Reverse Percentage Calculator

A price that already includes VAT or sales tax hides the pre-tax amount inside a single combined figure, and reconstructing that pre-tax number is exactly the reverse-percentage calculation — dividing the total by one plus the tax rate rather than trying to subtract a percentage of the total itself, which would use the wrong base and produce a slightly incorrect answer. This is the same underlying arithmetic as recovering a pre-discount price, just running in the opposite direction: a discount reduces a starting price down to a final one, while tax increases a pre-tax amount up to a final one, and reversing either one means correctly identifying which figure the percentage was originally calculated against.

Reverse-calculating the pre-tax subtotal from a total and a known tax rate, then comparing it against a subtotal figure printed separately on the same receipt or invoice, is a quick way to catch a tax calculated at the wrong rate or applied to the wrong line items — a discrepancy between the two numbers is a concrete signal something on the document does not add up correctly, worth querying before paying rather than after passing it along further.