How to Do a Reverse GST Calculation: The Divide-by-11 Method
When you have a GST-inclusive price and need to find the original amount plus the GST charged, you need a reverse GST calculation. Here's the simple method that every Australian accountant uses โ and how our free calculator does it instantly.
What is a Reverse GST Calculation?
A reverse GST calculation (also called "working back from GST") is when you start with a GST-inclusive price and calculate backwards to find the original price before GST and the exact GST component.
This is essential when you receive a receipt or invoice showing a total amount and need to separate the tax from the base price โ for example, when claiming GST credits on your BAS.
The Divide-by-11 Method
The simplest and most reliable way to find the GST component in any GST-inclusive price is to divide by 11. This works because of the mathematics of how GST is applied:
If Base Price = $100, then GST = $10, Total = $110
$10 is exactly 1/11th of $110
Therefore: GST = Total รท 11
Full Reverse GST Formula
Ex-GST Price = GST-Inclusive Total รท 1.1
GST Amount = GST-Inclusive Total รท 11
Verify: Ex-GST + GST = Original Total โ
Worked Examples
| GST-Inclusive Total | Ex-GST Price (รท1.1) | GST Amount (รท11) | Verify |
|---|---|---|---|
| $55.00 | $50.00 | $5.00 | $50 + $5 = $55 โ |
| $110.00 | $100.00 | $10.00 | $100 + $10 = $110 โ |
| $385.00 | $350.00 | $35.00 | $350 + $35 = $385 โ |
| $1,100.00 | $1,000.00 | $100.00 | $1,000 + $100 = $1,100 โ |
| $247.50 | $225.00 | $22.50 | $225 + $22.50 = $247.50 โ |
| $88.00 | $80.00 | $8.00 | $80 + $8 = $88 โ |
When Do You Need a Reverse GST Calculation?
- Claiming input tax credits: You receive a receipt showing a total and need to identify the GST for your BAS
- Checking supplier invoices: Verifying that the GST shown on an invoice is correct
- Business pricing: If you want to offer a specific GST-inclusive price but need to know the ex-GST amount for accounting
- Reimbursements: Splitting GST from expense reimbursements for employees
- Property transactions: Working out the GST component in property contracts
Use the Free Reverse GST Calculator
Instead of doing the maths manually, use our free Reverse GST Calculator. Enter any GST-inclusive amount and instantly see the ex-GST price and GST component โ perfect for BAS preparation and invoice checking.
The two reverse GST formulas are: (1) divide by 1.1 to get the original ex-GST price, and (2) divide by 11 to get the exact GST amount. Never subtract 10% โ this gives the wrong answer. For any amount, these two results should always add up to the original total as your verification check.