How to Calculate GST on an Invoice

A clear, compliant invoice shows three things separately: the base amount, the GST charged, and the total. Getting the order of operations right — especially when discounts are involved — keeps your billing accurate and audit-friendly. Here is the step-by-step process.

Step 1: Work out the taxable base

Start from the price of the goods or services before tax. If you offer a discount, apply it first — GST is charged on the discounted (net) price, not the original. Calculate the reduced price with the discount calculator, then use that as your taxable base.

Step 2: Add GST at the right rate

Multiply the base by the GST rate. For a ₹1,000 base at 18%: 1,000 × 0.18 = ₹180 GST, making the total ₹1,180. For multiple line items at different rates, calculate GST per line and sum them. The GST calculator handles add-or-remove in one step.

Step 3: Working backwards from a tax-inclusive price

If your listed price already includes GST, divide by (1 + rate ÷ 100) to find the base. For a ₹1,180 inclusive price at 18%: 1,180 ÷ 1.18 = ₹1,000 base, leaving ₹180 of GST. This "reverse GST" step is essential when your displayed prices are tax-inclusive.

Step 4: Show the breakdown

List the net amount, the GST (ideally split by rate, and into components like CGST/SGST where required), and the grand total. Transparent splits make invoices easier to reconcile and reclaim. For any percentage-based add-ons or partial figures, the percentage calculator is handy.

New to GST itself? Start with our companion guide, What is GST?, for the rates and fundamentals.