How to Calculate GST: Add It, Remove It, and Split CGST and SGST

Whether you're quoting a client, checking a bill or preparing an invoice, GST calculations come up constantly for Indian businesses. There are only two calculations to master (adding GST and removing it), plus knowing how the tax is split between the centre and the states.

Which rate applies?

The GST rate depends on the goods or service, and rates are revised by the GST Council from time to time. Always check the current rate for your product or service on the official GST portal or with your accountant. The calculations below work the same for any rate.

Adding GST to a price (exclusive → inclusive)

When your price doesn't include GST yet:

GST amount = price × rate ÷ 100 Total = price + GST amount

Example at 18%: a service priced at ₹10,000.

• GST = 10,000 × 18 ÷ 100 = ₹1,800.
• Total = ₹11,800.

Removing GST from an inclusive amount (inclusive → exclusive)

When an amount already includes GST, you can't just subtract the rate as a percentage of the total. Use:

Price before GST = total × 100 ÷ (100 + rate) GST amount = total − price before GST

Example at 18%: a bill of ₹11,800 including GST.

• Price before GST = 11,800 × 100 ÷ 118 = ₹10,000.
• GST = ₹1,800.

The common mistake

Taking 18% of ₹11,800 gives ₹2,124, which is wrong: that would be 18% of a price that already contains tax. Always divide by (100 + rate).

CGST, SGST and IGST

• Within one state (intra-state supply): GST is split equally into CGST (central) and SGST (state), or UTGST in union territories. At 18%: 9% CGST + 9% SGST.
• Between states (inter-state supply): the whole amount is IGST. At 18%: 18% IGST.

The total is the same; only the split differs.

Use the calculator

The GST Calculator adds GST to a price or takes it out of an inclusive amount, at a preset or any rate you enter, with CGST, SGST and IGST shown. Amounts are rounded to two decimal places (paise), as on invoices.

Put it on an invoice

The Invoice Generator creates invoices, quotes and receipts with tax, GST, discounts and your logo, then prints them or saves them as PDF. What an invoice must show (GSTIN, addresses, HSN or SAC codes, invoice numbering, place of supply) depends on the rules that apply to you; check the official requirements or ask your accountant.

Discounts and GST

When a discount is given on the invoice before tax, GST is calculated on the discounted price. For example, ₹10,000 less 10% discount = ₹9,000; GST at 18% = ₹1,620; total ₹10,620. Percentages in general, including discounts, are easy to check with the Percentage Calculator.

Rounding

Calculate GST per line or on the total consistently, and round to paise. Small differences between line-by-line and total-based calculations are normal; follow your accounting software's or accountant's method.

Common questions

Is GST charged on the discounted price? For discounts shown on the invoice at the time of supply, yes.

How do I find the GST in an MRP? MRP is generally inclusive of taxes: use the "removing GST" formula with the applicable rate.

Are my amounts stored? No. The calculation happens in your browser.

This article explains the arithmetic; it isn't tax advice. For your specific situation, consult a qualified professional.

Summary

• Add: price × rate ÷ 100.
• Remove: total × 100 ÷ (100 + rate).
• Intra-state: CGST + SGST; inter-state: IGST.