A discount and a tax rate both sound like simple percentage math — until you're combining two discounts, or trying to work backward from a total that already includes tax. Here's where the confusion actually comes from.
Why Stacked Discounts Don't Just Add
Each discount applies to whatever price remains after the previous one, not to the original price. A $100 item at 20% off drops to $80; a further 10% off applies to that $80, not the original $100 — landing at $72, a combined 28% off rather than the 30% you'd get by simply adding the two percentages. Stacked percentage discounts are always slightly less generous than adding them together suggests.
Tax Before or After a Discount?
In most jurisdictions, sales tax is calculated on the discounted price, applied after the discount, not before. This reflects the general legal principle that sales tax should apply to what was actually paid, not the item's original list price. A $100 item at 20% off ($80) with 8% tax comes to $86.40 — not $108 taxed down afterward, which would be the wrong order.
Finding the Pre-Tax Price
To reverse a tax-inclusive total back to the pre-tax price, divide by (1 + the tax rate as a decimal) — for an 8% rate, divide by 1.08. A $54 total at 8% tax means the pre-tax price was 54 ÷ 1.08 = $50. This is the same reversal logic used for percentage increases generally: dividing by 0.92 (treating it like reversing a discount) would give a different, incorrect number.
Why Tax Rates Vary So Much
In many places, sales tax stacks across multiple government levels — state, county, and city rates can each apply independently and combine into one total rate. This is why the same purchase can carry a noticeably different total tax rate just a short drive away, across a county or city line, even within the same state.
Calculating It Instantly
Work out a discounted price, stack a second discount, or reverse from a sale price with our free Discount Calculator, or add, reverse, or total sales tax with quick-fill U.S. state rate presets using the Sales Tax Calculator.
FAQ
Does stacking a 20% discount and a 10% discount give you 30% off? No — each discount applies to whatever price remains after the previous one, not to the original price. A $100 item at 20% off becomes $80, and a further 10% off applies to that $80, not the original $100, bringing it to $72 — a combined 28% off, not 30%. Stacked percentage discounts are always slightly less generous than simply adding the percentages together.
Does sales tax apply before or after a discount? In most jurisdictions, sales tax is calculated on the discounted price, not the original price — the discount is applied first, then tax is calculated on what remains. This is generally required by law in most places, since sales tax is meant to apply to what was actually paid for the item, not its original list price.
How do I figure out the pre-tax price from a total that already includes tax? Divide the tax-inclusive total by (1 + the tax rate as a decimal). For an 8% tax rate, divide by 1.08. A $54 total at 8% tax means the pre-tax price was 54 / 1.08 = $50 — dividing by 0.92 (as if reversing a discount) would give the wrong number, since tax and discounts use inverse formulas.
Why do sales tax rates vary so much between different areas? In many countries, sales tax is set at multiple government levels — state, county, and city rates can all stack on top of each other, and each jurisdiction sets its own rate independently. This is why the same purchase can carry a meaningfully different total tax rate just a short distance away, across a city or county line.