Question 1

What is the difference between "add tax" and "remove tax"?

Accepted Answer

"Add tax" starts from a pre-tax price and calculates what the tax and final total will be — useful when you're quoting or invoicing. "Remove tax" (reverse or back-calculation) starts from a tax-inclusive total, such as a receipt total or a price already displayed with tax, and works backward to find the original pre-tax amount and the tax portion hidden inside it.

Question 2

Why can't I just multiply the total by the tax rate to reverse it?

Accepted Answer

Multiplying a tax-inclusive total by the rate over-counts, because the rate applies to the pre-tax amount, not the total. The correct reverse formula divides the total by (1 + rate/100) to find the pre-tax amount first, then subtracts that from the total to get the tax portion. For an 18% tax, a $118 total is not $118 × 18% = $21.24 of tax — it is $18 of tax on a $100 pre-tax amount.

Question 3

Does this work for GST, VAT, and US sales tax?

Accepted Answer

Yes — the math is identical for any single-rate tax added on top of a price. GST and VAT are typically applied the same way in most countries, and this calculator includes the common Indian GST slabs (5%, 12%, 18%, 28%) as quick presets. For US sales tax, just enter your state or local combined rate instead.

Question 4

What about compound or multi-rate taxes (e.g. CGST + SGST)?

Accepted Answer

Where a tax is split into components that add up to a single combined rate — like CGST + SGST in India, which together equal the total GST rate — enter the combined percentage here and the result will match. This tool does not model taxes that compound on top of each other (a tax charged on a price that already includes a different tax).

Question 5

Why might this differ slightly from a rounded invoice?

Accepted Answer

Some invoicing systems round the tax amount to two decimal places at each line item before summing, which can produce a total a cent or two different from calculating tax on the grand total directly. Treat this tool as an accurate planning and verification calculation rather than a substitute for your accounting software's line-item rounding rules.

Question 6

How is IGST different from CGST + SGST on an interstate sale?

Accepted Answer

For a sale within the same Indian state, GST is split into CGST and SGST — two components, each half the total rate, one going to the central government and one to the state government. For a sale between two different states, the same total rate is instead charged as a single IGST (integrated GST) line, collected by the central government and settled with the states afterward. Either way, the combined rate the customer pays is identical — enter that combined percentage here regardless of whether the invoice itself shows it as one IGST line or a split CGST + SGST.

Question 7

Can I use this to calculate UK or EU VAT?

Accepted Answer

Yes. VAT is added and reverse-calculated with exactly the same math as GST — a single percentage applied on top of a pre-tax price. Just switch the rate to your country's VAT rate (20% standard rate in the UK, and ranging roughly 17–27% by country within the EU) instead of using the Indian GST presets.

Pre-tax amount	$1,000.00
Tax at 18%	$180.00
Total	$1,180.00

Item	Value
Total (tax-inclusive)	$118.00
Tax rate	18%
Pre-tax amount	$100.00
Tax portion	$18.00

Region / tax	Typical rate
India GST — essentials	5%
India GST — standard slabs	12% / 18%
India GST — luxury/sin goods	28%
UK VAT — standard rate	20%
US sales tax	Varies by state — no single national rate

GST / Sales Tax & Reverse Tax Calculator

How each direction is calculated

A $118 receipt total at 18% GST

Common tax rates by region

Common questions