PercentFox is a small, free set of tools for the percentage math that comes up in everyday life — tips, discounts, price changes, grades, growth rates, and comparisons. The goal is a calculator that shows its work: every result comes with the formula it used and a running example, so you can learn the method, not just copy the answer.
How the calculators work
All calculations run in your browser using plain arithmetic — nothing is sent to a server. The formulas are the standard ones taught in school:
- Percentage of a number: part = (percent / 100) × whole
- Percentage change: (end − start) / |start| × 100
- Increase / decrease: base × (1 ± percent / 100)
- Percentage difference: |a − b| / ((a + b) / 2) × 100
Results are rounded to two decimal places for display. The underlying calculation is not rounded.
Who makes this
PercentFox is an independent project maintained by a small team. It is funded by advertising, which keeps the tools free to use with no sign-up.
Accuracy and disclaimer
We work to keep the formulas and examples correct, but the Site is provided for general reference and education only. It is not financial, tax, accounting, or investment advice. Always double-check a figure before relying on it for anything important.
Feedback
Found a mistake or want a calculator we don’t have yet? The contact page has an email address — suggestions are welcome.