What is the PSB file format?
.psb files contain a large Photoshop document project saved in a proprietary binary format developed by Adobe. The format is designed for documents that exceed the .psd format's dimensional limit of 30,000 × 30,000 pixels - .psb can store images up to 300,000 × 300,000 pixels. For smaller projects the .psd format is used. Both formats are native to Photoshop and are quite similar in structure, yet .psb extends dimension and data-offset fields to 64-bit values, making it suitable for very large images.
Detailed specifications for .psb are published on the Adobe developer website. A .psb file is organized into the following main sections:
- Header
- Color data
- Image resources
- Masks and layers
- Image data
Unlike .psd, which has a 2 GB file-size ceiling, .psb uses 64-bit length values for large data sections, removing that cap and allowing working files well beyond 2 GB. Both formats share the 8BPS magic signature; the version field at byte offset 4 distinguishes them (0x0001 for PSD, 0x0002 for PSB).
A .psb document can store the same rich project data as a .psd file:
- Images
- Layers and masks
- Transparency settings
- Filters
- Text
- Alpha channels and color data
In Adobe Photoshop, saving in Large Document Format must be enabled under Edit > Preferences > File Handling before .psb files can be created. Projects saved as .psb must be exported to a compatible format - such as TIFF or PNG - to open them in applications that do not natively support the format.
Security & safety
RISK: LOWA PSB is passive image data - layers and pixels, not executable code - so opening one is safe. The real-world concerns are practical, not malware: PSB files are often enormous and can be slow or impossible to open on machines with limited RAM/disk; and exporting to a flattened format (JPG/PNG) permanently discards the editable layers, so keep the PSB master. As always, only the usual caution applies - be wary of a file that claims to be an image but has a different real extension.
Format details
in a nutshellPrograms that open PSB files
Windows4 apps
Adobe Photoshop
Paid The native editor. File > Open the .psb to edit all layers. Creative Cloud subscription. 
GIMP
Open-source Free editor; File > Open imports the PSB (some Photoshop-only features may be flattened/dropped). 
Photopea
Free Free in-browser Photoshop clone; File > Open the .psb to view/edit layers - no install, no Photoshop needed.
Affinity Photo
Paid Affordable one-time-purchase editor; opens large PSD/PSB files with layers. Good Photoshop alternative. 
macOS3 apps
Affinity Photo
Paid Open large PSD/PSB with layers; one-time purchase Photoshop alternative for Mac. 
Linux2 apps
GIMP
Open-source File > Open imports the PSB on Linux (some Photoshop-specific features may not survive).
Photopea
Free Upload/open the .psb in the browser; the closest free thing to Photoshop, layers preserved. Technical details
deep spec| File signature | `8BPS` (hex `38 42 50 53`) at offset 0, followed by version bytes `00 02` at offset 4 |
| MIME type | `image/vnd.adobe.photoshop` |
| Format type | Binary layered raster image; proprietary Adobe format |
| Max canvas dimensions | 300,000 × 300,000 pixels per axis |
| File size limit | No hard cap - 64-bit data-length fields remove the 2 GB ceiling imposed by PSD |
| Version discriminator | 2-byte version field at offset 4: `0x0002` for PSB, `0x0001` for PSD (both share the `8BPS` signature) |
| Color modes | Bitmap, Grayscale, Duotone, Indexed Color, RGB, CMYK, Lab, Multichannel |
| Bit depth | 1, 8, 16, and 32 bits per channel |
| Max channel count | Up to 56 channels total, including alpha channels |
| Data length fields | 64-bit in PSB versus 32-bit in PSD - the core structural difference enabling large-document storage |
| Layer support | Unlimited layers, layer groups, adjustment layers, fill layers, and smart objects |
| Supported compression | Raw (none), PackBits RLE, ZIP without prediction, ZIP with prediction |
| Photoshop enable step | Must be turned on via Edit > Preferences > File Handling > Enable Large Document Format (`.psb`) |
| Smart object support | Embedded and linked smart objects stored as sub-documents within the `.psb` container |
| Typical use cases | Large-format print, billboard design, panoramic photography, video game texture atlases - any canvas exceeding PSD's 30,000 px limit |
| Released | 2002 (Photoshop 7.0, with the 'Large Document Format' / Big option) |
| Specification | www.adobe.com |
PSB conversions
Community Q&A
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