What this site is
Redmoon Calculators is a free collection of text, readability, and writing-analysis tools. It exists because the existing free options online are usually one of three things: a single calculator embedded next to a wall of ads, a paywalled SaaS app with a free tier designed to expire, or a tool that quietly uploads your text to a server you can't audit. We wanted something different: a wide library of formulas, all open, all running in your browser, with the math visible on every page.
Today the site hosts more than seventy calculators across readability scoring, lexical analysis, style checks, and writing utilities — Flesch, Gunning Fog, SMOG, Dale–Chall, Coleman–Liau, ARI, LIX, lexical density, passive voice, hedge detection, keyword density, reading time, and more. Each tool ships with its formula, a worked example, an explanation of when it's the right choice, and a list of the mistakes it most commonly makes.
How it works
Every calculation runs locally in your browser. The text you paste into a tool is processed in JavaScript on your device; it is not uploaded, logged, or stored on a server. That isn't a marketing claim — it's a consequence of how the site is built. You can open the developer console on any tool page and watch the calculation happen.
The formulas are implemented from the original papers wherever possible. Where a tool requires a word list (Dale–Chall, hedge detection, cliché lists, etc.), the list is included in the page bundle and viewable in the page source.
Who builds it
Redmoon Calculators is written and maintained by Paul Clark at Redmoon Software, an independent developer studio. Every tool page carries that byline and the date its formula or rates were last verified, so you can see who stands behind a number and how recently it was checked. We also run Redmoon Randomizers, a sister site of dice rollers, random generators, and decision tools.
Where the numbers come from
The readability tools implement named formulas from published research — Flesch (1948), the Kincaid Navy report (1975), McLaughlin's SMOG (1969), Coleman–Liau, Dale—Chall, Spache and the rest — and each tool page cites the specific work it implements rather than asserting a score and expecting you to take it on faith. Where a paper sits behind a publisher paywall we cite it in text instead of linking to a URL we cannot vouch for.
The selling calculators are different: they encode fee schedules and royalty rates that change. Those pages link the operator's own current fee page — Etsy, Amazon KDP, ACX — and say plainly that the rates drift, because a pricing calculator with no source and no date on it is worse than no calculator at all. Tools whose output is pure arithmetic over your own inputs, like dice odds or fabric yardage, cite nothing, and their pages say so by simply not carrying a sources block.
How it's funded
The site is supported by display advertising. Ads are clearly labelled and never disguised as content or results. We do not sell user data, and we do not run sponsored "calculators" promoted as if they were neutral. If a tool is here, it's because we think it's useful — not because someone paid us to add it.
Talk to us
Found a bug, want a new calculator, disagree with how we implement a formula, or want to write about us? Use the button in the footer or on any tool page. We read every message.