How to Calculate GST: Add or Remove GST in Seconds

Whether you're writing an invoice, checking a bill or pricing a product, you'll need to do two GST calculations again and again: adding GST to a price, and pulling GST out of a price that already includes it. The second one is where most people go wrong.

Adding GST to a price

If a price excludes GST, multiply it by the rate:

GST amount = price × rate ÷ 100
Total = price + GST amount

Example at 18%: a service costs 10,000 before tax.

• GST = 10,000 × 18 ÷ 100 = 1,800
• Total = 11,800

Removing GST from a GST-inclusive price

If a price includes GST, you can't just take 18% off. Divide instead:

Price before GST = total ÷ (1 + rate ÷ 100)
GST amount = total − price before GST

Example at 18%: the total is 11,800.

• Price before GST = 11,800 ÷ 1.18 = 10,000
• GST = 1,800

The common mistake

Taking 18% off 11,800 gives 2,124, which is wrong. GST was calculated on 10,000, not on 11,800, so subtracting 18% of the total removes too much. Always divide by 1.18 (or 1.05, 1.12 and so on) to remove GST.

Quick multipliers

Rate	To add GST, multiply by	To remove GST, divide by
5%	1.05	1.05
12%	1.12	1.12
18%	1.18	1.18
28%	1.28	1.28

CGST, SGST and IGST (India)

In India, GST on a sale within one state is split equally into CGST (central) and SGST (state). For 18% GST, that's 9% + 9%. For a sale between states, the whole 18% is charged as IGST. The total tax is the same; only the split and where it's paid differ.

Do it in one step

The GST calculator adds or removes GST at any rate and shows the CGST/SGST split. For VAT, as in the UAE, Saudi Arabia or the UK, the maths is identical: use the VAT calculator. For US-style sales tax added at the till, try the sales tax calculator.

Tips for invoices

• Show the price before tax, the tax rate, the tax amount and the total on separate lines.
• Round the tax per invoice according to your local rules, not per item, unless required otherwise.
• Keep your GST/VAT registration number on every tax invoice.

Rates and rules differ by country and product and they change over time, so check the current rate for your item with your tax authority or accountant.