Separate tax from a tax-inclusive total
A reverse sales tax calculation works backward from a total that already includes tax. It estimates the pre-tax amount and the tax portion when the combined tax rate is known.
Formula
Worked example
For a $108 total that includes 8% tax, the pre-tax amount is $100 and the included tax is $8. Subtracting 8% of $108 would be incorrect because the rate applies to the smaller pre-tax base.
Use the combined rate that applied to the transaction
A location may combine state, county, city, district, or other rates. Enter the actual combined rate shown for the transaction rather than a statewide rate that omits local tax.
Some products, services, buyers, or holidays may be exempt or taxed differently. The calculator performs a mathematical reversal; it does not determine whether a transaction was legally taxable.
Rounding can create a small difference
Point-of-sale systems may calculate tax per item, per line, or on the order subtotal. Rounding each line can produce a result a few cents different from reversing the order total.
For bookkeeping, retain the original receipt or platform report and use its stated tax amount when available. Use this estimate when the total and rate are known but the tax split is not shown.
Inputs to verify
- Confirm the total already includes sales tax.
- Use the combined rate for the actual sale location.
- Enter the rate as a percentage, not as a currency amount.
- Compare the estimate with the original transaction record.
Last reviewed September 8, 2026. This calculator provides an estimate for planning and education. Verify current rates, rules, and account-specific terms with the relevant official source.