How to calculate GST, with examples

GST is a percentage added to the price. The only tricky parts are whether your price already includes GST, and whether the sale is within your state or to another state.

Current GST rates

Since 22 September 2025, most goods and services fall into two slabs: 5% and 18%. A special 40% rate applies to a short list of luxury and sin goods. The old 12% and 28% slabs were abolished, with most items moving to 5% or 18%. Check your item's current rate with your CA if it used to be taxed at 12% or 28%.

When your price is before GST

Multiply the amount by the rate. On ₹10,000 at 18%, GST is ₹10,000 × 18 ÷ 100 = ₹1,800, so the total is ₹11,800.

When your price already includes GST

Divide out the tax: GST = total × rate ÷ (100 + rate). On ₹11,800 at 18%, GST is ₹11,800 × 18 ÷ 118 = ₹1,800, and the taxable value is ₹10,000. A common mistake is taking 18% of ₹11,800, which gives ₹2,124 and overstates the tax.

CGST + SGST or IGST

For a sale within your own state, the GST splits equally: 18% becomes 9% CGST + 9% SGST, so ₹1,800 is shown as ₹900 + ₹900. For a sale to another state, the full 18% is charged as IGST, shown as one line of ₹1,800. The total paid by the buyer is the same either way.

Several items at different rates

Calculate GST for each item at its own rate, then add them up. An invoice for a ₹2,000 service at 18% and ₹1,000 of goods at 5% has ₹360 + ₹50 = ₹410 of GST. BillKaro does this per item and groups the tax by rate on the invoice.

Last updated 2026-10-03. General information, not tax advice.