GST calculator: add or remove GST, CGST and SGST split
Add GST to a base price or take it out of a price that includes it, at any rate, with the CGST and SGST halves for a sale within a state or IGST for a sale across states.
Type the amount and the rate, say whether GST is already in it, and press Calculate.
The two directions
Adding GST is straightforward: tax = base × rate ÷ 100, and the invoice total is base + tax. Taking GST out of a price that already includes it is where people slip. The tax is a share of the base, not of the total, so you divide rather than subtract a percentage:
base = total ÷ (1 + rate ÷ 100) tax = total − base
A bill of Rs 1,180 with 18% GST in it has a base of 1,180 ÷ 1.18 = Rs 1,000 and tax of Rs 180. Taking 18% off 1,180 (which gives 967.60) is wrong by Rs 32.40, and the error grows with the rate.
CGST, SGST and IGST
GST is one tax collected in two ways. On a sale within a state, the rate is split in half: Central GST and State GST, each at half the rate, both on the same base. On a sale from one state to another, the whole rate is charged as Integrated GST. The total tax is the same either way; the split decides which government's account it goes to and which lines appear on the invoice. The calculator shows the split for the supply type you pick; on your invoice the supply type follows the place of supply rules, which for goods is usually where the goods are delivered and for services usually the customer's registered location.
Worked examples
| Case | Base | Tax | Total |
|---|---|---|---|
| Rs 1,000 plus 18%, within a state | 1,000.00 | CGST 90.00 + SGST 90.00 | 1,180.00 |
| Rs 1,000 plus 18%, across states | 1,000.00 | IGST 180.00 | 1,180.00 |
| Rs 2,499 including 5% | 2,380.00 | 119.00 | 2,499.00 |
| Rs 649 including 12% | 579.46 | 69.54 | 649.00 |
| Rs 1,00,000 plus 28%, across states | 1,00,000.00 | IGST 28,000.00 | 1,28,000.00 |
Finding the right rate
GST rates are set by notification against the HSN code of goods or the SAC code of services, and the schedule changes from time to time; the slabs in force and any cess for a particular item are on the GST Council's and CBIC's pages, and on the invoice of anyone already selling that item. This page deliberately carries no rate table, so it cannot go stale; type the rate you have confirmed. If you sell, your invoice must also show the GSTIN, the HSN or SAC, and the place of supply, none of which a calculator can supply.
Rounding
The GST rules allow rounding the tax on an invoice to the nearest rupee, and most billing software rounds each tax line separately. The calculator shows paise so you can see the exact split; round at the end, per line, the way your software does.