settings, your customer record and your product catalogue.
HSN/SAC codes: how to handle them
Every product (goods) gets an HSN code; every service gets a SAC code. The number of digits required depends on your turnover — smaller businesses can get away with 2 or 4 digits, larger ones need 6 or 8. The exact threshold changes occasionally, so check current rules with your CA.
Practically: enter the HSN/SAC once when you set up a product or service, and the system prints it on every invoice. Don’t do it bill-by-bill. You’ll forget, and the bills will be inconsistent.
Intra-state vs inter-state: CGST/SGST vs IGST
This trips people up. The rule is simple:
- If the buyer is in the same state as you — split GST into CGST + SGST (e.g. 9% + 9% = 18%)
- If the buyer is in a different state — charge IGST (e.g. 18%)
Software should detect this from the customer’s state and split tax automatically. You should never have to think about it during a bill.
Retail receipts vs tax invoices
For most over-the-counter retail sales below the threshold, a simple retail receipt (also called a Bill of Supply for exempt items, or a simple receipt for small B2C) is enough. For B2B sales or larger B2C, you issue a full tax invoice.
Your software should let you switch between these formats with a single setting — ideally per bill. Don’t lock yourself into one format.
Discounts and tax: the order matters
This is a subtle one. If you give a discount before tax, the taxable value drops and so does the GST. If you give the discount after tax, the customer pays full GST and you absorb the rest. Both are legal, but you must be consistent and the invoice must show clearly which one you did.
Good software handles this transparently — the line item shows price, discount, taxable value, and tax separately. Don’t use tools that hide the math.
Reconciliation: where things go wrong at filing time
At GSTR-1 / GSTR-3B filing time, your CA reconciles your sales summary against the invoices you raised. The two have to match. Common reasons they don’t:
- Bills edited or deleted after the fact, without trail
- Mixed tax rates in one bill, computed inconsistently
- Returns and credit notes not recorded properly
- Wrong customer state, leading to wrong CGST/SGST vs IGST
Software with an immutable bill history, proper credit notes and automatic state-based tax routing makes filing a non-event. Software without it makes filing a fire-drill.
The shortcut: pick GST-native software
You can manage GST in a generic invoicing tool — but you’ll spend hours customising templates and double-checking math. India-built billing software handles HSN/SAC, GSTIN, CGST/SGST/IGST and tax invoice formats by default. DiraFlow is one example. The point isn’t which tool — it’s that you shouldn’t be doing GST math by hand in 2026.
Get the software right once, and GST stops being a thing you worry about.