Reverse Sales Tax Calculator
Extract the GST already included in a tax-inclusive price
Frequently Asked Questions
When you have a GST-inclusive price and need to know how much sales tax is embedded in it, the calculation isn't as simple as multiplying by the tax rate the way you would to calculate tax forward — this calculator does the reverse arithmetic correctly.
Why you can't just multiply by the tax rate
If a price already includes 17% GST, the tax portion is not 17% of that total price — it's 17% of the original pre-tax price, which is a smaller base. Multiplying a GST-inclusive total by 17% overstates the actual tax embedded in it, because you're applying the rate to a number that's already inflated by the tax itself. The correct approach divides the inclusive price by (1 + tax rate) to recover the pre-tax base first, then calculates the tax as the difference — a two-step process that's easy to get subtly wrong doing it by hand.
A worked example
A retail price of Rs 11,700 that already includes 17% GST does not contain Rs 1,989 of tax (11,700 × 0.17) — it actually contains Rs 1,700 of tax, because the true pre-tax price was Rs 10,000, and 17% of that smaller number is the correct tax amount. The error from the naive approach compounds as prices get larger, which is exactly why this calculator exists rather than leaving people to do (what looks like) simple percentage math by hand.
How this connects to your other calculations
This is the mirror image of the forward calculation on the Sales Tax Calculator — use that one when you're starting from a pre-tax price and need to add GST; use this one when you're starting from a GST-inclusive price (a retail receipt, an all-in quoted price, an imported invoice already showing a tax-inclusive total) and need to extract the tax component or the underlying pre-tax value.
Common mistakes
The single most common error, by far, is exactly the naive-multiplication mistake described above — applying the tax rate directly to an already-inclusive price instead of dividing by (1 + rate) first. This shows up constantly in manual invoice reconciliation and is worth double-checking with a calculator rather than trusting a quick mental estimate, especially on larger invoice amounts where the error compounds into a real rupee discrepancy.