The GST Calculator adds GST to a net price or strips it out of a GST-inclusive price, at whatever rate your country applies. Enter the amount, set the rate — 10% in Australia, 15% in New Zealand, 5% in Canada, 18% on most Indian goods and services, or your local figure — choose Add or Remove, and it returns the net amount, the tax amount, and the gross amount, each rounded to the cent.
How It Works
Adding tax is straightforward multiplication. Removing it is where errors happen, because the tax inside a gross price is not the rate times the gross price — the tax was calculated on the smaller net amount:
A useful consequence: at 10% GST, the tax inside any inclusive price is exactly 1/11th of the total; at 15%, it is 3/23rds; at 5%, 1/21st. Accountants use those fractions as a mental check on the calculator's answer.
Worked Example 1: Removing GST (Australia, 10%)
An invoice totals **220 ÷ 1.10 = 220 − 20.00 — the amount you claim as an input tax credit on your BAS. The trap this avoids: subtracting 10% of 198 and a phantom GST of $22, overstating your credit by a tenth.
Worked Example 2: Adding GST (New Zealand, 15%)
A service quoted at **349 × 15% = 401.35. Run the same numbers backwards as a check: 349.00 exactly.
Worked Example 3: A Higher Rate (India, 18%)
A gross price of ₹11,800 at 18%: net = ₹11,800 ÷ 1.18 = ₹10,000, GST = ₹1,800. Higher rates widen the subtraction trap — "removing 18%" from the gross would understate the net by over ₹320.
Where Each Direction Is Used
Add belongs on the sales side: quoting ex-GST prices, preparing invoices, and setting shelf prices from a net margin target. Remove belongs on the purchases side: extracting the claimable GST from receipts and supplier invoices for your activity statement or VAT return, and recovering the true net cost of an expense quoted inclusive. Profitability work should always run on net figures — the GST you collect is never revenue; it is the tax office's money passing through your account, and margins computed on gross prices flatter every product you sell.
Limitations
The calculator handles a single amount at a single rate. Real invoices mix rates and GST-free lines (basic food, many health and education services, and exports in Australia are GST-free), and tax authorities differ on whether GST is rounded per line or per invoice — on multi-line documents the two methods can differ by cents. It does not determine whether a supply is taxable in the first place, handle margin schemes, or replace your accounting software's BAS/VAT reporting; it makes the arithmetic on any one figure exact.
Frequently Asked Questions
Why can't I just subtract the percentage to remove GST?
Because the percentage was applied to the net, not the gross. Tax is rate × net, and net is smaller than gross, so rate × gross always overstates the tax. Division by (1 + rate) reverses the original multiplication exactly — it is the only correct inverse, at any rate, in any country.
What fraction of a GST-inclusive price is the tax?
Tax ÷ gross = rate ÷ (1 + rate). At 10% that is 1/11 (about 9.09% of the total), at 15% it is 3/23 (about 13.04%), at 20% VAT it is 1/6 (about 16.67%). If your computed GST is a bigger share of the total than that fraction, something has gone wrong.
Does this work for VAT as well?
Yes. VAT and GST are the same consumption-tax mechanism under different names, and the add/remove arithmetic is identical — enter your VAT rate (20% in the UK, for example) and use it the same way. What differs between countries is which supplies are taxable and at which rate, not the math.