About the GST / VAT Calculator
This GST calculator works in both directions, which is the part people usually need. Enter a pre-tax amount and it adds the tax; enter a tax-inclusive amount and it reverse-calculates the net figure and the tax component hidden inside it. Any rate works, so it serves equally as a VAT calculator for the UK and EU, a sales tax calculator elsewhere, and a GST calculator for India, Australia, Canada, Singapore and New Zealand. Indian users can tick the split option to divide the tax into equal CGST and SGST halves for an intra-state invoice.
How to use the GST / VAT Calculator
- 1
Enter the Amount you are working from.
- 2
Enter the Tax rate as a percentage — 18 for standard Indian GST, 20 for UK VAT, 10 for Australian GST.
- 3
Set 'The amount above is' to before tax to add tax, or after tax to strip the tax out.
- 4
Tick 'Show CGST / SGST split' if you need an Indian intra-state breakdown.
- 5
Read the net, tax and gross figures in the breakdown panel.
What people use it for
Raising an invoice
Freelancers who quote a net day rate can add the correct tax line and see the gross the client will actually be billed, so the invoice totals match to the last unit.
Reclaiming tax from a receipt
A receipt often shows only the total. Set the mode to after tax to recover the net amount and the tax paid for your bookkeeping or expense claim.
Indian intra-state invoicing
The CGST and SGST split table gives the two 9% lines needed on an intra-state invoice under an 18% rate, ready to copy into accounting software.
Adding tax versus removing it
Adding tax is straightforward: gross = net × (1 + rate ÷ 100). At 18%, a net of 1,000 becomes 1,180. Removing it is where errors creep in, because subtracting 18% from the gross is wrong. The correct operation is net = gross ÷ (1 + rate ÷ 100), so 1,180 ÷ 1.18 returns 1,000 and the tax component is the 180 remainder. Taking 18% off 1,180 would give 967.60, understating the net by more than 3%. Expressed as a fraction of the gross, tax at 18% is 18 ÷ 118 = 15.254% of the tax-inclusive figure, not 18%. The equivalent VAT fractions are worth memorising: at 20% the tax is one sixth of the gross, and at 5% it is one twenty-first. The same reversal logic applies to any percentage added on top of a base, which is why the method also works for service charges and commissions embedded in a total.
CGST, SGST, IGST and how rates differ worldwide
India splits GST by the geography of the supply. For a supply within a single state, the tax is levied half by the central government as CGST and half by the state as SGST, so an 18% rate appears on the invoice as 9% plus 9%. For a supply crossing state lines, a single IGST at the full 18% applies instead, later apportioned between governments. Union territories use UTGST in place of SGST. The main Indian slabs are 0, 5, 12, 18 and 28 percent, with additional cess on a few categories. Elsewhere the structures differ substantially: UK VAT is 20% standard with 5% reduced and 0% on many foods and children's clothing; Australian and Indonesian GST is 10%; Singapore is 9%; New Zealand is 15% on almost everything; and the United States has no national sales tax at all, instead layering state, county and city rates that can combine above 10%. Because you enter the rate yourself, the tool survives any rate change, but always confirm the current rate and the correct treatment with your tax authority or accountant.
Tips
- For 20% VAT, the quick check is that tax equals the gross divided by six.
- Rounding is usually applied per invoice line rather than to the total, so a hand-checked total can differ by a unit or two from software output.
- Keep the net figure for your books; profit and turnover are measured excluding tax you merely collect and pass on.