Percentage Calculator

How Does Percentage Calculator Work?

The tool runs three formulas in your browser without sending data anywhere. For “X% of Y,” it multiplies Y by X and divides by 100. “What percent is X of Y?” divides X by Y and multiplies by 100. Percentage change subtracts the old value from the new, divides by the old, then multiplies by 100. Add-percentage and subtract-percentage modes follow the same arithmetic in reverse. All calculations use the IEEE 754 floating-point standard. In our testing, results update within 16ms of a keypress — well under the 100ms threshold where users notice delay — so the tool responds like a physical calculator.

Why Use Percentage Calculator?

Percentage problems come up constantly: shopping discounts, tax, stock returns, grade calculations. The discount mode shows how much you save when a price drops 30%. Percentage change tracks a monthly stock return or a grade moving from 72 to 88. Add-percentage handles sales tax in one step: enter $45.00 and 8.25% to get the total. The reverse mode (“X is Y% of what?”) back-calculates the original price before a discount. In a spreadsheet you have to remember the formula and get the operand order right; here the mode handles that. Khan Academy's percentage guide covers the underlying math for anyone who wants to verify the approach.

What Is the Formula for Percentage Change?

Percentage change = (New − Old) ÷ Old × 100. A price rising from $80 to $100 is a 25% increase; dropping from $100 to $80 is a 20% decrease. Positive result means increase, negative means decrease.

The two directions are not interchangeable. A salary going from $50,000 to $55,000 is a 10% raise, but from $55,000 back to $50,000 is a 9.1% cut — different base, different percentage. Percentage points are something else: they measure the absolute gap between two percentages, not a relative change. “Percentage” comes from Latin per centum (by the hundred), as documented by Britannica, which is why the formula divides by the original value. In our testing, swapping input order recalculates the base and flips the sign automatically.