Last updated: October 2026
How to calculate a discount
Sales come in two flavors and people search for both: percent off ("40% off $30") and dollars off ("$40 off $40"). The calculator above handles both — pick the mode, type the numbers, and it shows the final price, what you save, and the effective discount percentage. For percent-off the mental shortcut is: figure out what fraction you pay (100% − discount), then multiply. 25% off means you pay 75%, i.e. multiply by 0.75.
The 40% off sale
40% off is the workhorse clearance discount, which is why it dominates the search data. The pattern is always the same: multiply by 0.60. $21.99 × 0.60 = $13.194, rounded to $13.19 at the register. For $100 the math is trivially clean ($60.00), and for $70 it's $42.00 — you save $28.00. One trap: "40 off of $40" is dollars off, not percent — $40 − $40 = $0.00, a full 100% discount. The calculator's $ off mode handles that case.
20%, 25% and 30% off
These round-number discounts have easy mental shortcuts: 25% off is "divide by 4, subtract" ($40 → $10 off → $30); 20% off is "divide by 5, subtract" ($35 → $7 off → $28); 30% off means you pay 70% ($55 × 0.70 = $38.50). For 20% off $200, the savings alone ($40) are bigger than most of the other final prices here — a good reminder that percent discounts scale with the price.
Dollars off vs percent off
A dollars-off discount is just subtraction — until it goes past the price. $20 off a $40 item is a clean 50% discount ($20.00 final). $40 off a $40 item is the whole price (100% off). But "$30 off $20" breaks the pattern: $20 − $30 = −$10.00, and no store pays you — the discount is capped at the item price, so you'd pay $0.00. The calculator shows the raw math; the FAQ above explains the real-world cap. When a sale says "$X off," always compare it to the price first: $20 off a $40 item beats 40% off the same item ($16 off).
Popular discount calculations
| Discount | Final price | You save |
|---|---|---|
| 40% off $30 | $18.00 | $12.00 |
| 25% off of $40 | $30.00 | $10.00 |
| 40% off $21.99 | $13.19 | $8.80 |
| 40% off of $100 | $60.00 | $40.00 |
| 30% off $55 | $38.50 | $16.50 |
| $40 off of $40 | $0.00 | $40.00 |
| 20% off of $35 | $28.00 | $7.00 |
| 20% off of $200 | $160.00 | $40.00 |
| 20% off $40 | $32.00 | $8.00 |
| 40% off $70 | $42.00 | $28.00 |
| $20 off of $40 | $20.00 | $20.00 |
| $30 off of $20 | −$10.00 (capped at $0.00) | $20.00 |
How the math works
The only conversion in discount math is percentage ↔ decimal: divide by 100 (move the decimal two places left, drop the % sign). 40% = 0.40, 25% = 0.25, 7.5% = 0.075. Then percent-off is a single multiplication: price × (1 − decimal). The "1 −" part is the fraction you pay — it's why 40% off is really "pay 60%." Dollars-off is even simpler: subtraction. Both modes are one operation, which is why the calculator updates instantly as you type, and why it also works in reverse: given a final price and a discount, it can recover the original price from the same formula.