How to Calculate GST: Adding and Removing GST With Examples
There are only two GST calculations you will ever need: adding GST to a price that does not include it, and taking GST out of a price that already includes it. The second one trips people up, because you cannot just subtract the rate. This guide shows both, step by step.
Part 1: Adding GST to a price
Use this when the price is "before tax" or "plus GST".
- Find the GST amount: GST = Price × Rate ÷ 100
- Add it to the price: Total = Price + GST
You can also do it in one step by multiplying the price by (1 + rate/100). At 18% that factor is 1.18; at 5% it is 1.05.
Example. A laptop bag costs ₹1,500 before tax and the rate is 18%.
- GST = ₹1,500 × 18 ÷ 100 = ₹270
- Total = ₹1,500 + ₹270 = ₹1,770
Part 2: Removing GST from a GST-inclusive price
Use this when the price already includes GST, for instance a printed MRP or a final bill, and you want to know the base price and the tax inside it.
- Find the base price: Base = Total ÷ (1 + Rate/100)
- Find the tax: GST = Total − Base
Example. A bill totals ₹11,800 and includes 18% GST.
- Base = ₹11,800 ÷ 1.18 = ₹10,000
- GST = ₹11,800 − ₹10,000 = ₹1,800
The most common mistake
Many people remove GST by taking 18% off the total. That gives ₹11,800 − 18% = ₹9,676, which is wrong. The 18% was calculated on the base price, not on the total, so the tax inside ₹11,800 is ₹1,800, not ₹2,124. Always divide by (1 + rate), never subtract the rate directly.
Splitting GST into CGST and SGST
For a sale within one state, the GST is split equally. At 18% the split is 9% CGST and 9% SGST. In the laptop bag example the ₹270 tax is ₹135 CGST and ₹135 SGST. For a sale to another state, the whole ₹270 appears as IGST. The split does not change the total you pay.
Quick reference for common rates
| Rate | Multiply the base by | Divide a GST-inclusive total by |
|---|---|---|
| 5% | 1.05 | 1.05 |
| 18% | 1.18 | 1.18 |
| 40% | 1.40 | 1.40 |
As of the rate changes that took effect on 22 September 2025, the main slabs are 5%, 18% and 40%, with 0% for exempt goods. Rates can be revised by the GST Council, so confirm the rate that applies to your product.
A worked example with a higher slab
A product has a base price of ₹1,00,000 and falls in the 40% slab. GST is ₹1,00,000 × 40 ÷ 100 = ₹40,000, so the buyer pays ₹1,40,000. Going the other way, if you see ₹1,40,000 inclusive of 40% GST, the base is ₹1,40,000 ÷ 1.40 = ₹1,00,000.
Checking your work
- Base + GST must equal the total, whichever direction you calculated.
- GST divided by the base price must give back the rate. In the first example, 270 ÷ 1500 = 0.18.
- Round to the nearest paisa only at the end, not in the middle of a calculation.
If you would rather not do the arithmetic by hand, the GST Calculator adds or removes GST and shows the CGST and SGST split in one click. For the bigger picture of how the tax works, read What Is GST?