Sales tax calculator

The reverse direction is the one worth bookmarking

Adding tax to a price is easy enough to do in your head. The direction that earns this page a bookmark is the second one: starting from a receipt total and backing out the pre-tax price. Anyone who files expense reports, itemizes business purchases, or reconciles a cash box eventually needs it, and almost everyone does it wrong the first time. The wrong way is subtracting the tax percentage from the total. The right way is dividing the total by one plus the rate, and the two answers are not the same. On an $86.60 receipt at 8.25%, dividing by 1.0825 gives the correct $80.00 pre-tax price. Subtracting 8.25% from the total gives $79.46, which is 54 cents wrong on a single receipt and compounds into real error across an expense report.

Why subtraction fails: the tax was calculated as a percentage of the smaller pre-tax number, so removing it requires dividing by the factor that created it, not lopping the same percentage off the bigger number. The calculator shows the division it performed under the result so you can paste the working into a spreadsheet.

One practical note on rates: the number you need is your combined state, county, and city rate, and it changes at municipal boundaries. The rate printed on any local receipt is the authoritative source for your location, better than any lookup table.

Both directions, worked

Forward: a $129.99 purchase in a 7.5% jurisdiction. Enter 129.99 and 7.5 with the direction set to add. Tax is 129.99 x 0.075 = $9.75, and the total is $139.74. Reverse: your card statement shows $139.74 and you need the pre-tax figure for reimbursement. Flip the direction, enter 139.74 and 7.5. The calculator divides by 1.075 and returns $129.99 with $9.75 of tax, recovering the original sticker price exactly.

What the calculator actually does

In add mode, tax equals the price times the rate over 100, and the total is their sum. In remove mode, the pre-tax price is the total divided by (1 + rate/100), and the tax is whatever remains. The input labels swap automatically when you change direction, so the amount field always names what it expects. Values display to the cent using standard rounding; there is no state lookup or geolocation, the rate is purely what you type, which is also what makes the tool work for VAT and GST anywhere in the world.

Limits worth knowing

Real receipts are messier than one rate times one price. Groceries, clothing, and medicine are exempt or reduced in many states, so a mixed shopping trip has no single rate that backs out cleanly; the reverse calculation is only exact when everything on the receipt was taxed at the same rate. The tool also rounds at the end, while some registers round line by line, which can produce penny-level differences on multi-item receipts. Neither of those affects the common cases, a single purchase or a same-rate receipt, where the math here matches the register.