How to Calculate GST: Formulas and Worked Examples

There are only two calculations, and everything else in this article is about feeding them the right numbers. To add GST to a base price: GST amount = (Original cost × GST rate) / 100, and Net price = Original cost + GST amount. To pull the tax back out of a price that already includes it: GST amount = Original cost − [Original cost × (100 / (100 + GST rate))], which is the same thing as saying the base value equals the inclusive amount divided by (1 + rate/100). If you want the arithmetic done for you, the GST calculator handles the forward direction and the reverse GST calculator extracts tax from an inclusive figure.

The two formulas you actually need

Take a base price of 10,000 and assume a notional rate of 18%. The forward calculation is 10,000 × 18 / 100 = 1,800, and the net price is 10,000 + 1,800 = 11,800. At a notional 5% the same base gives 10,000 × 5 / 100 = 500 and a net price of 10,500. Nothing about this changes with the size of the transaction; it is a straight proportion.

The reverse calculation is where people go wrong. Given an inclusive 11,800 and a notional 18% rate, the base is 11,800 × (100 / 118) = 10,000, so the tax is 11,800 − 10,000 = 1,800. The more compact form is to divide by 1.18. A useful sanity check: the tax component of an 18% inclusive price is not 18% of that price but 18/118, about 15.25% of it. At a notional 5% the fraction is 5/105, about 4.76%.

Both formulas assume you already know the correct rate. That is the genuinely hard part, and it depends on the classification of the item rather than on any calculation. Look the item up in the HSN code list with GST rates, and read how the current rate slabs are structured before you assume a figure. Every worked example below uses a notional rate purely to demonstrate the arithmetic — confirm the rate actually notified for your goods or services before you put it on an invoice.

Exclusive or inclusive: which price were you quoted

An exclusive price is the value of the supply before tax; GST is added on top and shown as a separate line. An inclusive price already contains the tax, and the base value has to be extracted from it. The same rupee figure means two different things depending on which convention is in play, and a 10,000 quote is worth 10,000 or 8,475 to the seller depending on the answer.

In practice, B2B quotations in India are almost always exclusive, because the buyer recovers the tax as input credit and only cares about the base. Retail prices, restaurant menus in some formats, and anything printed with an MRP are inclusive. When a quotation is ambiguous, three things settle it: the phrase used (plus GST, exclusive of taxes against inclusive of all taxes), whether the seller is registered at all, and whether the figure has the shape of a round base price. A quote of 11,800 is almost certainly 10,000 plus 18% tax; a quote of 10,000 flat is almost certainly exclusive.

Splitting the tax into CGST and SGST, or a single IGST

The rate you look up in a notification is the total rate. How it appears on the invoice depends on whether the supply is intra-state or inter-state, which is settled by comparing the place of supply with the location of the supplier — see the place of supply rules for how that determination is made when goods move or services are delivered remotely.

On an intra-state supply, the total rate is split down the middle. Each half is the full rate divided by two, and both halves are computed on the same taxable value. CGST is not calculated on a value that includes SGST or the other way round; they are parallel levies on one base. At a notional 18% on a base of 10,000, that is 9% CGST of 900 and 9% SGST of 900, totalling 1,800. At a notional 5% on the same base, it is 2.5% CGST of 250 and 2.5% SGST of 250, totalling 500.

On an inter-state supply there is no split. A single IGST line carries the full rate: 1,800 at a notional 18%, or 500 at a notional 5%, on the same base of 10,000. The arithmetic total is identical, which is the point of the design — the tax burden does not change with the direction of the supply, only the way it is divided between governments. Our guide to the difference between CGST, SGST and IGST covers why that split exists and how the credit flows behave differently.

What goes into the value you multiply by the rate

The formulas take a taxable value as their input, and Section 15 of the CGST Act decides what that value is. The starting point is the transaction value: the price actually paid or payable, where the supplier and recipient are unrelated and price is the sole consideration. Section 15(2) then adds several things back in.

• Taxes, duties, cesses and fees charged under any law other than the GST Acts, where they are charged separately by the supplier.
• Amounts the recipient was liable to pay which the supplier has incurred and not already included in the price.
• Incidental expenses — packing, commission, and anything the supplier does in respect of the supply at or before delivery and charges to the recipient.
• Interest, late fee or penalty charged for delayed payment of the consideration. This surprises people: the interest you charge a slow-paying customer carries GST at the rate of the underlying supply.
• Subsidies directly linked to the price, except subsidies given by the central or a state government, which stay out of the value.

Freight and insurance

Whether transport charges enter the value turns on what the contract says about delivery, not on how the line is labelled. If you have contracted to deliver at the buyer premises, transport and insurance are incidental to the supply of goods and form part of the value under Section 15(2), taxed at the rate applicable to the goods. Freight on a consignment of goods at a notional 5% is taxed at 5%, not at the rate that would apply to a standalone transport service. If the sale is ex-works and the buyer arranges collection, the freight never enters your value at all — the transport is a separate supply between the buyer and the transporter, often falling under reverse charge where a goods transport agency is involved.

Income-tax TCS

Tax collected at source under the Income-tax Act sits oddly across the two systems, and the asymmetry is deliberate. GST is not computed on income-tax TCS, because TCS is an interim levy on the buyer rather than a charge forming part of the consideration. Income-tax TCS, on the other hand, is generally computed on the amount received, which includes GST. So the two levies are calculated on different bases within the same invoice, and neither compounds on the other. Confirm the current position with your auditor, as the treatment has been clarified by circular rather than being obvious from the statute.

Calculating GST when a discount is involved

A discount shown on the face of the invoice is simply deducted before tax is computed. On a list price of 10,000 with a 10% trade discount, the taxable value is 9,000 and, at a notional 18%, the tax is 1,620 rather than 1,800. Section 15(3)(a) excludes a discount given before or at the time of supply where it has been duly recorded in the invoice, so it never enters the value in the first place.

A discount agreed after the supply is a different problem. A quarterly volume rebate, a year-end incentive or a price adjustment made once targets are met can only be excluded from value under Section 15(3)(b) if two conditions hold: the discount was established in terms of an agreement entered into at or before the time of supply and is specifically linked to the relevant invoices, and the recipient has reversed the input tax credit attributable to it. Miss either limb and the tax already charged stands, no matter how commercially real the rebate is. Where the conditions are met, the adjustment is made through a credit note against the original invoices rather than by recalculating them, which is why the linkage to specific invoice numbers matters so much.

The related timing question — what happens when money changes hands before the supply does — is covered separately in our note on GST on advance payments.

Working out the GST hidden inside an MRP

Maximum retail price is, by definition under the Legal Metrology packaged commodities framework, inclusive of all taxes. Adding GST to an MRP is not a rounding error, it is a legal impossibility, and yet it is one of the most common mistakes in retail billing. The reverse formula is the only correct approach.

Take a printed MRP of 499 and assume a notional 18%. The base price is 499 / 1.18 = 422.88. The tax component is 499 − 422.88 = 76.12. On an intra-state sale that appears as 38.06 CGST and 38.06 SGST; on an inter-state sale as a single IGST line of 76.12. The retailer margin, the distributor margin and the manufacturer price are all being carved out of that 422.88, not out of the 499.

The same logic applies to a discounted MRP. If the item sells at 449 against an MRP of 499, the taxable value is derived from what the customer actually pays: 449 / 1.18 = 380.51, with tax of 68.49. The discount reduces the consideration, and the tax follows the consideration. Feeding the figure into the reverse GST calculator is quicker than doing this at the till.

A worked multi-line invoice

Tax is computed line by line, at each item own rate, and only then totalled. It is never computed by applying a single blended rate to the invoice total. Here is an intra-state invoice with three items sitting in three different slabs, all rates notional.

Each line takes its own rate: 12,000 × 5% = 600, split into 300 CGST and 300 SGST; 8,500 × 12% = 1,020, split into 510 and 510; 25,000 × 18% = 4,500, split into 2,250 and 2,250. The tax columns are then added down, giving 3,060 CGST and 3,060 SGST on a taxable value of 45,500, and an invoice total of 51,620.

Applying the highest rate on the invoice to the whole taxable value would have produced 45,500 × 18% = 8,190 of tax against a correct figure of 6,120 — an overcharge of 2,070 on a single invoice, and a mismatch that surfaces later in reconciliation because the rate-wise summary in your return will not tie back to the invoice. The rate-wise breakup is a mandatory part of the tax invoice format precisely so this is visible.

The compounding trap, cess and rounding

Never subtract the rate from an inclusive price

The single most expensive arithmetic error in GST is reversing an inclusive price by subtracting the rate. Take the inclusive 11,800 from earlier. Subtracting 18% gives 11,800 × 0.82 = 9,676, and implies a tax of 2,124. The correct base is 10,000 and the correct tax is 1,800. The method is 324 out on one transaction, and it is wrong in a consistent direction, so it accumulates.

The reason is that the 18% was originally applied to 10,000, not to 11,800. Taking 18% off the larger number removes 18% of a base that already contains the tax — you are deducting tax on the tax. Multiplication by 1.18 is undone by division by 1.18, not by multiplication by 0.82. The two only coincide when the rate is zero.

Compensation cess

On notified goods, compensation cess is an additional line computed on the same taxable value as GST, not on the GST-inclusive amount. On a taxable value of 10,000 with a notional 18% GST and a notional 12% cess, you get 1,800 of GST and 1,200 of cess, for a total of 13,000. Cess never compounds on GST. It also sits in its own credit pool: cess credit can only be set off against cess liability, never against CGST, SGST or IGST. Check whether cess applies to your item, and at what rate, in the cess notification for that HSN.

Rounding

The CGST Act requires the tax amount to be rounded off to the nearest rupee, per invoice or other document. Where the amount contains paise, fifty paise or more rounds up to the next rupee and less than fifty paise is ignored. Round at the point of stating the tax payable on the document, not at every intermediate step — rounding each line separately and then adding produces a different total from computing precisely and rounding once, and a paise-level difference between your invoice and your return is enough to open a reconciliation query.

Frequently asked questions