Most online image compressors work the same way: you upload your file to someone’s server, their backend runs the compression, and they send you back a smaller file. Simple enough. But that means your images — product shots, client photos, confidential mockups — are sitting on a stranger’s server for some amount of time. You’re trusting their retention policy. And their security.
Squoosh does it differently. Every byte of compression happens inside your browser tab. Nothing leaves your machine. And somehow, despite running entirely client-side, it produces smaller files than most server-based alternatives.
What’s Actually Happening Under the Hood
The reason Squoosh can run professional-grade compression without a server is WebAssembly. Google Chrome Labs compiled codecs like MozJPEG, OxiPNG, libwebp, libavif, and JPEG XL directly into WASM modules that execute in the browser at near-native speed.
This is the same MozJPEG that Mozilla developed to improve on the original JPEG encoder. The same libavif used by production systems. Squoosh isn’t using a JavaScript reimplementation — it’s running the actual compression libraries, just compiled to a different target. The quality you get is equivalent to what you’d get from command-line tools.
For the privacy angle: because there’s no upload, there’s no server, no retention policy to read, no third party involved at all. The file you compress never leaves your device. That’s not a marketing claim — it’s a technical consequence of how the tool works.
The Formats It Supports (And Why It Matters)
Here’s where Squoosh pulls ahead of simpler tools. Most online compressors handle JPEG and PNG. Some handle WebP. Squoosh supports:
| Format | Encoder | Best For |
|---|---|---|
| JPEG | MozJPEG | Photos, high-color images |
| PNG | OxiPNG | Screenshots, graphics with transparency |
| WebP | libwebp | Web images, good browser support |
| AVIF | libavif | Modern browsers, best compression ratios |
| JPEG XL | jxl | Future format, excellent quality |
| WebP2 | Experimental | Research/testing only |
AVIF is worth understanding. It’s derived from the AV1 video codec and consistently produces files 20-50% smaller than WebP at equivalent visual quality, and 50%+ smaller than JPEG. Google’s research on AVIF shows it outperforming other formats for most image types. Browser support now covers Chrome, Firefox, Safari, and Edge — meaning you can use it in production today.
Squoosh lets you encode directly to AVIF without installing anything. TinyPNG doesn’t offer AVIF. Convertio does, but requires uploading to their servers. This distinction matters if you’re working with images you’d rather keep private.
Side-by-Side Comparison: The Feature Nobody Else Has
The most useful thing about Squoosh isn’t even the codec selection. It’s the comparison slider.
When you open an image in Squoosh, you get a split view: original on one side, compressed on the other. You drag a divider left and right to compare. The file sizes update in real time as you adjust quality settings. You can see exactly where compression artifacts start to appear, then back off the quality slider until they disappear.
This sounds simple. It is simple. But no other no-login image tool does it this well. TinyPNG compresses automatically with no quality control — you get what you get. Convertio lets you set quality numerically but gives you no visual feedback. Squoosh shows you the tradeoff in real time, which means you can make an informed decision rather than guessing at numbers.
The file size display shows both absolute sizes (e.g., “1.2 MB → 340 KB”) and percentage reduction. This is the information you need to make a decision. Not “optimized!” — actual numbers.
How It Compares to the Common Alternatives
When you need to compress an image for a web project, the usual suspects are TinyPNG, Convertio, iLoveIMG, and similar services. All of them require uploading your file. All of them have file size limits on free plans. Most have usage caps.
TinyPNG is fast and produces good results for PNG and JPEG, but it uses its own compression algorithm without exposing quality controls. The free tier caps at 20 files per month. It doesn’t support AVIF or JPEG XL. And your files go to their servers in the Netherlands.
Convertio supports a huge range of formats and is genuinely useful for format conversion tasks. But free accounts cap at 10 conversions per day and 100 MB file size. The conversions happen server-side, which is fine for non-sensitive files.
Squoosh has no file size limit, no conversion cap, and no account requirement. It doesn’t even have a concept of “free tier” because there’s no server infrastructure to pay for. The only constraint is your browser’s memory, which becomes relevant for very large images (think 50+ megapixel RAW files).
Where Squoosh falls short: it’s one file at a time. If you need to compress 200 product photos in one session, the web interface isn’t the right tool. For batch compression, the Squoosh CLI solves this — it’s available as an npm package (npx @squoosh/cli) and supports the same encoders as the web UI.
The Open Source Argument
Squoosh is open source on GitHub under the Apache 2.0 license. The codebase is public, the compression happens locally, and Google Chrome Labs has been maintaining it as a showcase of what WebAssembly can do in browsers.
This matters for a few reasons. You can verify the tool isn’t doing anything unexpected with your files — there’s nothing to hide because there’s no server. You can host your own instance if you want. And the project being open source means the community can audit improvements to the WASM codec implementations.
If you want to go deeper on browser-based tools built with WebAssembly, the Datasette Lite project is another example — a full SQLite database that runs in your browser tab. The WASM trend is worth paying attention to. It’s enabling a class of no-login tools that simply couldn’t exist before.
When to Use Squoosh vs. Something Else
Squoosh is the right choice when:
- You’re working with photos or graphics you’d prefer not to upload anywhere
- You need AVIF or JPEG XL encoding without installing software
- You want to visually verify the quality/size tradeoff before downloading
- You need to squeeze every possible byte out of a file
For batch processing, the Squoosh CLI is the answer. For format conversion beyond images (documents, audio, video), Convertio covers more ground. For SVG specifically, SVGOMG runs locally in the browser and handles SVG optimization better than Squoosh.
The scenario where Squoosh uniquely wins: you have a single high-quality image, you care about compression quality, and you’d prefer the file not leave your computer. Product photography before sending to a client. Medical images. Personal photos. Legal documents that happen to be images. For these cases, uploading to a third-party service just to resize a file is a bad trade.
A Tool That Respects Your Files
Most no-login tools are “no-login” because they’re simple enough that accounts don’t make sense. Squoosh is different — it’s technically sophisticated enough that it could require an account and server infrastructure, but it was deliberately built to run client-side. That’s a design choice, not a limitation.
The web is increasingly capable of running real software without relying on backend servers. Squoosh has been demonstrating this since 2018. The codecs keep improving, browser support for AVIF keeps spreading, and the gap between “upload to a server” and “run locally” keeps closing.
If you haven’t used it yet, squoosh.app opens instantly without signup. Drop an image, compare the results, export. That’s it. For more tools that work this way — no account, no upload, no tracking — the nologin.tools directory catalogs hundreds of them across every category.