GST Reverse Calculator
Reverse-calculate GST from an inclusive amount to find base price, GST amount, and total.
Last reviewed by the Radiatus Cloud team
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Find the GST inside a total price
When a price already includes GST, working backwards to the base price and the tax is not as simple as taking the percentage off. This reverse calculator takes a GST-inclusive amount and finds the base price, the GST amount and the total, so you can see exactly how much tax is inside a figure.
Why you cannot just subtract the percentage
The common mistake is to take 18% off an inclusive price to find the base, which is wrong, because the 18% was added to the base, not to the total. To reverse it you divide by one plus the rate, so an inclusive amount is the base times 1.18, and the base is the inclusive amount divided by 1.18. Getting this right matters for invoicing, expense claims and any time you need to separate tax from a quoted price.
Accurate tax, either direction
The calculation runs entirely in your browser, so your figures are never uploaded and stay on your own machine, which matters for anything involving money.
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Frequently Asked Questions
How do I find the base price from a GST-inclusive amount?
Divide the inclusive amount by one plus the GST rate. For 18% GST, divide by 1.18; the result is the base and the rest is the GST.
Why can I not just subtract the percentage?
Because the GST was added to the base, not the total. Subtracting the rate from the inclusive price overstates the base and understates the tax.
When is this useful?
For invoicing, expense claims and reconciliation, wherever you have a tax-inclusive figure and need to separate the base price from the GST inside it.
Does it handle any GST rate?
Yes. Enter the rate and it reverses the calculation correctly for that rate.
Is my data uploaded?
No. The calculation runs entirely in your browser.
Privacy & Security
Calculated locally in your browser. No data is sent to any server.
How to Use
Enter total (GST inclusive) and GST rate to get base and tax.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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