GST / VAT Calculator
Add tax to a net price or work backwards out of a tax-inclusive one — the reverse calculation people get wrong most often.
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How it works
Adding tax is simple multiplication. Removing it is where errors creep in: subtracting 18% from a tax-inclusive price does not give the net amount, because the 18% was calculated on the smaller net figure, not on the total.
The correct reverse calculation divides by 1 plus the rate. On a 1,180 gross price at 18%, dividing gives 1,000 net and 180 tax. Subtracting 18% would have given 967.60 — wrong by more than 30, and wrong in a direction that under-declares tax.
Add: gross = net × (1 + r) · Remove: net = gross ÷ (1 + r)
- Net — the price excluding tax — the figure that counts as revenue
- Gross — the price the customer pays, tax included
- r — the tax rate as a decimal (18% = 0.18)
Worked example
Removing 18% GST from a tax-inclusive price of 1,180
Inputs
Results
Subtracting 18% instead would give 967.60 — understating the net by 32.40 and the tax with it.
Frequently asked questions
How do I remove GST from a price?
Divide by 1 plus the rate, not by subtracting the percentage. At 18%, divide the gross by 1.18. The tax is the difference between the two figures.
How is a split rate handled?
Where a rate is divided between two authorities — central and state, or federal and provincial — the total rate is what matters for the calculation. The split field divides the resulting tax between them for your invoice.
Which figure is my revenue?
The net amount. Tax collected is held on behalf of the authority and passed on, so including it in revenue overstates both turnover and margin.
Is my data sent anywhere?
No. The calculation runs entirely in your browser. Nothing you type is transmitted to us or stored.
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