About the gst calculator
GST arithmetic is easy in one direction and quietly wrong in the other. Adding tax to a price is a multiplication anyone can do. Stripping it out of a tax-inclusive figure is where people go astray, because the instinct is to subtract the rate — and subtracting 18% from an inclusive ₹118 gives ₹96.76 rather than the ₹100 you started with. The tax-inclusive amount has to be divided by 1.18, not reduced by 18%, and the gap widens as the rate does.
The slab structure changed materially on 22 September 2025. The 56th GST Council merged the 12% slab into 5% and 18%, and replaced the old 28%-plus-compensation-cess tier with a flat 40% for luxury and sin goods. Most of what used to sit at 12% moved down to 5%; most of the 28% band moved down to 18%. Anything written before that date — including a great many calculators still online — will quote the old five-tier list.
The 12% and 28% rates are still available here, because a credit note raised today against an invoice from before the changeover carries the rate that invoice was raised at. They are labelled as withdrawn so nobody reaches for them by accident on new billing.
Which taxes apply depends on whether the supply crosses a state line, not on how big it is. Within a state the tax splits into CGST and SGST, half to the centre and half to the state. Between states it is a single IGST at the full rate. The total is identical either way, so the split matters for your returns rather than for the customer's bill. Where the tax works out to an odd number of paise the two halves are not equal — one carries the extra paisa — because two rounded halves would not add back to the total.
Rates here are the headline slabs. Actual classification is by HSN or SAC code and there are plenty of items that sit somewhere unexpected, so check the CBIC rate finder for anything you are billing rather than assuming the obvious slab.