Skip to content
Free Tool Arena

Source + transparency

What runs in your browser, where the formulas come from.

Free Tool Arena is built on open-source libraries, ships every formula with primary-source citations, and runs entirely in your browser. Here's how to verify all of that yourself.

Verify what runs in your browser

You don’t have to trust our claim that tools are local-only. Open your browser’s DevTools (Cmd/Ctrl + Option/Shift + I) → Network tab → run any tool. The only requests you’ll see are the static HTML, our own JS chunks, Vercel Analytics (privacy-friendly), and Google AdSense if not blocked.

No tool inputs leave your machine. No API call to compute your loan payment, tip split, or password strength. Disconnect from the internet after the page loads — every tool keeps working.

Open-source libraries we build on

  • Next.js 14 + React 18 for the site framework
  • pdf-lib for client-side PDF editing (merge, split, watermark, redaction)
  • pdf.js (Mozilla) for PDF reading + table extraction
  • tesseract.js for in-browser OCR
  • qrcode for QR generation
  • heic2any for HEIC → JPG conversion
  • Web Crypto API (built into your browser) for password generation, hashing, encryption

Published formulas + sources

Every calculator with meaningful math has a “Show the math + sources” panel below the inputs. The formula in plain text, what the tool assumes, authoritative source URLs (IRS, CDC, SEC, CFPB, peer-reviewed papers), and date of last verification.

Examples: mortgage, loan, compound interest, BMI, calorie, paycheck, tax, macros.

Site code

The full site repository is currently private while we polish for release. The per-tool methodology pages publish the actual logic — the formula, sources, and assumptions — which is what matters for verifying that a calculator gives correct answers.

If you find a methodology error, please contact us — we treat methodology bugs the way other sites treat security bugs: top priority.

What we don’t do

  • No tracking pixels, no fingerprinting, no third-party trackers beyond AdSense
  • No telemetry on tool inputs (we don’t know what loan amounts you’re calculating)
  • No accounts, no email capture, no “upgrade to remove watermark”
  • No file upload to a server — anything that processes a file does it locally

See also: Trust & safety, Security, Tool methodology, Accessibility. Last reviewed 2026-05-02.