GST Calculator

GST arithmetic is simple in one direction and easy to get wrong in the other. Adding tax to a base price is a multiplication most people can do in their head. Extracting the tax already contained in a final price is not, and subtracting 18% from an inclusive amount — the intuitive move — gives the wrong answer every time.

This calculator works in both directions at every notified slab, at any custom rate you type, and shows the tax split the way the invoice has to state it: CGST and SGST for a supply within a state, a single IGST line for one that crosses a state border.

Everything runs in your browser. No figure you enter is uploaded, stored or logged, which matters when the numbers are somebody's invoice.

Base amount = inclusive amount x 100 / (100 + rate). The GST is whatever is left over.

For an item priced at 1,180 rupees inclusive of 18% GST, the base is 1,180 x 100 / 118 = 1,000 and the tax is 180. Subtracting 18% of 1,180 instead gives 967.60 — wrong by more than 3%, and it will not reconcile against any invoice.

The same formula is what makes reverse-calculating a supplier invoice worthwhile. Enter the invoice total, choose the slab that should apply, and compare the extracted tax against what the invoice states, because that figure is what you claim as input tax credit. A mismatch is worth raising before you file.

It is decided by the place of supply, not by where your office is or where the payment comes from. If the supplier's location and the place of supply are in the same state, the levy is intra-state and splits evenly between the centre and the state: an 18% supply is 9% CGST and 9% SGST, on two separate invoice lines. If they are in different states, it is inter-state and the whole 18% is charged as one IGST line, collected centrally and apportioned later.

The customer pays exactly the same amount either way, which is why the toggle here changes the labels and the line items but never the total. What it does change is what you can do with the credit afterwards, and what a mislabelled invoice costs to fix.

The place of supply is defined by statute and is not always the delivery address — bill-to and ship-to differing, services, and goods supplied on a vessel or an aircraft all have their own rules. This calculator does the arithmetic for whichever you select; deciding which one applies is a question for the transaction, not for a tool.

The rate depends on the HSN or SAC code of what is being supplied, not on the amount. Essentials and many food items sit at 5%, a large middle band of goods and most services at 18%, and a small set of luxury and demerit goods at 28% plus cess.

The custom rate field exists for the cases the four buttons cannot cover. Compensation cess stacks on top of the slab, so a combined effective rate on a demerit good can be well above 28%. Some supplies are notified at rates outside the four — and slabs are revised from time to time, so a rate that is current when you read this may not be the one on the invoice you are checking.

This calculator does not classify goods — that is a decision for your invoice, and getting it wrong is a compliance problem no tool can fix. It computes the arithmetic accurately once you know which rate applies.

Under the reverse charge mechanism the liability moves from the supplier to the recipient: the recipient pays the tax to the government directly instead of paying it to the supplier as part of the price. It applies to certain notified supplies, and to purchases from unregistered suppliers in the cases the law specifies.

The arithmetic is untouched. Same base, same rate, same tax, same split between IGST and CGST plus SGST — every figure this calculator shows is the figure that applies. What changes is the paperwork and the cash flow: the supplier issues the invoice without collecting the tax and states that GST is payable on reverse charge, and the recipient deposits that amount in cash rather than settling it out of accumulated input tax credit.

The credit itself is not lost. Once paid, the recipient claims it in the normal way if the supply is eligible, which is why reverse charge is usually a timing and compliance burden rather than a cost — provided it is identified and paid on time.