Sales tax math comes up in two directions: adding tax to a price to find the checkout total, and stripping tax out of a total to find the original price. Both are simple once you see the pattern, and both are handy for budgeting and expense reports.
TL;DR — Choose add or remove, enter the amount and rate in the sales tax calculator, and see the net, tax and total.
Adding tax
To add sales tax, multiply the price by the tax rate and add it on. A $100 item at an 8% rate has $8 of tax, for a $108 total. This is the everyday case — figuring out what you will actually pay at the register, or building a budget that includes tax.
Removing tax
The reverse is less obvious but just as useful. Given a tax-included total, divide by 1 plus the rate as a decimal to recover the original price. A $108 total at 8% divides by 1.08 to give back the $100 pre-tax price, leaving $8 of tax. This is exactly what you need for expense reports or bookkeeping, where the tax portion has to be separated out.
Why rates differ everywhere
In the US, the sales tax you pay is usually a combination of state, county and city rates, so it varies from place to place — sometimes between neighboring towns. A handful of states charge no sales tax at all. Your receipt lists the combined rate, or your state’s revenue site has a lookup. Enter it in the sales tax calculator to get exact figures either direction.