GST Calculator

The reverse calculation everyone gets wrong

Extracting tax from an inclusive price is the single most common arithmetic error in everyday business, and it always takes the same form. Given an inclusive price of 118 at 18% GST, the instinct is to take 18% of 118 — which gives 21.24 — and conclude that is the tax. It is not. The 18% was applied to the base price of 100, producing 18 of tax and a total of 118. To recover the base you divide by 1.18, not subtract 18%. The error is small on a single invoice and compounds badly across a year of bookkeeping, and it is why the reverse calculation deserves its own button rather than being done in your head.

What GST replaced

Before 2017, India taxed goods through a thicket of overlapping central and state levies — excise duty, VAT, service tax, octroi, entry tax and more — each applied at a different point in the chain and frequently charged on top of one another. That cascading, tax-on-tax structure inflated final prices in ways nobody could easily trace, and it made moving goods between states an administrative ordeal. GST consolidated them into a single destination-based tax with input credits, so that businesses reclaim the tax paid on their inputs and the levy falls, in effect, only on the value added at each stage. That is the whole design goal: tax the value added once, rather than taxing the tax.