Percentage Calculator
Switch modes to answer everyday percentage questions — discounts, exam scores, price hikes, and growth rates — without memorizing formulas.
Inputs
Result
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Percentage formulas (quick reference)
Part from percent: To find what P% of Y equals, compute Y × (P/100). That is the discount amount if Y is list price, or the tip if Y is the bill before tip — always confirm whether tax is included.
Ratio to percent: “X is what percent of Y?” answers (X / Y) × 100 when Y ≠ 0. Use it for conversion rates, test scores, or market share.
Increase/decrease by a percent: Increasing Y by P% yields Y × (1 + P/100); decreasing uses Y × (1 − P/100). Watch for compound successive changes — two 10% increases are not the same as one 20% increase.
Percent change between two values: From X to Y, use ((Y − X) / |X|) × 100. The result is positive if Y > X and negative if Y < X. If the starting value is zero, percent change is undefined — we surface an error instead of dividing by zero.
Frequently asked questions
How do you calculate percentage change from X to Y?
Percent change is ((Y − X) divided by the absolute value of X) times 100 when X is not zero. If X is zero, relative percent change is undefined.
Why does percent change show undefined?
Starting from zero, there is no meaningful ratio of change, so the tool asks you to describe the situation in absolute numbers instead.
Do you apply bank-style rounding?
We show enough decimal places for clarity; institutions may round currency differently.
Can inputs be negative?
Yes in modes where it makes sense—for example comparing a loss to a baseline—as long as denominators stay valid.
What is “percent of” mode?
It computes P% of a base value by multiplying the base by P divided by 100.
Are numbers uploaded?
No. All modes run locally in JavaScript.