Large image files slow down websites, fill up storage, and make sharing over email a pain. Mac has built-in tools that handle basic compression without installing anything — and there are free apps for batch work.
Short Answer: The quickest way to compress images on Mac is using Preview (built-in): open the image → Tools → Adjust Size → reduce dimensions or resolution → Export as JPEG with lower quality. For batch compression of many images, use Squash or the free Squoosh website. No extra software needed for occasional compression.
Method 1: Preview (Built-in, Free)
Preview is macOS’s default image viewer and it has surprisingly capable export options. No download required.
Reduce file size via export quality
- Open the image in Preview (double-click any image file)
- Go to File → Export
- Change the Format to JPEG
- Drag the Quality slider to the left — watch the file size estimate at the bottom change
- Set the quality to around 60–80% for a good size/quality balance
- Click Save
📸 SCREENSHOT NEEDED: Mac Preview Export dialog showing Format set to JPEG and the Quality slider with file size estimate
Reduce dimensions (resize)
- Open the image in Preview
- Go to Tools → Adjust Size
- Change width or height (keeping Scale proportionally checked)
- The new file size estimate shows at the bottom
- Click OK, then File → Save
Quick compress in Preview:
Open image → File → Export → Format: JPEG → Quality: 70% → Save
Method 2: Squoosh (Free Web App)
Squoosh (squoosh.app) is a browser-based image compressor made by Google. Drag your image in, pick the output format (WebP is usually smallest), adjust the quality slider, and download. No account needed, no file size limits.
It’s especially good for converting PNG screenshots to WebP for web use — often 80% smaller than the original PNG with no visible quality loss.
📸 SCREENSHOT NEEDED: Squoosh web app showing before/after image comparison with file size reduction displayed
Method 3: Automator (Batch Resize — Built-in)
Automator is Mac’s built-in automation tool and it has a “Scale Images” action that lets you batch resize a folder of images in seconds:
- Open Automator (search with Spotlight)
- Create a new Quick Action
- Set “Workflow receives” to image files in Finder
- Add the action Scale Images from the library
- Set your target size (e.g., 1920px wide)
- Save the Quick Action with a name like “Resize Images”
- Now right-click any images in Finder → Quick Actions → Resize Images
📸 SCREENSHOT NEEDED: Mac Automator showing Quick Action workflow with Scale Images action configured
Warning: Automator’s Scale Images action modifies files in place by default. Either make copies first, or add a ‘Copy Finder Items’ action before the Scale Images action to work on copies instead.
Method 4: ImageOptim (Free App)
ImageOptim (imageoptim.com/mac) is a free Mac app that strips metadata and applies lossless compression — it reduces file size without changing quality at all. Just drag images onto it and it processes them. Great for web images where you want smaller files without any quality trade-off.
Comparing the Methods
- Preview Export: Best for single images, quick, built-in
- Squoosh: Best for web images, WebP conversion, browser-based
- Automator: Best for batch resizing, no extra software
- ImageOptim: Best for lossless compression, no quality loss
Quick checklist:
- ✅ Used Preview Export with 70–80% JPEG quality for quick single-image compression
- ✅ Tried Squoosh for converting screenshots to WebP for web use
- ✅ Set up an Automator Quick Action for recurring batch resize tasks
- ✅ Made backup copies before batch processing with Automator
Frequently Asked Questions
What’s the best image format for the web — JPEG, PNG, or WebP?
WebP is best — it’s smaller than both JPEG and PNG at the same quality. Use JPEG for photos, PNG for screenshots/graphics with text, and WebP for everything when targeting modern browsers (all major browsers support WebP since 2020).
How much can I compress a JPEG without visible quality loss?
Most people can’t distinguish between quality 80% and 100% on a typical screen. Quality 60–70% still looks good for web display. Below 50%, compression artefacts become visible, especially in areas of fine detail or sky gradients.
Does macOS have a built-in batch image converter?
Sort of — Automator handles batch resizing (as above) and Preview can export multiple images at once: select them all in Preview’s sidebar, then File → Export All Images. However, for format conversion with fine-grained quality control, Squoosh or a dedicated app like Permute is more capable.
Conclusion
For occasional compression, Preview’s Export function handles it without installing anything. For web work where you need WebP output, Squoosh is a bookmark worth keeping. ImageOptim is worth the download if you regularly process images for a website.
For more Mac productivity tips, see our guides on Mac tips and tricks and the best free Mac software.