Percentage Calculator
Calculate percentages three ways: find X% of Y, find what percent X is of Y, or calculate the percentage change between two numbers.
What is X% of Y?
X is what % of Y?
% change from X to Y
How It Works
The Percentage Calculator solves the three percentage problems you actually encounter day to day, all on one page. Mode 1: ‘What is X% of Y?’ — applied for tips, discounts, sales tax, or scaling a value. Mode 2: ‘X is what percent of Y?’ — applied for grading, proportions, market share. Mode 3: ‘What is the percentage change from old to new?’ — applied for growth rates, price changes, performance comparisons. Each mode takes two numeric inputs and produces the answer instantly; switching modes does not lose your input values. The math is plain decimal arithmetic with full floating-point precision, and the result is shown rounded to a reasonable number of digits with the full precision available on copy. Negative numbers and decimals work as expected: a percentage change from 100 to 80 reports −20%, and a percentage of 0 is rejected with a clear error rather than silent NaN. Everything runs in your browser, which means even quick calculations involving salary, invoice totals, or financial figures stay private.
Use Cases
- Calculating a tip or service charge on a restaurant bill
- Working out a discount or sale price
- Finding what share of a budget a line item represents
- Computing year-on-year growth rates for a report
Frequently Asked Questions
- Why are there three modes?
- Because the same word 'percentage' covers three different operations. Listing them explicitly avoids the confusion of trying to back-derive what you meant from a single input field.
- How does the change-percentage mode treat negative numbers?
- A change from 100 to 80 reports −20%; a change from 80 to 100 reports +25%. The tool computes (new − old) ÷ |old| × 100.
- What if the original value is zero?
- Percentage change from zero is mathematically undefined (division by zero); the tool reports an error rather than infinity or NaN.
- How precise is the output?
- Calculations use IEEE 754 double precision (about 15 decimal digits). The display rounds to 2–4 decimals; the underlying number is more precise.
- Are my numbers sent anywhere?
- No. Calculations run entirely in your browser.