Reverse Sales Tax Calculator
Extract the sales tax from a tax-included total to find the pre-tax price and the exact tax paid.
How it works
Price before tax = total ÷ (1 + rate) · Tax = total − price before tax
To pull sales tax out of a tax-included total, you divide by (1 + the rate), not subtract the rate from the total. That is because the tax was originally charged on the smaller pre-tax price, not on the grand total. Subtracting 7.5% of the total removes too much and gives the wrong answer — a common bookkeeping mistake when reconciling receipts or logging expenses.
Worked example
You paid $107.50 including 7.5% tax. Pre-tax price = 107.50 ÷ 1.075 = $100.00, so the tax was $7.50. Note that subtracting 7.5% of $107.50 ($8.06) would have been wrong — the tax is a percentage of $100, not of the total.
Frequently asked questions
Why can’t I just subtract the tax rate from the total?
Because the tax was computed on the pre-tax price, which is smaller than the total. Subtracting the rate from the total over-removes tax. Dividing by (1 + rate) reverses the original calculation exactly.
When is a reverse sales tax calculation useful?
For expense reports and bookkeeping when only the gross amount is known, for pricing an item so the round-number total includes tax, and for reclaiming or reporting the tax portion of purchases.
Does this work for VAT or GST too?
Yes — the math is identical for any tax added as a percentage of a base price. Only the name and rate change; dividing the gross by (1 + rate) always recovers the base.
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This calculator is for educational purposes. Double-check important results before acting on them.