GST Calculation Formula: Add and Remove GST

The two GST formulas

Add GST:     GST = price x rate / 100
             total = price + GST

Remove GST:  price = total x 100 / (100 + rate)
             GST = total - price

Use the first when you know the price before tax, for example a rate you quote to a customer. Use the second when the amount already includes GST, such as an MRP or the total on a bill.

Example: adding 18% GST

An item costs ₹1,000 before tax and the rate is 18%. The GST is 1,000 x 18 / 100 = ₹180, so the customer pays ₹1,180.

Example: removing GST from a total

A bill says ₹1,180 including 18% GST. The price before tax is 1,180 x 100 / 118 = ₹1,000, and the GST is ₹180. For ₹500 including 5% GST: 500 x 100 / 105 = ₹476.19, so the GST in it is ₹23.81.

Do not take 18% off the total. 18% of ₹1,180 is ₹212.40, which would give ₹967.60. That is wrong, because the GST was 18% of the lower price, not of the total. Always divide by (100 + rate).

CGST and SGST, or IGST?

The rate stays the same; only the way it is split changes. When the buyer is in your own state, the GST is split in half: CGST goes to the centre and SGST to the state. When the buyer is in another state, the whole amount is IGST.

The calculator works out CGST and SGST separately at half the rate each and rounds each to the paisa, the way billing software does. That means the two halves can be 1 paisa away from the IGST figure. For ₹847.50 at 18%, CGST and SGST are ₹76.28 each (₹152.56 together), while IGST is ₹152.55. Both are correct.

When you remove GST from a total, the parts always add up to the total you typed. For ₹500 including 5%, the GST of ₹23.81 is split as CGST ₹11.91 and SGST ₹11.90.

GST on ₹2,500 at the main rates

The calculator has buttons for 0.25, 3, 5, 12, 18, 28 and 40%, and you can type any rate from 0 to 100. Since 22 September 2025 most goods and services are at 5% or 18%, with 40% on a few luxury and sin goods and special rates such as 3% on gold. The 12% and 28% buttons stay for bills made before that change. Which rate applies to your item depends on its HSN or SAC code, so check the CBIC website or ask your accountant. The official notification decides, not the calculator.

Adding up a bill with two rates

On a bill, the calculator groups the items by rate, works out the GST once for each rate, then rounds the grand total to the nearest rupee with a round-off line. Here is a bill for a buyer in the same state, with prices before GST:

5% group:   taxable ₹900.00    CGST ₹22.50   SGST ₹22.50
18% group:  taxable ₹1,405.50  CGST ₹126.50  SGST ₹126.50

Taxable value  ₹2,305.50
Total GST      ₹298.00
Total          ₹2,603.50
Round off      +₹0.50
Grand total    ₹2,604.00

Some billing software works out GST on each line instead of on each rate. Its total can then differ by a few paise. That is normal.

GST and your profit

The GST you collect is not your income. You pay it to the government after taking off the GST you paid on your purchases (input tax credit). So work out profit on prices before GST. If an item costs you ₹800 before GST and you sell it for ₹1,000 before GST, the profit is ₹200: a 20% margin on the selling price, or a 25% markup on the cost. The customer pays ₹1,180 at 18%. The Profit margin part of the calculator does this for you.

Common questions

How do I calculate 18% GST on ₹1,000? Multiply by 18 and divide by 100: ₹180. The total is ₹1,180, which is CGST ₹90 + SGST ₹90 in your state, or IGST ₹180 for another state.

What is the reverse GST formula? Price before GST = total x 100 / (100 + rate). For ₹1,180 at 18%, that is ₹1,000.

Does the calculator include cess, TCS or TDS? No. Compensation cess, TCS and TDS are not included, and it does not make a GST tax invoice.