A very common mistake when you want the price before tax is to subtract the tax rate from the total. If you paid $108.00 with 8% sales tax, the pre-tax price is not $108 × 0.92 = $99.36.

Why subtracting the percentage is wrong

Sales tax is charged on the pre-tax price:

total = price × (1 + rate)

So to go backwards you divide, you don't multiply by (1 − rate):

price = total / (1 + rate)
tax   = total − price

For $108.00 at 8%: 108 / 1.08 = $100.00, and the tax is $8.00. The "subtract 8%" method undercounts by 64 cents, and the error grows with the rate and the amount.

In code

function reverseSalesTax(total, ratePercent) {
  const rate = ratePercent / 100;
  const price = total / (1 + rate);
  // round to cents only at the end
  const priceCents = Math.round(price * 100);
  const taxCents = Math.round(total * 100) - priceCents;
  return { price: priceCents / 100, tax: taxCents / 100 };
}

console.log(reverseSalesTax(108, 8));      // { price: 100, tax: 8 }
console.log(reverseSalesTax(54.27, 7.25)); // { price: 50.6, tax: 3.67 }

Working in integer cents for the final split guarantees that price + tax always equals the receipt total exactly, which matters if you show both numbers to a user.

Gotchas

• Combined rates. In the US the rate on a receipt is usually state + county + city (+ special district). Use the combined rate, not just the state base rate.
• Rounding per line vs per receipt. Stores often round tax per item, so reversing a whole-receipt total can be off by a cent or two.
• Exempt items. Groceries, clothing or prescriptions are exempt or taxed differently in many states, so a mixed receipt can't be reversed with one rate.
• VAT/GST works the same way. UK VAT at 20%: divide by 1.2. Australian GST at 10%: divide by 1.1 (i.e. the GST is 1/11 of the total).

If you just need a quick number, I keep a free sales tax calculator here: https://clearcalctools.com/calculators/sales-tax-calculator/ (free, no sign-up).

How do you handle per-line rounding when reconstructing receipts? Curious what others do.