How to Calculate Sales Tax (Forwards and Backwards)
The one-line formula for adding sales tax, the reverse calculation almost everyone gets wrong, and why a state rate never matches your receipt.
Two numbers on a receipt, and plenty of people can’t reliably get from one to the other. Adding tax is easy. Taking it back out is where things go sideways.
Adding tax
Multiply, divide by 100, add. A $50 order at 8.25%:
50 × 8.25 ÷ 100 = 4.13
Total: $54.13. That’s all there is to it.
Removing tax is not subtraction
This is the one that costs money. A supplier invoices $107.25 including 7.25% tax and you need the net figure for the books.
Wrong: 107.25 − 7.25% = $99.47 Right: 107.25 ÷ 1.0725 = $100.00
Fifty-three cents on one line. Across a hundred lines of an expense claim it stops looking like a rounding difference. The reason is simple enough: the 7.25% was charged on the smaller number, so subtracting 7.25% of the total takes a percentage of the wrong base. The Sales Tax Calculator has a reverse mode for exactly this.
Why the state rate never matches your receipt
There’s no federal sales tax in the US. Forty-five states plus DC levy one, and most of them let counties, cities, and special districts stack their own rates on top.
Texas is “6.25%”, but a purchase in Houston lands at 8.25% once the city and the transit authority take their cut. Colorado has the lowest state rate among states that charge anything, 2.9%, and Denver still ends up past 8%.
Local rates add. They don’t compound. A 6.25% state rate plus 2% city is 8.25%, not 8.375%. Put the state figure in one field and the sum of everything local in the other.
Five states have no statewide sales tax: New Hampshire, Oregon, Montana, Alaska, Delaware. Alaska is the trap. No state rate, but plenty of boroughs and cities charge their own, so a purchase in Juneau is taxed anyway.
Discount first, tax second
Take the discount off, then tax what’s left. A $100 item at 20% off in an 8% jurisdiction: $80 net, $6.40 tax, $86.40 due.
Manufacturer coupons are the exception in several states. Because the retailer gets reimbursed for the coupon’s value, tax is charged on the full price rather than what you handed over. Shipping is its own mess: taxable in some states, not in others, and in a few only when the goods themselves are taxable.
If you sell online
South Dakota v. Wayfair (2018) ended the rule that you only collected tax where you had a physical presence. States now set economic nexus thresholds, commonly $100,000 in sales or 200 transactions into that state per year. Cross one and you register and collect there. The rate usually follows the buyer’s address rather than yours, because most states source sales to the destination.
Check it against a real receipt
If you have both a subtotal and a total, the rate falls out. Subtract, divide by the subtotal, multiply by 100. An $84.00 subtotal and $90.93 paid gives 6.93 ÷ 84 × 100 = 8.25%. Useful for catching a point-of-sale system charging the wrong jurisdiction.
Open the Sales Tax Calculator, choose add or reverse, and the figures update while you type. Nothing gets sent anywhere.