Why "percentage calculator" is actually three different tools#
"What is 20% of 50?", "10 is what percent of 50?", and "what is the percent change from 50 to 60?" are three structurally different calculations that happen to share the word "percent" — the first multiplies, the second divides one way, and the third divides a different way and needs both a starting and ending value. A single generic form that asks for two numbers and guesses which calculation you meant is guessing wrong for at least two out of three visitors. This tool keeps the three modes explicit and separate.
Why percent change uses the absolute value of the starting number#
Percent change is defined relative to where you started — (new − old) ÷ old — but if the starting value happens to be negative (a common case in finance, like a loss shrinking toward zero), dividing by a negative number flips the sign of the result in a way that reads backwards to a human. This tool divides by the absolute value of the starting number instead, so a move from -50 to -40 correctly reads as a 20% improvement, not a confusing -20%.
A percentage can validly go over 100%#
"60 is what percent of 50?" is a completely valid question with an answer over 100% (120%) — the part is simply larger than the whole. This tool does not clamp or flag that as an error, since exceeding 100% is often exactly the point (a metric that grew past its baseline, a quota exceeded).