Reverse GST Calculator NZ

Enter a GST-inclusive amount and this calculator will extract the GST component and show the GST-exclusive price.

Understanding the Reverse GST Calculation

When you receive a receipt or invoice showing only the GST-inclusive total, you often need to determine how much of that amount is GST. This is where the reverse GST calculation comes in. Instead of adding GST to a base price, you're working backwards from the total to extract the tax component.

The 3/23 Method Explained

At the current 15% rate, the GST component of any inclusive amount is exactly 15/115 of the total. This fraction simplifies to 3/23. Inland Revenue prescribes this method because it produces mathematically exact results without rounding errors. To find the GST in a $575.00 inclusive total: $575.00 x 3 / 23 = $75.00 GST. The remaining $500.00 is the GST-exclusive amount.

When You Need This Calculator

Common scenarios include reconciling till receipts that only show totals, preparing GST returns from bank statements, checking supplier invoices for correct GST treatment, and splitting mixed expenses (such as a meal with both business and personal components). If you're registered for GST, accurately extracting the GST content is essential for claiming the correct input credits.

Historical Rates

If you're dealing with invoices from before 1 October 2010, the applicable rate was 12.5%. The fraction for extracting GST at 12.5% is 1/9 (or 12.5/112.5). For Australian GST at 10%, the fraction is 1/11 (or 10/110). This calculator supports all three rates.

For the full suite of GST tools, return to the GST.co.nz homepage. For other NZ calculators, visit Calculate.co.nz.