Add or extract GST — both ways

To add GST, multiply the price by 1.1 — $1,000 becomes $1,100. To find the GST already inside a total, divide by 11 rather than taking 10% of it: the GST in a $1,100 invoice is $100, not $110. That second one is the calculation people get wrong, because 10% of the GST-inclusive figure is not the GST.

Because the total is 110% of the original price, so the GST is 10/110 of it — one eleventh. Taking 10% of the gross overstates the GST by 10% and leaves your ex-GST figure short, which then flows into your BAS and your margins.

Only if you are registered. Registration is compulsory once your GST turnover reaches the ATO threshold, and optional below it — many sole traders register voluntarily so they can claim GST credits on tools and materials. If you are not registered you must not charge GST, and your invoice is a regular invoice rather than a tax invoice.

No. Some supplies are GST-free and others are input-taxed, and the treatment of things like residential property differs from ordinary trade work. For everyday contracting the 10% standard rate is what applies, but check the ATO guidance or your accountant for anything unusual — this calculator is arithmetic, not tax advice.

For a tax invoice the ATO expects your identity and ABN, the date, a description of what was supplied, the GST amount (or a statement that the total includes GST), and the buyer’s identity or ABN once the sale passes the threshold where that becomes required. Getting the GST figure right is the part this tool handles; the rest is a template question.