How to calculate GST and remove it from a bill
Adding GST to a price is easy. Taking it back out of a total is where most mistakes happen, and a wrong method can leave your base amount out by hundreds of rupees. This guide shows both directions with real numbers, and how to check a bill. Updated 21 October 2026.
What GST is on a bill
GST is charged as a percentage of the base amount, which is the price of the goods or service before tax. The buyer pays the base amount plus the tax, and the bill shows all three lines: the base, the tax, and the total. Everything else about GST arithmetic follows from that one fact, and nearly every mistake comes from forgetting it.
Adding GST to a price
Adding is the easy direction. With the base amount and the rate:
- GST = base × rate ÷ 100
- Total = base + GST, which is the same as base × (1 + rate ÷ 100)
The same ₹1,000 base gives a different bill at each rate:
| Rate | Base | GST | Total on the bill |
|---|---|---|---|
| 5% | ₹1,000 | ₹50 | ₹1,050 |
| 18% | ₹1,000 | ₹180 | ₹1,180 |
| 40% | ₹1,000 | ₹400 | ₹1,400 |
If a price is quoted “plus GST”, it is a base amount and you add the tax on top. If it is quoted “inclusive of GST”, the tax is already inside it, which is the case below.
Removing GST from a total
This is where people go wrong. Given a bill total, the base amount is total ÷ (1 + rate ÷ 100), and the GST is whatever is left over. You divide. You do not subtract the percentage.
Take a bill of ₹2,360 that includes 18% GST:
- Correct: 2,360 ÷ 1.18 = ₹2,000 base, so the GST is ₹360.
- Wrong: taking 18% off the total, 2,360 × 0.82 = ₹1,935.20. That is ₹64.80 short of the true base.
The subtraction fails because the 18% was charged on the base, not on the total. The total is bigger than the base, so 18% of the total is more than the tax that was actually charged. A quick check for any answer: multiply your base by the rate, add it back, and you should land exactly on the bill.
How much of an inclusive bill is tax
Because the tax sits on top of the base, it is a smaller share of the total than the headline rate suggests. On a GST-inclusive bill the tax share is rate ÷ (100 + rate):
| Rate | Share of the total that is GST |
|---|---|
| 5% | 4.76% (50 of 1,050) |
| 18% | 15.25% (180 of 1,180) |
| 40% | 28.57% (400 of 1,400) |
So “the price includes 18% GST” means a little over 15% of what you pay is tax, not 18%.
CGST, SGST and IGST
The rate does not depend on where the buyer is. Only the way it is written on the bill does. When the seller and the buyer are in the same state, the tax is split in half: for 18% on a ₹1,000 base that is ₹90 CGST and ₹90 SGST. When the sale crosses a state border the whole ₹180 appears as one IGST line. The buyer pays the same total either way.
Which rate applies
After the September 2025 revision, most goods and services sit at 5% or 18%, exempt items at 0%, and a short list of luxury and harmful goods at 40%. The older 12% and 28% rates still appear on invoices raised before the change.
The rate belongs to the item, through its HSN code for goods or its SAC code for services, and rates are revised from time to time. This guide explains the arithmetic. It cannot tell you which rate your product attracts, so confirm that with the official rate finder or your accountant before you put a figure on a real invoice.
A checklist for checking a bill
- For each line, multiply the taxable amount by the rate. It should equal the tax shown on that line.
- If the seller and buyer are in the same state, look for equal CGST and SGST lines. If not, look for a single IGST line. Seeing both kinds on one line is a red flag.
- Add the lines up. The total should equal the base plus all the tax. A difference of a few paise is normal, because tax is rounded on each line.
- If a discount is printed on the invoice, it reduces the base before the tax is added, so the tax is charged on the smaller figure.
- For a business purchase, make sure the bill carries the seller’s GSTIN, since you will need it to claim input tax credit.
A bill you are unsure about is worth a conversation with the seller or your accountant. A guide can check the arithmetic, not the facts behind the bill.
Work it out on your own figures
The GST Calculator adds or removes GST at any rate, shows the working under the answer, and splits the tax into CGST and SGST or a single IGST line. It runs in your browser, so nothing you type is uploaded.