🧾 How Sales Tax Is Calculated (Forward and Reverse)

Sales tax math goes two directions: adding tax onto a price you already know, or working backward from a total to figure out what the price was before tax. Both come up constantly — one at checkout, the other when you're trying to expense a receipt or budget from a final price.

Calculating Tax Forward

A $100 item with 8.5% sales tax costs 100 × 1.085 = $108.50 at checkout. The tax amount itself is just the pre-tax price times the tax rate: $100 × 0.085 = $8.50.

Calculating Tax in Reverse

If you only know the final, tax-included total — common on a receipt that just shows one number, or a price quoted "all in" — you can work backward to find the original pre-tax price:

A $108.50 total at 8.5% tax means the pre-tax price was 108.50 ÷ 1.085 = $100.

A Common Mistake

It's tempting to just subtract the tax rate percentage directly from the total (multiplying by 0.915 instead of dividing by 1.085) — but that gives a slightly wrong answer, because the tax rate applies to the pre-tax price, not the total. Always divide the total by (1 + rate), don't multiply it by (1 − rate).

Why Sales Tax Rates Vary So Much

Sales tax is typically set at the state, county, and sometimes city level, which is why the same purchase can carry a noticeably different tax rate just a few miles apart — always check the specific local rate rather than assuming a flat national number.

Step-by-Step: Calculate Sales Tax

1. Choose whether you're adding tax to a known price, or extracting it from a known total
2. Enter the price (or total) and the local tax rate
3. Get the tax amount and the other price instantly