Three common percentage calculations in one tool
This calculator handles the three percentage questions people actually ask: what is X% of Y, X is what percent of Y, and the percentage change from X to Y. Each lives on its own tab, so you pick the shape of your question instead of rearranging a formula. All the math runs locally in your browser — nothing you type is sent anywhere.
Enter two numbers and hit Calculate (or press the space bar). The result comes with a plain-English explanation, and the percent-change tab labels the answer as an increase or decrease.
Going from 80 to 100 is a 25% increase, because the gain of 20 is measured against the start of 80. Going back from 100 to 80 is only a 20% decrease, since the same 20 is now measured against 100. That's why a stock that drops 50% needs a 100% gain to recover. The change tab always divides by the starting value, which is also why that value can't be zero.
The first finds a part from a percentage (25% of 200). The second finds a percentage from two values (50 is what percent of 200). The third measures relative change between an old and a new value.
If a rate moves from 4% to 5%, that's a rise of 1 percentage point but a 25% relative increase. The change tab computes relative change, so entering 4 and 5 gives +25%.
Percentage change divides the difference by the starting value, and dividing by zero is undefined — any increase from nothing is infinite in percentage terms.
Use the first tab to find the discount amount, then subtract it from the price. For 30% off a $90 item: 30% of 90 is 27, so you pay 90 - 27 = $63.