What it calculates
A GST tax invoice shows the taxable value of what you sold and the tax charged on it.
How it works
Taxable value = quantity × unit price − discount. GST is charged on the taxable value and split by type of supply.
Formula
Total = (Q × P − D) × (1 + Rate ÷ 100)
- Q = quantity, P = unit price, D = discount shown on the invoice
How to use it
- Enter quantity, unit price and any discount.
- Pick the GST rate and type of supply.
- Choose whether to round the total.
For 10 units at ₹1,250 with 18% GST within the state:
Why it helps
- Avoids arithmetic errors on invoices.
Limitations
- One line only.
- An invoice must also show the particulars required by the CGST Rules (GSTINs, HSN/SAC, place of supply, invoice number and date).
Frequently asked questions
Is GST charged before or after discount?
A discount shown on the invoice at the time of supply reduces the taxable value, so GST is charged on the discounted amount.
Should I round off the invoice total?
Many businesses round the total to the nearest rupee and show the difference as a separate round-off line, as this calculator does. Tax amounts themselves are calculated to the paisa.
Sources
- GST rate rationalisation effective 22 September 2025 (e.g. Notification No. 9/2025-Central Tax (Rate), 17 Sep 2025) — checked via published summaries; confirm on cbic-gst.gov.in — CBIC (checked 2026-10-09)
Arithmetic aid only; not tax advice. Your invoice must also meet the CGST Rules on tax invoices.