Updated 8 November 2026

GSTCal is a free Australian GST calculator. It adds GST to a price, removes GST from a GST-inclusive price, and shows the formula behind every answer so you can check it yourself.

We built it for the people who work with GST every day: small business owners, sole traders, freelancers, contractors, bookkeepers and anyone checking a quote or a receipt.

GSTCal is run by Simsim, who builds the calculators and writes and reviews every page. Simsim tests each GST formula against hand-worked examples and checks each explanation against the Australian Taxation Office’s (ATO) published guidance before it goes live. Read more on Simsim’s author page.

• Calculators you can check. Every result shows the GST, the amount excluding GST and the amount including GST, worked out to the cent, with the formula used.
• Both directions. Use the Add GST calculator for prices before GST and the Remove GST calculator for prices that already include it.
• Plain-English explanations. Our formulas, quick reference table and FAQs explain how GST works, based on the ATO’s published guidance.
• Private by design. Calculations run in your browser. The amounts you type are never sent to us.

• Not the ATO or any government agency. We link to the Australian Taxation Office’s own guidance throughout.
• Not a tax agent or accountant. Our calculators and guides are general information, not advice about your situation.

The GST maths is kept in code, separate from the content system, and covered by automated tests with hand-worked examples, including rounding and amounts with cents. Read our editorial guidelines for the full process.

Spotted a mistake or have a question? Email us at hello@gstcal.com.au. Corrections to figures are our first priority.