How to Convert Images to JPEG XL in Your Browser (Step by Step)
Reviewed by the OnlineFree.app team · Updated
Key points
- Custom PRD build converts one image to or from JPEG XL in the browser and never uploads the file.
- The tool reports original bytes, converted bytes and the exact percentage delta for your image.
- Lossless is on by default for PNG and JXL input, and off for JPEG and WebP input.
- Encoder effort maps to libjxl levels 3, 5 and 7, and the max setting can take ten seconds or more.
- Files above 25 MB or 40 megapixels are rejected, and only one image converts at a time.
Convert an image to JPEG XL in four steps
Custom PRD build is a free in-browser converter that changes one image between JPEG, PNG, WebP and JPEG XL and then shows the exact byte count before and after. The encoding runs on your own machine, so the file is never uploaded, and there is no account, extension or install of any kind.
The flow takes four moves. First, drop a JPEG, PNG, WebP or .jxl file onto the drop zone, or browse for it. Second, check the target format: it defaults to JPEG XL unless the input is already .jxl, in which case it defaults to JPEG. Third, decide between lossless output and a quality setting. Fourth, download the result, which arrives with the original filename and the new extension.
Conversion runs as soon as the file lands, and changing any setting re-runs it, so both panels stay in sync: drop zone and controls on the left, live preview and byte bars on the right. Keeping them visible side by side is the point, because the useful output is the comparison, not just the converted file. The whole toolset lives at FreeOnline.fyi.
Reading the size-compare bars
The right-hand panel reports three numbers: original bytes, converted bytes, and the delta as a percentage, plus a plain-language verdict line such as “JXL is 34% smaller at equal quality.” The bars above the numbers are drawn to scale, so a halving of size is immediately visible without reading the digits.
You can reproduce the percentage yourself with (original − converted) ÷ original × 100. A 1,000,000-byte JPEG that lands at 660,000 bytes is 34% smaller. If the converted file is larger, the delta goes negative, which is normal — exporting a small JPEG to PNG or to lossless WebP will always add bytes.
One caution: the verdict describes a size difference at the settings you chose, not a guarantee of identical perceived quality. Use the side-by-side preview at matching zoom, ideally at 100%, and look at fine texture and smooth gradients, where aggressive compression shows up first.
Which quality and effort settings should you pick?
Defaults are sensible for a first look: target .jxl, lossless on for PNG and .jxl input, lossless off for JPEG and WebP input, quality 90, encoder effort balanced, and keepMetadata on. The quality slider is ignored whenever lossless is on, so switching lossless off is what re-activates it.
Encoder effort is the setting people overlook. Fast, balanced and max map to libjxl effort levels 3, 5 and 7, and the interface warns that max may take ten seconds or more. Balanced is the right choice for quick decisions about whether JPEG XL is worth pursuing; max makes sense when you are producing one final file and want the last few percent of compression.
For lossy delivery, quality 90 is a solid starting point and 80–90 covers most photographic needs. Push below roughly 75 on smooth skies or skin tones and you will usually see banding in the preview before you see it anywhere else, which is exactly why the preview sits next to the slider.
Is JPEG XL worth using in 2026?
Viewing .jxl files outside a tool like this is still uneven as of 2026. Chrome removed built-in JPEG XL decoding in 2023, and support has varied by browser and platform since; the Wikipedia entry on JPEG XL tracks the current state. A .jxl dropped into a plain <img> tag may render for some visitors and not others.
That makes the format strongest for archiving, local photo libraries, and delivery pipelines you fully control. For public websites, the safer pattern is to keep JPEG or WebP as your default source and offer .jxl through a <picture> element with a fallback, so no visitor ends up with a broken image.
JPEG XL is standardized as ISO/IEC 18181, which is why encoder behaviour and these effort levels are consistent across implementations. If Custom PRD build reports a saving of only a few percent, changing your workflow probably is not worth it; on large photo sets with 30%+ reductions, the archive case is compelling.
Limits, privacy, and what to verify
Custom PRD build handles one image at a time, capped at 25 MB and 40 megapixels. Larger files are rejected rather than silently resized. Because everything runs locally in the browser, nothing is uploaded — which is good for privacy, but also means a slow device produces slow conversions, particularly at max effort.
Metadata handling is deliberately conservative: keepMetadata defaults to on and preserves EXIF orientation and ICC profile where the target format allows. Converting to a format that cannot carry a given field will drop it, so check orientation and colour on the result if those matter for your project.
Verify anything important before you replace originals. Download the file, confirm its size with your operating system or a terminal listing, and compare the preview at 100% zoom. Keep the source image until you are satisfied — the converted file is a new artefact, not an edit of the old one.
Frequently asked questions
Is Custom PRD build free, and does it upload my images?
Custom PRD build is free to use and never uploads your image. The JPEG XL encoder runs inside your browser, so the file stays on your device and no account, plugin or install is required. That also means conversion speed depends on your own hardware: a fast desktop finishes quickly, while a low-power laptop takes longer, especially at the max effort setting.
How much smaller will my image get with Custom PRD build?
There is no fixed number, because the saving depends on the image and the settings you choose. Custom PRD build answers this for your actual file, showing original bytes, converted bytes and the percentage delta the moment conversion finishes. A lossless PNG-to-JXL conversion usually saves less than a lossy re-encode at quality 90, and some images grow slightly instead of shrinking.
Can I convert a .jxl file back to JPEG or PNG?
Yes. Drop a .jxl file into Custom PRD build and the target format defaults to JPEG, with PNG and WebP also selectable. The pixels come back as they were stored, though the exported file is re-encoded in the new format, so byte-for-byte identity with an older JPEG or PNG is not guaranteed. Keep your originals for anything important.
What are the file limits on Custom PRD build?
Custom PRD build accepts one image at a time, up to 25 MB in file size and 40 megapixels in resolution. Anything larger is rejected rather than automatically resized. Inputs and outputs cover JPEG, PNG, WebP and JPEG XL. Batch conversion is not offered, so a folder of images has to be handled one file at a time.
Do I need to install libjxl or a browser extension?
No. Custom PRD build runs the encoder in the browser, so there is no libjxl install, command-line step, extension or account. That is the main advantage over running cjxl locally: you drop a file and get a size comparison in seconds. The trade-off is a firm ceiling on file size, since browser memory is finite.