By Flavio Copes

I launched a new tools section on the site: 90 free calculators and utilities for developers, running entirely in your browser. No signup, no ads.

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I just launched a new section on this site: flaviocopes.com/tools.

It’s a collection of 90 free tools for developers. Calculators, utilities, little helpers for things I kept doing by hand or with random sites full of ads.

Why build them? To be useful. I’ve been writing tutorials for years to help people learn. Tools are the same idea in a different shape: instead of explaining something, I give you something you can use right away.

A few principles I followed:

• Free, no signup. Open the page, use the tool, done.
• Everything runs in your browser. Your data never leaves your machine. Paste a JWT in the decoder or a prompt in the token counter, and nothing gets sent to a server.
• No ads, no tracking, no dark patterns.

The tools are grouped by category. Some highlights:

AI and LLMs: an inference cost calculator to estimate your monthly LLM bill, a token counter that shows what a prompt costs on every major model, a subscription vs API break-even calculator, a context window fit checker, and an AI agent cost visualizer that shows why multi-step agent runs get expensive.

JavaScript and TypeScript: a regex tester with a plain-English explainer, a JSON formatter, a JWT decoder, a JSON to TypeScript converter, and an event loop visualizer that steps through microtasks vs macrotasks.

CSS and HTML: a flexbox playground, a CSS grid generator where you paint named areas, a color converter with WCAG contrast checking, and a clamp() calculator for fluid typography.

Git: a command finder where you describe the goal and get the command, and a “Git oh no” recovery tool for when things go wrong.

Databases: a database chooser, a SQL formatter, and a SQL to Prisma to Drizzle schema converter.

Web platform: a CORS debugger, a Cache-Control header builder, a cron expression builder with next-run preview, and a timestamp converter.

Business and writing: a freelance rate calculator, a SaaS MRR calculator, and a payment platform fee comparison for Paddle, Stripe, Gumroad, Polar, and Lemon Squeezy.

And a lot more: the full list is on the tools page, organized by category.

One small exception to the “browser only” rule: the app idea generator has an optional AI mode that calls a small model on the server. It’s rate limited so it stays free, and the curated ideas work without it.

If there’s a tool you’d love to have, let me know. If it’s useful to you, it’s probably useful to a lot of people.

Check them out at flaviocopes.com/tools.

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