What Is a PSD File? Layers, PSD vs PSB, and How to Open One Without Photoshop
What Is a PSD File? The Short Answer
A PSD file (Photoshop Document, extension .psd) is Adobe Photoshop's native, layered image format. If you're wondering what a PSD file is because a designer sent you one or a mockup download is full of them, here's the core idea: a PSD isn't a finished picture. It's the working file, with every layer, mask, text box, and effect kept separate so the image can still be edited.
Photoshop has saved to PSD by default since version 1.0 in 1990. Every PSD starts with the four-byte signature 8BPS, and Adobe publishes the full format specification, which is why free editors can read it.
Three facts explain most PSD questions:
- It keeps everything editable. Layers, adjustment layers, masks, live text, smart objects, and channels all survive a save and reopen.
- It usually carries a flattened copy. Alongside the layers, Photoshop stores a single merged image. That copy is what Preview, Quick Look, and most non-Adobe apps actually display.
- It's big. Layers are stored at full resolution with fairly light compression, so PSDs are often 5 to 50 times larger than a JPEG of the same image.
What Is a PSD File Made Of?
Adobe's Photoshop File Formats Specification splits every PSD into five sections, in this order:
- File header. Signature, version, width, height, bit depth (1, 8, 16, or 32 bits per channel), and color mode (RGB, CMYK, grayscale, Lab, indexed, duotone, and others).
- Color mode data. Only used by indexed and duotone images; empty for everything else.
- Image resources. Non-pixel information: resolution, ICC color profile, paths, guides, slices, thumbnails, and metadata.
- Layer and mask information. Every layer, its masks, blend modes, effects, and settings. This is usually the largest section.
- Image data. The flattened composite of the whole document.
Here's what those layers can contain:
| Element | What it stores | Effect on file size |
|---|---|---|
| Pixel layers | Ordinary image content, each with its own transparency | Large: up to the full canvas per layer |
| Adjustment layers | Curves, Levels, Hue/Saturation and other edits saved as settings, not pixels | Tiny |
| Type layers | Live, editable text plus the font name | Tiny, but the font must be installed to edit or render it exactly |
| Shape layers and vector masks | Vector paths that stay sharp at any size | Small |
| Layer masks | A grayscale image that hides or reveals parts of a layer | Medium |
| Smart objects | An embedded copy of another file (a photo, a PSD, even an Illustrator file) that can be scaled and filtered non-destructively | Can be very large, because the full source file lives inside the PSD |
| Layer styles | Drop shadows, strokes, glows, and other effects, stored as settings | Tiny. Our Photoshop drop shadow guide shows them in use |
| Channels | The color channels plus extra alpha channels (saved selections) and spot color channels for print | Each extra channel adds a full-resolution grayscale image |
One thing a PSD does not store is your undo history. Close the file and the History panel starts empty next time.
The composite image and "Maximize Compatibility"
The last section, the composite, matters more than it looks. Apps that can't interpret Photoshop layers, including macOS Preview, Quick Look, Finder thumbnails, and Lightroom, read that flattened image instead. Adobe's spec notes that when Maximize Compatibility is turned off, the composite may not be created, so a reader would have to rebuild the image from the layers itself.
This is controlled by Photoshop > Settings > File Handling > Maximize PSD and PSB File Compatibility (Edit > Preferences on Windows), with options Always, Ask, and Never. Adobe says Never reduces file size and save time but stops older applications from opening your files, and that layered PSDs must be saved with the setting on to work in Lightroom.
If a PSD shows up blank or white in Quick Look, a missing composite is the usual cause. Changing the setting doesn't fix files already saved: open the PSD in Photoshop, set the preference to Always, and use File > Save As to write a new copy.
PSD vs PSB: What's the Difference?
PSB is Photoshop's Large Document Format. It holds exactly the same layers and features as a PSD, but with much higher limits. Under the hood, the header says version 2 instead of 1, and several internal length fields are twice as wide, which is what lets the file grow past 2 GB.
| PSD | PSB | |
|---|---|---|
| Maximum width or height | 30,000 pixels | 300,000 pixels |
| Maximum file size | 2 GB | No practical limit for normal work |
| Features | Full | Full (a few plug-in filters are unavailable above 30,000 px) |
| Opens in | Photoshop, Photopea, GIMP, Affinity, Krita, Preview, many viewers | Photoshop CS and later, plus a shorter list of other apps |
| Typical use | Almost everything | Panoramas, billboards, large-format print, huge layered composites |
Photoshop tells you when you've outgrown PSD: saving fails with a message that the document exceeds the 2 GB limit. Use File > Save As and pick Large Document Format. Stay with PSD whenever the file fits, because anything you send to other people or other apps is more likely to open.
Why Are PSD Files So Large?
We took a real 10.2-megapixel phone mockup PSD (3680 × 2760, 8-bit RGB, four layers including two smart objects) and measured where the bytes go:
| Part of the file | Size |
|---|---|
| Whole PSD | 29.5 MB |
| Layers, masks, and embedded smart objects | 18.1 MB |
| Flattened composite image | 11.4 MB |
| Image resources (profile, thumbnail, metadata) | 0.02 MB |
Roughly 38% of this file is the composite that exists only so other apps can display it. The rest is layer data. Photoshop compressed both with RLE (run-length encoding), a fast, simple lossless method that does well on flat colors and empty areas but poorly on photographic detail. For comparison, a lossless PNG of the same flattened image was 5.4 MB, and a quality-85 JPEG was 0.64 MB.
What makes a PSD grow: more full-size pixel layers, embedded smart objects (each carries its entire source file, which may be a 50-megapixel raw photo), higher bit depth (16-bit doubles the pixel data), a bigger canvas, and the composite on top of it all.
How to make a PSD file smaller
- Delete hidden layers you don't need. Hidden layers are saved just like visible ones.
- Rasterize or delete unused smart objects. Right-click a smart object and choose Rasterize Layer once you're sure you won't need to re-edit the source. Rasterizing drops the embedded file but makes the layer destructive.
- Crop away canvas outside the frame. Use the Crop tool with Delete Cropped Pixels checked; otherwise the hidden pixels stay in the file.
- Delete extra alpha channels in the Channels panel if they are old saved selections.
- Drop to 8-bit (Image > Mode > 8 Bits/Channel) when heavy editing is done.
- Flatten a copy for delivery. Image > Flatten Image, then Save As a new file, keeps your layered master untouched.
- Consider turning off Maximize Compatibility only for files that never leave Photoshop. You save the composite's space, but you lose Finder and Quick Look previews, Lightroom import, and recovery options if the file is damaged.
In File Handling, leave Disable Compression of PSD and PSB Files unchecked. It speeds up saving very large documents, but users report it makes files noticeably bigger.
How to Open a PSD File Without Photoshop
You have two kinds of options: viewers that show the flattened composite, and editors that read the layers.
To view it:
- Mac: double-click to open in Preview, or select the file and press Space for Quick Look. Both show the composite only.
- iPhone and iPad: the Files app can usually preview a PSD the same way.
- Windows: there's no built-in PSD viewer. Use one of the editors below or a viewer such as IrfanView or XnView.
To see and edit layers (all free):
- Photopea runs in any browser and reads layers, masks, smart objects, and text. It's the closest free match to Photoshop. Files open locally in your browser, but check its current privacy terms for sensitive client work.
- GIMP (Mac, Windows, Linux) imports layers and masks. Adjustment layers, smart objects, and some layer styles may not come through exactly.
- Krita opens PSD layers and is geared toward painting.
- Affinity, which Canva relaunched as a free all-in-one app for Mac and Windows in late 2025, opens PSDs with layers and adjustment layers. Smart objects are rasterized on import by default.
- Pixelmator Pro (paid, Mac) opens and saves layered PSDs.
Smart objects, newer adjustment types, and missing fonts cause most rendering differences. When in doubt, the composite in Preview shows what the designer saw.
PSD to PNG: Keep the Transparency
PSD to PNG is the conversion to pick when the design has transparent areas, such as a logo, an icon, a cutout product shot, or a UI element. PNG keeps the alpha channel, so transparent pixels stay transparent. JPG has no transparency at all and fills those areas with white. PNG is also lossless, so text and sharp edges stay crisp.
Converting to PNG always flattens the layers. Hidden layers are left out, and live text becomes pixels. Keep the original PSD if you might edit again.
1. Preview on a Mac
- Open the PSD in Preview.
- Choose File > Export.
- Set Format to PNG and leave Alpha checked to keep transparency.
- Click Save.
2. Photopea or Photoshop (to export one layer or artboard)
Use File > Export As > PNG. In either app you can hide layers first, or export a single layer or artboard, which Preview can't do because it only sees the composite.
3. Batch convert a folder in Terminal
macOS includes sips, which reads PSD composites. From the folder containing your PSDs:
mkdir -p pngs
sips -s format png *.psd --out pngs/
On our test files this converted each PSD to a same-size PNG in well under a second, preserving transparency where the PSD had it. If you have ImageMagick installed, magick "design.psd[0]" design.png does the same; [0] tells it to read the composite instead of each layer.
Both tools rely on the composite, so a PSD saved without Maximize Compatibility may come out blank. For JPG output, Automator, Photoshop's Image Processor, GIMP, and online converters, see our PSD to JPG guide.
PSD vs PNG vs JPG vs TIFF
| PSD | PNG | JPG | TIFF | |
|---|---|---|---|---|
| Purpose | Editing master | Graphics, transparency, screenshots | Photos for sharing and the web | Print, scans, archives |
| Compression | Lossless (RLE or ZIP) | Lossless | Lossy | None, lossless, or lossy |
| Editable layers | Yes, all features | No | No | Photoshop layers, other apps see flat |
| Transparency | Yes | Yes | No | Yes |
| Bit depth | 8, 16, 32-bit | 8, 16-bit | 8-bit | 8, 16, 32-bit |
| CMYK | Yes | No | Limited support | Yes |
| Opens in browsers | No | Yes | Yes | Safari only |
| Our 10.2 MP test file | 29.5 MB | 5.4 MB | 0.64 MB | 5.7 MB (LZW) |
The pattern: edit in PSD, deliver in PNG or JPG. TIFF is the middle ground when a printer wants a high-quality flat file, and our TIFF file explainer covers it in depth. For choosing between the two delivery formats, see JPEG vs PNG.
PSD is a pixel format, even when it contains vector shape layers. Logos and illustrations made for any-size scaling usually live in vector formats instead: see what an EPS file is and SVG vs PNG.
Where Compresto Fits
Compresto doesn't open, edit, or flatten PSD files. Photoshop, Photopea, Preview, or sips handle that part. Compresto is useful right after export, when you have a folder of PNGs or JPGs that are still too heavy to email, upload, or put on a website.
Drop the exported images into Compresto and it compresses them in a batch on your Mac, with nothing uploaded. It can shrink PNGs while keeping their transparency, convert them to JPEG, WebP, or AVIF when transparency isn't needed, resize them in the same pass, and aim for a target file size when a site or form has a limit. Our guides on compressing PNG files, resizing PNGs, and reducing file size without losing transparency go deeper.
Frequently Asked Questions
What is a PSD file?
Adobe Photoshop's native layered format. It saves layers, masks, adjustment layers, editable text, smart objects, channels, and color profiles, plus a flattened copy of the image that other apps can preview.
How do I open a PSD file without Photoshop?
On a Mac, use Preview or Quick Look to view the flattened image. To work with layers for free, use Photopea, GIMP, Krita, or Affinity. Windows has no built-in PSD viewer.
What is the difference between PSD and PSB?
PSB is the Large Document Format. PSD tops out at 30,000 pixels per side and 2 GB; PSB goes to 300,000 pixels and drops the 2 GB limit. Fewer apps outside Photoshop open PSB.
Why won't my PSD file preview on a Mac?
Preview and Quick Look display the PSD's stored composite. If the file was saved with Maximize Compatibility set to Never, the composite is missing or blank. Re-save a copy in Photoshop with the setting on Always.
How do I convert a PSD to PNG?
In Preview, choose File > Export, pick PNG, and keep Alpha checked. In Photopea or Photoshop, use File > Export As > PNG. For a folder, run sips -s format png *.psd --out pngs/ in Terminal.
Why are PSD files so large?
Every layer is stored at full resolution, smart objects embed their whole source files, and a second flattened copy of the image is saved for compatibility, all with light compression. Delete hidden layers, rasterize unused smart objects, and flatten finished copies to shrink them.
The Short Version
A PSD file is Photoshop's editable working file: layers, masks, text, smart objects, and a flattened composite that other apps use for previews. Switch to PSB only when you pass 30,000 pixels or 2 GB. Open PSDs without Photoshop in Preview, Photopea, GIMP, or Affinity, export to PNG when you need transparency or JPG for photos, and keep the layered original as your master.