Reverse GST Calculator
Enter the total amount (including GST) and the GST rate to find the base amount and the GST component. Useful when you have an invoice total and need to back-calculate the original price.
How Reverse GST Calculation Works
When a price includes GST, you can't simply subtract the GST percentage from the total because the percentage applies to the base amount, not the total. The correct formula is:
Base Amount = Total Amount ÷ (1 + Rate ÷ 100)
For example, if your total is ₹11,800 at 18% GST:
Base = ₹11,800 ÷ (1 + 18 ÷ 100)
Base = ₹11,800 ÷ 1.18
Base = ₹10,000
GST = ₹11,800 − ₹10,000 = ₹1,800
A common mistake is calculating 18% of ₹11,800 = ₹2,124, which is wrong. The correct approach divides by (1 + rate), not multiplies by rate on the total.