Price Change Calculator
Two questions shoppers and sellers keep meeting: "the price changed — by what percentage?" and "what did it cost before?" The first tool compares an old and a new price in both directions; the second runs a known increase or discount backwards to recover the original price.
Between two prices
Example: 80 → 100 is a 25% increase.
The price before a change
Example: 51 after 15% off (−15) was originally 60.
Percentages are one-way streets
A rise from 80 to 100 is 25%, but the return trip is -20% — both computed at build time by the engine behind the forms. The difference of 20 hasn't changed; the baseline has. This asymmetry is why a stock that drops 50% needs a 100% gain to recover, and why comparing two price changes only works when they're measured from the same direction.
Recovering the price before
To undo a percentage change you divide by the growth factor: original = current ÷ (1 + change ÷ 100). The sale tag reading 51 after 15% off reverses to 60, and the discount was worth 9 — not 15% of the sale price. The same division handles increases: it answers "what was the bill before the 8% rise?", which is how you check whether the quoted percentage matches the money actually added. For finding a sale price from a known discount (the forward direction), the discount calculator is the tool, and a package that shrank instead of repricing is a job for the unit price calculator.
When you just need the math
Percentage change isn't only about prices. For the abstract versions — what percent X is of Y, changes between any two quantities, adding a percentage to a number — the percentage calculator handles the general cases with the same engine discipline; this page stays focused on price tags, bills, and the trick questions they generate.
Frequently asked questions
A price went from 80 to 100. Why is that +25% but the way back −20%?
Each direction measures against its own starting price. Going up, the 20 difference is compared with 80 (+25%); coming down, the same 20 is compared with 100 (-20%). Same money, different baselines — which is why "prices rose 25%, so they'd have to fall 25% to recover" is wrong.
An item costs 51 after 15% off. What did it cost before?
Divide by the remaining fraction: 51 ÷ 0.85 = 60. Adding 15% back onto 51 gives 58.65 — short of the truth, because the discount was 15% of the original 60, not 15% of 51. The "price before the change" tool does this division for any percentage.
How do I undo a price increase?
Same principle, upward: divide the new price by (1 + increase ÷ 100). A bill of 259.20 after an 8% increase started at 259.20 ÷ 1.08 = 240. Subtracting 8% of the new bill instead would land below the true original.
The price rose 10% twice. Is that 20% overall?
Slightly more — increases compound. 100 rises to 110, and the second 10% acts on that larger figure, ending at 121: a 21% total increase. The same effect in the other direction is why stacked discounts save less than their sum.
Is this the same as the percentage calculator's change mode?
The underlying arithmetic is, and for abstract before-and-after numbers that page is the general tool. This one is built for prices specifically: it reports both directions at once and adds the reversal mode for recovering pre-increase and pre-discount prices.
All calculations run locally; no prices are transmitted or stored. The change and reversal formulas are tested, typed functions — display rounding is documented on the methodology page.