Four common percentage problems in one place — pick the one that matches what you need.
Each mode is a different rearrangement of the same relationship: part = percent × whole. Depending on which two values you already know, the calculator solves for the third — the percentage, the part, or the whole. The percentage-change mode instead compares two numbers directly, showing how much one has grown or shrunk relative to the other.
A percentage is just a fraction out of 100 — "20%" means 20 out of every 100, or 0.20 as a decimal. That simple idea shows up everywhere: sale discounts, tax rates, tips, interest rates, exam scores, and statistics in the news all lean on percentages to make numbers easier to compare at a glance.
Most percentage problems boil down to one of three questions: finding a part when you know the percentage and the whole (like calculating a 20% tip), finding what percentage one number is of another (like a test score), or finding the whole when you know a part and its percentage (like figuring out the original price before a discount). Once you spot which question you're actually asking, the math is the same formula rearranged.
It's easy to mix up "percentage change" with "percentage points," and the difference matters. If a value goes from 80 to 100, that's a 25% increase (a 20-point difference relative to the original 80). But if an interest rate goes from 5% to 7%, that's a 2 percentage-point increase — which is also a 40% relative increase. Being clear about which one you mean avoids a lot of confusion in finance and statistics.