A Privacy-Respecting Percent Calculator

A free percentage calculator I used for years has never offered a paid option, though I would have paid for one. It’s a simple little troika of scripts to calculate percentages, easier to use than to build myself. But when my ad blocker blocked their scripts from harvesting information about me that would help them throw ads at me, suddenly I was confronted with a full-screen alert, disguised as a technical error on my end.

TL;DR, here is a totally free-forever version, with no cookies or trackers.

The Offending Issue

If you visit percentagechange.net with an ad blocker active, you see an error message crafted to look like there’s a technical problem with your computer or browser. The help contact is hosted at “error-report.com,” reinforcing the impression that you have an error to report.

If you’re not sure what it means, “Error: Failed to load script. Fallback failed” sounds like a very serious issue. One that they even tried to fix, but, alas, the fallback failed.

Stated like this, it is technically true but not helpful to the user, who may find it frightening.

It’s also fundamentally dishonest: rather than disclosing something like, “We support this service with ad revenues that allow us to keep this site free to the user,” they employ the visual grammar, user-experience, and sensibility of an error message alerting you of a problem.

Clicking through in this modal display (which otherwise blocks the whole page) brings up a “troubleshooting guide.”

Select your ad blocker from the list, and it walks you, step by step, through disabling it.

This is deceptive in specific, deliberate ways. The language frames your computer, not their business model, as the problem.

The interface may mimic technical support, but the “fix” requires you to allow not just ads but the trackers, data brokers, and other tools those ads carry.

When I wrote to tell them I objected to this change, and that I understood what was happening and did not want assistance, they emailed me twice more offering technical help in the form of instructions on how to disable my ad blocker.

That last email is worth examining. It misleadingly characterized disabling your ad blocker for their site only as “the most restrictive site-only allowlist,” which they said would “limit ad allowance” to “just” their website. And like all the others, it is polite, and signs off “best regards,” with no name or anything to let the uninitiated understand that this is a third-party service that sells this to websites to enable them to maximize ad revenue.

Framing surveillance as a measured, minimal concession is not a compromise. It is an intentional manipulation of the trust users have built with a site they’ve relied on. This email is a concrete manifestation of bad faith.

And as I confirmed in my discussion with “error-report.com”, they offer no paid ad-free alternative. The cost of using this site is that you submit to behavioral tracking in exchange for a percentage calculator.

The Solution

Building an equivalent took about an hour.

Use it here, with no cookies or trackers. Ever.