Updated July 9, 2026
I own UtilCalc and wrote the code behind every calculator on it. This page is the plain answer to a question I get more than you'd think: how do you know the discount calculator is actually doing the math right? Short version: I run it against the same arithmetic a math teacher would grade by hand, and I check it before anything publishes.
The arithmetic each tool follows
Nothing here is invented or approximated. The date calculator counts real calendar days, leap years included, using JavaScript's own date math rather than a fixed 365-day year. The percentage tools use the standard part-over-whole relationship in whichever direction you need it: finding a part, finding a percent, or finding a whole. The discount calculator applies a percent off sequentially, meaning a second discount is taken off the already-reduced price, not stacked onto the original. The fraction calculator reduces every result to lowest terms using the greatest common divisor, the same method taught in a fifth-grade classroom. The Roman numeral converter follows the standard subtractive notation (IV instead of IIII) across the conventional 1 to 3,999 range. The numerology calculator uses the Pythagorean letter-to-number method and reduces multi-digit sums the way that system has always worked; I flag it clearly as entertainment, not science, because it is one. The Robux tax calculator applies Roblox's published 30 percent marketplace cut and its own DevEx conversion rate, both of which I check against Roblox's current documentation rather than a number I memorized once.
How I check the numbers
Every worked example that appears in a guide gets typed into the live calculator before that guide goes out. If a guide says 20% off $80 is $64, I confirm the discount tool returns $64 for those exact inputs. That step catches typos and stale examples before a reader ever sees them. When a formula depends on a value that changes, like the Robux DevEx rate, I recheck it on a rolling basis rather than assuming a number written down months ago is still current.
Review schedule
I re-run every calculator's worked examples roughly once a quarter, and immediately any time I touch the underlying code or a referenced external rate changes. A guide that hasn't been reviewed in longer than that gets a note added rather than sitting untouched with a stale "last updated" date.
When something is wrong
If a number doesn't check out, tell me through the contact page and include the exact inputs you used. I read every message myself, since there's no larger team here to route it to. If the error is real, I fix the calculator or the guide and note what changed rather than quietly editing it away.
Who writes what
Jessica Martinez writes the guides that walk through the reasoning behind each formula. I maintain the fourteen calculators themselves and check her worked examples against them before anything ships. More on how that division works is on the authors page.
How this site makes money
UtilCalc runs on Google AdSense display ads. There is nothing else influencing what gets published or how a formula is explained.