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
X is Y% of what? (find the whole)
Apply % change to X
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.
Worked Example
All six modes with real numbers: 15% of 80 is 12 (a tip on an $80 dinner). 24 out of 60 is 40% (a test score). Going from 50 to 65 is a 30% increase (a price rise). If 12 is 15% of something, the whole is 80 (reverse-engineering a deposit). Applying a 20% discount to 250 gives 200. Each mode shows a progress bar visualising the proportion, and the copy button captures the full sentence — handy for pasting a calculation into an email or chat without retyping it.
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.