Most GST sums at a shop counter come down to two questions:
- "My price is ₹1,000 plus GST. What's the total?"
- "The customer paid ₹1,180 including GST. How much of that is tax?"
Here are the formulas, with examples. You can also use the free GST calculator.
Formula 1: adding GST to a price (exclusive)
GST = Price × Rate ÷ 100
Total = Price + GST
Example at 18%: price ₹1,000
GST = 1,000 × 18 ÷ 100 = ₹180
Total = ₹1,180
Formula 2: finding GST inside a price (inclusive)
GST = Total × Rate ÷ (100 + Rate)
Price before GST = Total − GST
Example at 18%: the customer paid ₹1,180
GST = 1,180 × 18 ÷ 118 = ₹180
Price before GST = ₹1,000
Example at 5%: the customer paid ₹525
GST = 525 × 5 ÷ 105 = ₹25
Price before GST = ₹500
A common mistake is taking 18% of ₹1,180 (that gives ₹212.40, which is wrong). Always divide by 100 + the rate.
Splitting into CGST and SGST, or IGST
- Sale within your state: split the GST equally. At 18%, that's 9% CGST + 9% SGST (or UTGST in a union territory).
- Sale to another state: charge it all as IGST, 18%.
| Rate | Within your state | To another state |
|---|---|---|
| 5% | 2.5% CGST + 2.5% SGST | 5% IGST |
| 18% | 9% CGST + 9% SGST | 18% IGST |
| 40% | 20% CGST + 20% SGST | 40% IGST |
On ₹1,000 at 18% within the state, the invoice shows CGST ₹90 + SGST ₹90.
Which GST rate applies?
From 22 September 2025, most goods and services fall in two main slabs, 5% and 18%, with a special 40% rate for a small list of luxury and sin goods. Some items are exempt (0%), and a few have their own rates, such as 3% on gold. Look up your items by their HSN or SAC code on the GST portal, or ask your accountant. Rates do change.
Rounding
Calculate tax on each line, round to two decimal places (paise), then add the lines. Many businesses round the final total to the nearest rupee and show the difference as "round off".
Let the app do the maths
When you create a GST invoice in SmartRaseed, you enter the amount and pick the rate. The app works out the taxable value, the CGST and SGST or IGST, and the amount in words, all on the invoice PDF. Related reading: GST invoice format.
