GST Calculator

For eight years GST had four rates — 5, 12, 18 and 28 — and almost everyone who deals with invoices learned them as a set. On 22 September 2025 that set stopped existing. The 56th GST Council meeting rationalised the structure into two working rates, a 5% merit rate and an 18% standard rate, with a special 40% rate for a short list of de-merit goods. Around 375 goods and services were repriced in the process.

Twelve per cent is the one that catches people out, because it did not simply become something else. Most items that sat at 12% moved down to 5%, but a number of them moved up to 18%, so you cannot guess the new rate from the old one. Twenty-eight per cent mostly collapsed into 18% — small cars are the widely quoted example. Tobacco was held back: it stayed at 28% plus compensation cess until the Centre had finished repaying the loans it took to compensate states. From 1 February 2026 cigarettes, pan masala and chewing tobacco moved to 40% and bidis to 18% — not a single blanket rate, which is worth checking before you assume. Compensation cess had already been discontinued for every other category from 22 September 2025.

The practical consequence is not academic. On a ₹10,000 base, still billing at 12% collects ₹1,200 of tax. If the item is now a 5% item you have charged your customer ₹700 too much, and they will claim input credit that does not match the rate notification. If it is now an 18% item you have collected ₹600 less than you owe, and the shortfall is yours to make up. This calculator used to offer 12% and 28% as buttons; it no longer does, and it will tell you if you type either from memory.

A tax-inclusive figure is not the base plus a percentage of itself — it is the base plus a percentage of the base. So working backwards means dividing by one plus the rate, not subtracting the rate. A ₹1,180 bill at 18% breaks into ₹1,000 and ₹180. Take 18% off ₹1,180 and you get ₹967.60, which is ₹32.40 short and would understate the tax on every line of a return.

At 5% the error is small enough that people get away with it for years. At the higher rates it stops being a rounding annoyance:

Seeing two tax lines instead of one convinces a lot of first-time invoice readers that a local purchase is being taxed twice. It is not. An 18% intra-state supply is 9% CGST plus 9% SGST; an 18% inter-state supply is 18% IGST. Same rate, same total, same amount leaving the buyer's account. The split only decides who receives it — the Centre and the state where the supply happened, or the Centre alone, which then apportions IGST to the destination state.

Where it does matter is credit. CGST, SGST and IGST sit in separate ledgers with rules about which can be set off against which, so a business buying mostly inter-state and selling mostly intra-state can end up with credit in the wrong pocket. That is a filing problem rather than a pricing one, and it is the reason place of supply is worth getting right on the invoice even though the customer pays the same either way.

Which of the two applies is decided by the place of supply, not by where you happen to be sitting. A service delivered to a client in another state is inter-state even if you never leave your desk.

Section 170 of the CGST Act requires tax, interest, penalty and any other amount payable to be rounded to the nearest rupee. That sounds trivial until you notice that on an intra-state supply CGST and SGST are two separate levies, each rounded on its own. GST of ₹152.54 does not become ₹76.27 twice — it becomes ₹76 of CGST and ₹77 of SGST, and the two lines on your invoice are a rupee apart for no reason the customer can see. The breakdown above shows the exact figures and the rounded ones side by side so you can tell which discrepancy you are looking at.

The rest of the gaps between this calculator and a real invoice come from things upstream of the tax calculation. Accounting software commonly rounds each line item before totalling rather than rounding the total, and the two orders do not always agree. If a whole invoice is out by a rupee or two, that is usually where it came from, not from the rate.

None of this is tax advice, and rates change by notification — the September 2025 rationalisation is the clearest possible demonstration of that. Confirm the rate against the current notification for your HSN or SAC before you file anything. The amounts you type here stay in your browser; they are not sent to ToolMintX.