About maratool

maratool is a collection of free, browser-based developer tools. Almost every tool runs entirely in your browser — no data is sent to any server, no sign-up is required, and there are no paywalls. The one exception is the Instagram video downloader, which has to proxy through a small server because Instagram blocks direct browser fetches.

The tools cover common tasks that developers do constantly: converting formats, generating tokens, checking regex, decoding JWTs, hashing passwords, building meta tags. The goal is to have the fastest, cleanest version of each of these tools on the internet.

Why it exists

There’s a class of task I kept running into. Calculate a percentage I’m too lazy to do in my head. Strip the background off an image or a GIF. Throw together a phone mockup. Stuff you do not want to install an app for and definitely do not want to make an account for. For years I’d google whichever site did the thing, ignore the ads, copy the result, close the tab. Repeat next week with a different one.

AI can do some of these, but not always well, and sending private data through an LLM just to extract text from a contract PDF is not what most people want. So I built maratool: a growing pile of small browser-only tools where the math is right, your data stays on your machine, and nothing is uploaded anywhere.

It used to be just for me. Today the source is on GitHub and contributions are open. The full story is on the blog.

Open source

The project is source-available under the O’Saasy License (MIT plus a clause preventing third parties from offering it back as a competing hosted SaaS). You can fork it, run it yourself, and send PRs at github.com/almeidamarcell/maratool. The health calculators in particular need clinicians to help maintain them.

Built by

Marcell Almeida. You can reach him at maratool@marcell.com.br or open an issue on github https://github.com/almeidamarcell/maratool.

Stack

  • Astro — static site generator, pre-rendered HTML
  • Vanilla JS — zero frameworks in the tool logic
  • Web APIs — Web Crypto, DOMParser, FileReader, Canvas — no unnecessary libraries
  • Cloudflare Pages — global CDN, deployed on every push to main

Business model

AdSense. No subscriptions, no freemium tiers, no data collection. The site needs traffic to generate revenue, which means the tools need to be genuinely useful and fast to find.

When was it started?

March 2026. The first set of tools launched on March 17, 2026. New tools are added regularly.