Updated July 9, 2026
I'm Chris Terry. I put this site together after watching a retail app apply a second discount percentage on top of the first one and land on a number that didn't match the receipt in my hand. This page explains how the arithmetic behind the calculator and the guides actually gets checked, so you aren't just trusting a green result box because it looks confident.
Who Writes What
I maintain the calculator itself, the three input modes plus the percent-change tool, and the formula reference dataset. Marcus Vance writes the guides that walk through the reasoning: the formula breakdown, the percentage-versus-percentage-point distinction, and the roundup of common mistakes. The authors page has more on where he's coming from.
How the Arithmetic Gets Checked
Every worked example in a guide gets run back through the live calculator before it publishes. If typing 35 and 260 into the tool doesn't return 91, the way the formula guide says it should, that guide doesn't go out until it's fixed. Percentage change and percentage difference get the same treatment against their own formulas, new minus old over old for change, absolute difference over the average for difference. None of that arithmetic was invented here; every one of those formulas is the same one you'd find in a standard statistics class. What actually gets checked is whether the calculator's code and the guide's wording agree with each other, number for number.
When a Number Turns Out Wrong
Readers occasionally catch something we missed, usually a typo in a worked example rather than a flaw in the underlying formula. Send it through the contact page and I read every message myself. If something we published turns out to be genuinely wrong, we fix it and note the change on the page rather than rewriting history quietly.
Advertising on This Site
The only revenue here comes from Google AdSense display ads. There are no affiliate links and no sponsored placements steering a reader toward one answer over another, and there's no advertiser in a position to ask for that even if they wanted to.