GST Invoice Calculator
Add or extract GST, apply retention, and find the balance due on a construction tax invoice.
General tool only. This calculator gives estimates based on the values you enter. Always check your contract terms and any state-specific rules before relying on the figures.
Amount already includes GST
GST will be added on top of the invoice amount.
Results
Enter an invoice amount to preview GST and totals.
How it works
Behind the numbers
Toggle whether the amount you've entered already includes GST. Retention is held against the subtotal, then GST is calculated on the full subtotal.
Formula
The maths, written out
If GST included: Subtotal = Amount ÷ 1.10 Else: Subtotal = Amount GST = Subtotal × 10% Net = Subtotal + GST − Retention Balance = Net − Already paid
Worked example
A quick scenario
$22,000 ex-GST with 5% retention = $1,100 retention + $2,200 GST = $23,100 net payable.
Frequently asked
Is this calculator free to use?
Yes. All calculators on Contractor Calc are free and do not require sign-up.
How accurate are the results?
The calculators use standard formulas. Results are estimates only and depend on the values you enter. Always check against your contract.
Can I export the results?
Most calculators include a download or copy-to-clipboard option for sharing with clients or storing in your records.