Calculate an invoice total from quantity, unit price and tax rate.
An invoice total is built in a sequence, and the sequence is what determines the number. Line amounts come first, each quantity times unit price. Line-level or invoice-level discounts come next, reducing the taxable value. Tax is then applied to that discounted subtotal, not to the gross. Any charges that are themselves taxable — freight or packing, commonly — should be added before tax rather than after. Round only at the end, and round once.
Two practical points cause most invoice disputes. Rounding each line separately and then summing gives a different total from summing exact line values and rounding once at the foot; the difference is small per line but visible on a long invoice, and consistency matters more than which convention you pick. And under GST, the tax must be shown correctly split as CGST plus SGST for an intra-state supply or as a single IGST for inter-state — a compliant tax invoice also needs the supplier and recipient GSTIN, an HSN or SAC code, a sequential invoice number and the place of supply, or the buyer's input tax credit can be challenged.
You sell 10 units at ₹2,500 each, so the subtotal is ₹25,000. You give a 10% discount, which brings the taxable value to ₹22,500. GST at 18% on that is ₹4,050, so the invoice total is ₹26,550.
The order matters. If GST were wrongly charged on the undiscounted ₹25,000 (₹4,500) and the discount taken off afterwards, the total would be ₹27,000, and you would collect and pay ₹450 more GST than you should. A discount shown on the invoice at the time of supply reduces the taxable value.
Last reviewed: 5 November 2026 by the YogiCalc team · How we calculate and verify · Spotted an error? Tell us
After. Discounts reduce the taxable value, so tax is applied to the discounted subtotal. Applying tax to the gross and then discounting overstates the tax and can create a compliance problem.
Round once at the end wherever possible. Rounding each line and then summing accumulates small differences that become visible on a long invoice. Whichever convention you adopt, apply it consistently.
Usually yes, where it forms part of the supply, so it is added to the taxable value before tax is calculated. Treatment can vary by the nature of the charge, so check your specific case for large or unusual amounts.
Supplier and recipient GSTIN, a sequential invoice number and date, HSN or SAC codes, taxable value, the correct CGST/SGST or IGST split, and the place of supply. Missing details can jeopardise the buyer’s input tax credit claim.
After, when the discount is given on the invoice itself at the time of sale. GST is charged on the value after that discount.
Key items include the supplier's and buyer's names and GSTINs, invoice number and date, HSN or SAC code, taxable value, GST rate and amount, and the place of supply. Check the current GST rules for the full list.