GST for Small Businesses: Rates, Invoices and the Reverse Calculation

GST is conceptually simple: a tax on the supply of goods and services, collected at each stage of the chain. In practice, most of the errors small businesses make are not about which rate applies, but about direction, base and split. Get those three right and the rest follows.

The slabs, in one place

India uses four main GST slabs, plus two special rates:

• 0.25% — rough diamonds
• 3% — gold and silver jewellery
• 5% — essentials such as packaged food, transport and restaurants
• 12% — processed goods and some services
• 18% — the default for most goods and services
• 28% — luxury and demerit goods

Guessing between 12% and 18% is a common and expensive habit. The rate depends on the HSN code for goods or the SAC code for services, and both are published. Check the code once, note it against the product and you never have to guess again.

Direction: the most common error

An exclusive price is the value of what you sold, with tax added on top. An inclusive price already contains the tax. If you sell something worth ₹1,000 at 18%, the exclusive calculation gives ₹1,180. But if a customer has agreed to pay ₹1,180 all in, the taxable value is ₹1,000 and the GST is ₹180 — you do not add 18% on top of ₹1,180.

To reverse an inclusive amount, divide by 1.18. Subtracting 18% instead gives ₹967.60, which understates the taxable value and overstates the tax. On a month of invoices, that single error can throw out your entire GSTR-1 reconciliation. Our GST calculator has a separate mode for each direction, so the two never get mixed up.

Split: CGST and SGST, or IGST

Where the buyer is in the same state as you, the tax splits equally into CGST and SGST — 9% each on an 18% supply. Where the buyer is in another state, or the supply is an import, a single IGST at the full rate applies. The total tax is identical; only the destination of the money changes.

This matters because the split determines what you report where. Getting it wrong does not change what you collect, but it does change how the return should look, and mismatches are a common reason notices are issued.

Discounts and the taxable base

A trade discount is normally deducted before GST is calculated. If you sell at ₹1,000 with a 10% trade discount, the taxable value is ₹900 and GST is charged on ₹900. Applying the discount after tax overstates the tax on the invoice and creates a mismatch with what the buyer expects to claim as input credit.

Cash discounts — a small reduction for prompt payment — behave differently depending on how they are documented and whether they are known at the time of supply. This is one of the few areas where professional advice is worth the money.

Input tax credit: what it is worth

GST paid on business purchases can be set off against GST collected on sales. That is the mechanism that stops tax from piling up at every stage. Keep the following in mind:

• Only GST on purchases used for business qualifies.
• The supplier must have filed their return and reported the invoice, or the credit will not appear for you.
• Some purchases are blocked from credit entirely.
• Composition dealers cannot charge GST or claim input credit, so if you invoice for one, do not add GST.

A simple invoicing checklist

• Confirm the HSN or SAC code and the rate for each line item.
• Decide whether your price is inclusive or exclusive, and be consistent.
• Apply any discount before calculating the tax.
• Split correctly into CGST and SGST for intra-state, IGST for inter-state.
• Show your GSTIN and the buyer GSTIN where required.
• Reconcile your invoices against GSTR-1 before filing, not after.

What actually goes wrong each month

Three patterns cause most of the pain. Sales invoices are recorded with inclusive amounts treated as exclusive. Credit notes and returns are not adjusted, so reported turnover is higher than actual. And invoices below the threshold that still carry GST create a mismatch with the buyer credit.

None of these are complex problems, and all of them are avoidable with a consistent process: the same rate table, the same direction, the same split rules, every invoice. Run the numbers through the GST calculator before raising the invoice rather than after, and the monthly return becomes a formality instead of a repair job.