Percentage Calculator

Calculate what X% of Y is, what percent X is of Y, and percentage increase or decrease with step-by-step formulas and decimal multipliers.

Category: Math

How to Use Percentage Calculator

  1. Select your calculation mode: 'What is X% of Y?', 'X is what percent of Y?', or 'Percentage Increase/Decrease from X to Y'.
  2. Enter numeric values for Value X and Value Y (decimals and negative numbers are supported).
  3. Click Calculate to view the percentage result, decimal multiplier, complement value, and step-by-step algebraic formula.

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Frequently Asked Questions

How do I calculate what X% of a number Y is?

Convert the percentage X into a decimal multiplier by dividing by 100, then multiply by Y: Result = (X ÷ 100) × Y. For example, 15% of 250 is (15 ÷ 100) × 250 = 0.15 × 250 = 37.5.

How do I find what percentage X is of Y?

Divide the part (X) by the whole base value (Y) and multiply by 100: Percentage = (X ÷ Y) × 100. For example, to find what percent 45 is of 180, compute (45 ÷ 180) × 100 = 25%. Note that the base value Y cannot be zero.

What is the formula for percentage increase or decrease from X to Y?

Subtract the initial value X from the final value Y, divide by the absolute initial value |X|, and multiply by 100: Percentage Change = ((Y − X) ÷ |X|) × 100. A positive result indicates a percentage increase, while a negative result indicates a percentage decrease.

Why can't percentage change be calculated when the starting value X is 0?

Percentage change measures relative growth divided by the initial value X. When X = 0, dividing (Y − 0) by 0 results in division by zero, which is mathematically undefined. Only the absolute change (Y − X) is defined when starting from zero.

Why is a 20% increase followed by a 20% decrease not equal to the original number?

Percentages apply to the current base at each step. Increasing 100 by 20% gives 120, but decreasing 120 by 20% subtracts 0.20 × 120 = 24, leaving 96 (a net 4% loss). In general, (1 + r)(1 − r) = 1 − r².