J2K to FITS Converter
Processing entirely on your device — large files may take a moment.
J2K is the raw codestream spelling of JPEG 2000 — the same wavelet image without the JP2 container wrapper. Convert FITS frames to PNG or JPG to preview telescope data, or pack them into a ZIP for sharing.
Because the whole conversion happens on your device, it is instant, unlimited and genuinely private — no accounts, no watermarks, no files on anyone's server. Multiple files are converted in one go.
How J2K compares to FITS
| J2K | FITS | |
|---|---|---|
| Compression | Wavelet (lossy + lossless) | None (optionally Rice/GZIP) |
| Transparency | Supported | Not supported |
| Animation | Not supported (MJ2 is separate) | Not supported |
| Metadata | XML boxes (EXIF/GPS) | Rich text header (WCS) |
| Best for | Archives, medical, cinema | Telescope frames, science data |
How to convert J2K to FITS
- 1
Select your files
Drag & drop your J2K files onto the area above, or click to browse. You can add multiple files at once for batch conversion.
- 2
Convert instantly
Click "Convert to FITS". The conversion runs entirely inside your browser using WebAssembly — no upload, no queue, no waiting.
- 3
Download results
Download each converted FITS file individually, or grab everything at once as a ZIP archive.
About the J2K format
JPEG 2000 (JP2) replaced JPG's DCT blocks with wavelet transforms: at the same bitrate it shows fewer blocky artifacts, and its lossless mode compresses better than JPG ever could. It was the format of early digital cinema and remains common in archives, medical systems and geospatial pipelines.
Browser support never arrived — Chrome, Firefox and Safari all skip it — so a JP2 file is often invisible on the web. Converting JP2 to JPG or PNG makes archival material viewable anywhere; converting the other way gives long-term storage better compression.
+Strengths
- ✓True lossless mode
- ✓Better quality per byte than JPG
- ✓ROI / progressive decoding built in
!Limitations
- −No browser support
- −Slow to encode compared to JPG
About the FITS format
FITS (Flexible Image Transport System) has carried professional astronomy data since 1981: every file opens with a plain-text header that records telescope, instrument, exposure, filters and world coordinates, followed by raw pixel arrays — often 16-bit or float, sometimes data cubes.
Values can be negative or far above display range, so FITS never looks right in a photo viewer. Converting to PNG applies a simple scaling pass that reveals the nebula or galaxy in the numbers — good for previews, while science pipelines keep the original.
+Strengths
- ✓Self-documenting science headers
- ✓High bit depth (16/32-bit, float)
- ✓The standard of every observatory
!Limitations
- −Not display-ready
- −Files are large and uncompressed
Why convert J2K to FITS?
Think of it as moving your file to the container that fits the job. No browser support FITS works differently: Self-documenting science headers If any of that sounds like your task, run the conversion — it takes seconds and nothing leaves your device.
Frequently asked questions
Is this J2K to FITS converter really free?
Yes — completely free, with no sign-up, no watermark and no daily conversion limits. The site is supported by advertising.
Are my files uploaded to a server?
No. The conversion runs entirely inside your browser using WebAssembly, so your files never leave your device — that is what makes it instant and private.
Does it work on Windows, Mac, iPhone and Android?
Yes. Anything with a modern browser works: Windows, macOS, Linux, Android and iOS. No software installation is required.
Will I lose quality converting J2K to FITS?
No — FITS is a lossless format, so every pixel of the converted image matches your original J2K file. You can also adjust the quality/size balance before converting when the target format supports compression.
Will transparent areas survive the conversion to FITS?
Transparency is kept whenever FITS actually supports an alpha channel. If the target format has none (like JPG), transparent areas are flattened onto a white background — that is a property of the format, not of this tool.
Can I convert several J2K files at once?
Yes — drop as many files as you like into the tool and they are converted in one batch. When more than one file comes out, everything is bundled into a ZIP for a single download.