GST Calculation After the 2025 Reform: Forward, Reverse, Split

A shopkeeper quotes ₹11,800 for a product taxed at 18%. A customer who wants the GST amount multiplies 11,800 by 18% and gets ₹2,124. The bill actually carries ₹1,800 of GST. The customer overstated it by ₹324, and the reason is one wrong assumption about what the ₹11,800 is. Most GST errors on invoices are this kind: the arithmetic is fine and the base is wrong.

This post covers how to calculate GST in both directions, how the tax splits between the Centre and the state, and the mistakes that show up most often on invoices. Every figure was computed in code rather than in my head, so you can check them with the GST calculator.

The rates you are actually working with now

The 56th GST Council meeting on 3 September 2025 restructured the rates, and the new structure took effect on 22 September 2025. The main slabs are now 0%, 5%, 18% and 40%. The old 12% and 28% slabs were largely removed: most items that sat at 12% moved to 5%, most at 28% moved to 18%, and a short list of sin and luxury goods, such as tobacco, aerated drinks and large cars, moved to 40%. Gold keeps a special 3% rate, and rough diamonds 0.25%. Premiums on individual life and health insurance were exempted.

Two words in that summary matter: "largely" and "most". Some items did not follow the pattern, and the rate on a specific product depends on its classification, not on a rule of thumb. For a given item, look up its HSN or SAC code against the current rate notification. A calculator can only tell you what 18% of a number is. It cannot tell you that your product is at 18%.

Exclusive price: the easy direction

You have a price before tax and you want the invoice total. GST is the price times the rate.

• Price ₹48,000 at 18%: GST = 48,000 × 18 / 100 = ₹8,640. Invoice total ₹56,640.
• Price ₹1,250 at 5%: GST = ₹62.50. Total ₹1,312.50.
• Price ₹1,00,000 at 40%: GST = ₹40,000. Total ₹1,40,000.

Nothing tricky here. The only way to go wrong is to apply the rate to something that is not the pre-tax price, which is the subject of the next section.

Inclusive price: where the 324 rupees went

When the displayed price already contains GST, the tax is not 18% of that price. It is 18% of the smaller, pre-tax amount hiding inside it. The pre-tax amount is the inclusive price divided by (1 + rate/100).

For ₹11,800 at 18%: base = 11,800 / 1.18 = ₹10,000. GST = 11,800 − 10,000 = ₹1,800. Check it forwards: 10,000 × 18% = 1,800, and 10,000 + 1,800 = 11,800. It closes.

Here are two more, with the wrong shortcut beside each, because people use these shortcuts.

• ₹2,36,000 inclusive at 18%. Base ₹2,00,000, GST ₹36,000. The shortcut of taking 18% of 2,36,000 gives ₹42,480, which is ₹6,480 too much.
• ₹5,900 inclusive at 5%. Base = 5,900 / 1.05 = ₹5,619.05, GST ₹280.95. If someone assumed this price was at 18% instead, they would get a base of ₹5,000 and GST of ₹900. Same number, wrong rate, ₹619 of difference in the tax shown.

A related mistake is subtracting 18% from the inclusive price to get the base. That gives 11,800 − 2,124 = ₹9,676, when the true base is ₹10,000. A good test: whatever base you compute, multiply it by (1 + rate) and see whether you land back on the inclusive price. If not, something is off.

The same invoice, split into CGST, SGST and IGST

The rate does not change with the direction of the sale, but the labels do. If the seller and the place of supply are in the same state, the tax is charged as CGST plus SGST, split equally. If they are in different states, it is charged as IGST at the full rate.

Take that ₹48,000 sale at 18%, total GST ₹8,640:

Sale	CGST	SGST	IGST	Total GST
Within one state	₹4,320 (9%)	₹4,320 (9%)	none	₹8,640
Between states	none	none	₹8,640 (18%)	₹8,640

The buyer pays the same. What changes is which ledger the seller credits and which return line it lands in. A 5% rate splits as 2.5% and 2.5%. A 40% rate splits as 20% and 20%. When a software tool shows a half rate that looks odd, such as 2.5%, that is nothing wrong. It is half of 5%.

Mistakes I would check for on any invoice

1. The old 12% or 28% rate on a new invoice. Billing software that was set up years ago may still carry the old rate against an item. Say a seller bills ₹50,000 of goods that now sit at 5%. If the template still says 12%, GST is shown as ₹6,000 instead of ₹2,500, a ₹3,500 overcharge. Go the other way and an item that moved up to 40% but is billed at the old 28% on ₹1,00,000 undercharges by ₹12,000, which the seller may have to pay out of pocket later. After a rate change, update the item master and check the first few invoices by hand.

2. Applying the rate to the wrong base. This is the ₹11,800 example. It happens in both directions: multiplying an inclusive price by the rate, or dividing an exclusive price by 1.18. Before you calculate, decide whether the number in front of you contains tax already.

3. One rate for a mixed invoice. If one bill carries a 5% item and an 18% item, each line is taxed at its own rate. Averaging them is wrong even if it lands close.

4. Rounding at the wrong place. Section 170 of the CGST Act says amounts of tax payable are to be rounded off to the nearest rupee, with 50 paise or more rounded up and less than 50 dropped. What I could not verify is a single rule for how each invoice line should be rounded before it is totalled, so I will only show you why it matters. Ten lines of ₹10.25 at 18% each carry ₹1.845 of tax. Rounded line by line to two decimals, that is ₹1.85 per line and ₹18.50 in total. Computed once on the invoice total of ₹102.50, it is ₹18.45. A difference of five paise, but a seller who files with the first and a buyer who books the second will see a mismatch. Pick one method in your software, use it consistently, and ask your accountant which they expect.

5. Showing GST on a supply that is not taxed. With individual life and health insurance premiums now exempt, a quote that still lists 18% for them is stale. Same for any item you assume moved to 0%: confirm it before you remove the tax.

The cut-over date is not the whole story

Invoices dated after 22 September 2025 use the new rates. Invoices for supplies made before that date follow the old ones. The hard cases are the ones that straddle it, such as an advance taken in August with delivery in October. The rate there depends on time-of-supply rules, and I am not going to give you a one-line answer, because I have not verified one. If you hold a transaction like this, the GST notification or a chartered accountant is the right source.

A quick way to sanity-check any figure

Before you trust any number, do a rough check with the rate fractions. At 18%, GST on an exclusive price is a bit less than a fifth of it. On an inclusive price it is about 15.25%, because 18/118 is 0.1525. At 5% inclusive it is 4.76%. Those three percentages, 18%, 15.25% and 4.76%, cover most everyday checks. If you are looking at an inclusive bill of ₹11,800 and your GST figure is ₹2,124, ₹2,124 is 18% of the inclusive price, well above the roughly 15.25% it should be, so the base was wrong.

If percentage arithmetic still feels slippery, the guide to percentage mistakes covers the base-confusion problem in other settings. For GST returns and invoices that carry real money, use the calculator to cross-check, but let your accountant decide on classification and the date question.