Reverse sales tax isn't something you pay—it's something you calculate. For example, when you're going over your business receipts, you may want to know how much is actual income and how much is sales tax. Instead of calculating sales tax on the purchase amount, you have to figure out sales tax backward and subtract it from the total.

A reverse sales tax calculator is useful if you itemize your deductions and claim overpaid taxes on your tax returns. The only thing to remember about claiming sales tax and tax forms is to save every receipt for every purchase you intend to claim.

The formula is relatively simple. Divide your sales receipts by one, plus the sales tax percentage. Multiply the result by the tax rate, and you get the total sales-tax dollars. Subtract that from the receipts to get your non-tax sales revenue.
For example, suppose your sales receipts are $1,100, and the tax is 10 percent. Divide $1,100 by 1.1 and you get $1,000. Multiply that by 10 percent, and you get $100 sales tax, so your non-tax receipts are $1,000. You enter $1,000 in "sales" in your journal and $100 in "sales tax payable."