Mello is a directory of 122 browser-based utilities — formatters, converters, calculators, image and PDF tools, password and hash generators, date and time helpers, and more — that all share one property: they run entirely in the browser tab you have open. There is no upload, no account, no server-side processing, no third party between you and the tool. The JSON you format, the image you compress, the password you generate, the PDF you merge: all of it stays on your own machine.

That design choice matters more than it sounds. Most free online tools work by sending your input to a server, processing it there, and sending the result back. That is fine for harmless inputs, but the inputs you most often want to format, compress, decode, or convert are not harmless: an API response from production, a JWT containing user IDs, a scan of a contract, a photo with embedded GPS coordinates, a config file with secrets, an unannounced design asset. Pasting those into a server-side tool means handing them to an operator you have no agreement with. Mello avoids the question entirely by never sending anything anywhere.

What is here

The 122 tools are organized into 10 categories — each with its own page that explains what is in it and why. The largest groups by everyday use are developer utilities (JSON, regex, encoders, hashes, JWT decoding, subnet math), finance calculators (loan and mortgage amortization, ROI, margin, salary conversion, invoicing), image tools (compress, resize, crop, format conversion across every common pair, social media sizing), and PDF utilities (merge, split, rotate, image-to-PDF and back). The text and design groups cover the smaller daily jobs — case conversion, diffs, slug generation, color and palette work, contrast checking, CSS gradients and shadows.

Alongside the tools sit 13 long-form guides on the topics where a tool alone is not enough — choosing the right image format for the web, the differences between PNG, JPG, WebP, AVIF, and HEIC, how to compress images for email or reduce file size in general, and the social media image sizes each major platform expects. The guides cross-link to the tools that do the relevant work, so you can move from understanding the choice to making it without changing tabs.

How to find the right tool

The search box at the top matches tool names, taglines, and keywords — typing "compress", "convert", "color", "date", or any partial term surfaces every tool that mentions it. If you are browsing rather than looking for something specific, the category chips below the search box filter the page down to one category at a time, and the "Featured" section at the top highlights a hand-picked set of the most useful tools across the site. Each category page repeats this with a longer intro and the full set of tools in that group.

Privacy by default, not as a feature

Most online tools treat privacy as something to claim in the footer. Mello treats it as an architectural choice: the site is statically generated, every tool is a client-side JavaScript module, and no part of the workflow has a server-side endpoint that handles your input. That means there is nothing to leak. Close the tab and the JSON you pasted, the image you compressed, and the password you generated are gone. The site does serve ads (which is what keeps it free) — full details are on the privacy page.

For anything that needs more than a single utility — choosing between image formats, understanding how a particular conversion works, or working out which size your photos need to be for a specific platform — the guides are the longer-form companion to the tools. For everything else, pick a category below and start.