Short answer:
- Add GST (exclusive price): GST = price x rate / 100. Total = price + GST.
- Remove GST (inclusive price): base price = inclusive price x 100 / (100 + rate). GST = inclusive price - base price.
- Split: for a sale within the same state, half is CGST and half is SGST (or UTGST). For a sale to another state, the whole amount is IGST.
Check any figure instantly with the free GST calculator.
Formula 1: Adding GST to a price
Use this when your price is before tax.
GST amount = original price x GST rate / 100
Price including GST = original price + GST amount (or original price x (1 + rate/100))
Example: a service priced at Rs 10,000 at 18% GST.
- GST = 10,000 x 18 / 100 = Rs 1,800
- Total = Rs 11,800
Formula 2: Removing GST from an inclusive price
Use this when the price you have already includes tax (a retail MRP, a quote "all inclusive").
Base price = inclusive price x 100 / (100 + GST rate)
GST amount = inclusive price - base price
Example: Rs 11,800 inclusive at 18%.
- Base = 11,800 x 100 / 118 = Rs 10,000
- GST = Rs 1,800
Common mistake: taking 18% of the inclusive price (11,800 x 18% = 2,124) overstates the tax. Always divide by (100 + rate).
CGST, SGST and IGST
| Type of supply | Tax charged | Example at 18% on Rs 10,000 |
|---|---|---|
| Within the same state (intra-state) | CGST + SGST, each half the rate | CGST 9% = Rs 900, SGST 9% = Rs 900 |
| Within a Union Territory without legislature | CGST + UTGST | Rs 900 + Rs 900 |
| Between states, imports, exports (inter-state) | IGST, full rate | IGST 18% = Rs 1,800 |
The place of supply, not the location of your office, decides which applies.
Quick reference: multipliers
| GST rate | To add GST, multiply by | To find base from inclusive, divide by |
|---|---|---|
| 3% | 1.03 | 1.03 |
| 5% | 1.05 | 1.05 |
| 18% | 1.18 | 1.18 |
| 40% | 1.40 | 1.40 |
The rate list was restructured in September 2025, with most goods and services moving to 5% or 18% and a higher rate for a short list of luxury and "sin" goods. Rates depend on the HSN or SAC code of what you sell, so always confirm the current rate for your item on the official CBIC rate notifications before invoicing.
Worked examples
Discount, then GST
List price Rs 2,000, 10% discount on the invoice, 18% GST. Taxable value = 1,800. GST = 324. Total = Rs 2,124. Discounts shown on the invoice reduce the taxable value.
Several items at different rates
Calculate GST line by line at each item's rate, then add the totals. Do not apply one rate to the invoice total.
Rounding
Calculate to two decimals per line and round the invoice total to the nearest rupee if you wish, showing the round-off as a separate line.
Frequently asked questions
How do I calculate 18% GST on an amount?
Multiply the amount by 0.18 for the tax, or by 1.18 for the total including tax.
How do I find the original price from a GST-inclusive price?
Divide the inclusive price by (1 + rate/100). At 18%, divide by 1.18.
Is GST calculated on the price after discount?
Yes, if the discount is given before or at the time of supply and shown on the invoice. Post-sale discounts have specific conditions.
When is IGST charged instead of CGST and SGST?
When the supplier and the place of supply are in different states, and on imports and exports.
Calculate it now
Use the free GST calculator to add or remove GST at any rate and see the CGST/SGST split. Need GST-ready invoicing built into your business software? See our fintech software development services.