Percentage Calculator
X% of Y, X is what percent of Y, percentage increase, and percentage decrease
Frequently Asked Questions
Percentage calculations cover several genuinely distinct question types — "what is X% of Y," "X is what percent of Y," percentage increase, and percentage decrease — and each uses a different rearrangement of the basic percentage relationship, which is exactly why a single "percentage formula" memorized for one question type often gets misapplied to a different one.
Why these are different calculations, not one formula applied four ways
"X% of Y" is a straightforward multiplication (percentage as a decimal, times the base value). "X is what percent of Y" reverses that — dividing X by Y and converting the result to a percentage, answering a proportion question rather than calculating a portion. "Percentage increase/decrease" is different again — it measures the relative size of a change between an original and new value, calculated as the difference divided by the original value (not the new value), expressed as a percentage. Each of these four question types requires identifying which two numbers play which role (base value versus part, or original versus new value) before applying the correct rearrangement — using the wrong formula, or dividing by the wrong one of two given numbers, is where most percentage-calculation errors actually happen.
A worked example
Calculating percentage change (increase or decrease) between two values by dividing by the new value instead of the original value produces a different, incorrect percentage — the convention specifically divides by the original (starting) value, since percentage change describes how large the change is relative to where the value started, not relative to where it ended up.
How this connects to your other everyday calculations
Percentage math underlies several other everyday calculators on this site — the Discount Calculator applies percentage-decrease logic specifically to pricing, and the Percent Error Calculator applies a related but distinct relative-deviation calculation to measurement accuracy rather than everyday value changes.
Common mistakes
Dividing by the new value instead of the original value when calculating a percentage increase or decrease is the single most common percentage-math error — always confirm which of the two numbers involved is the "starting point" (the denominator) before calculating a percentage change, since swapping them produces a different and incorrect result.