Batch Resize Photos on Mac: Step-by-Step

This guide shows you how to resize multiple photos at once on a Mac using the built-in Preview app and Finder, so you can quickly shrink large images for email, web uploads, or saving disk space—no extra software needed.

What You’ll Need

• A Mac running macOS Catalina (10.15) or later (the steps work on older versions too, with slight menu differences).
• The photos you want to resize, saved in a single folder or easily selectable.
• Enough free disk space for the new resized copies (if you choose to duplicate).

Step-by-Step

1. Locate the photos in Finder. Put them all in one folder if they aren’t already, so you can select them easily.
2. Select the photos you want to resize. Click the first one, then hold the Command key and click each additional photo. To select a contiguous group, click the first, hold Shift, and click the last.
3. Open them in Preview. With the photos selected, right-click (or Control-click) on any selected file, hover over Open With, and choose Preview. All selected photos will open in a single Preview window, with a sidebar listing each image.
4. Select all images in Preview. In the Preview sidebar, click the first thumbnail, then press Command+A to select every image. If the sidebar isn’t visible, go to View > Thumbnails (or press Command+Alt+2).
5. Open the resize tool. With all images selected, go to Tools > Adjust Size in the menu bar. A dialog box appears.
6. Choose your resize settings. In the dialog, you’ll see fields for Width and Height. By default, the aspect ratio is locked (the chain icon is closed). Enter a new value in either Width or Height—the other will adjust automatically. You can also choose a preset from the Fit into dropdown (e.g., 800×600, 1024×768).
7. Set the scale (optional). If you want to reduce by a percentage instead of exact pixels, change the dropdown next to Width/Height from pixels to percent and enter a value like 50.
8. Decide whether to resample. Leave Resample image checked (it should be by default). This changes the actual pixel dimensions. Unchecking it would only change the DPI metadata, not the file size.
9. Click OK. Preview will resize all selected images at once.
10. Save the changes. Go to File > Save All (or press Command+S). This overwrites the original files. If you want to keep the originals, choose File > Export Images instead, pick a destination folder, and click Export.

Common Mistakes

• Forgetting to select all images in Preview—if you only click one thumbnail, only that image gets resized.
• Overwriting originals accidentally—using Save All replaces your original files. Always use Export Images if you might need the full-resolution versions later.
• Not locking the aspect ratio—if the chain icon is open, changing width without height will distort the image. Make sure it’s closed.
• Resizing to a larger size—upscaling a small image makes it blurry. Only shrink photos unless you have a specific reason to enlarge.
• Using DPI instead of pixels—changing DPI (dots per inch) affects print size, not file size or screen dimensions. For web use, adjust the pixel width/height.

Tips

• Work on copies—duplicate the folder before resizing (select folder, press Command+D) so you have a backup.
• Use a consistent width—for email or web, 1600px wide is usually plenty; for thumbnails, 400px works well.
• Check file size after—after resizing, select the new files in Finder and press Command+I to see the total size. If still too large, resize again to a smaller dimension.
• Batch rename after resizing—right-click the resized files in Finder, choose Rename, and add a suffix like “_small” to keep them distinct from originals.
• Use Automator for repeat tasks—if you resize photos often, create an Automator Quick Action that does this in one right-click, but the Preview method is fine for occasional use.

Troubleshooting

• Only one photo gets resized—you likely selected only one thumbnail in Preview. Click the first thumbnail, then press Command+A to select all, then redo Tools > Adjust Size.
• The “Adjust Size” option is grayed out—this happens when no image is selected. Click any thumbnail in the sidebar first.
• Resized photos are still large—check the actual pixel dimensions after resizing. A 4000px wide photo resized to 2000px is still fairly large. Try 1280px or lower. Also, if the original was a high-quality JPEG, the file may still be big due to quality settings; use Export and lower the quality slider.
• Photos open in separate windows—if each photo opens in its own Preview window, you didn’t select them all before opening. Close all, reselect in Finder, and use Open With > Preview again.
• Can’t find the sidebar—press Command+Alt+2 or go to View > Thumbnails to show it.
• Export Images option is missing—this appears only when multiple images are selected in the sidebar. If you have only one, select it, then go to File > Export (singular) instead.

Frequently Asked Questions

Can I resize photos without losing quality?

Shrinking a photo always loses some data, but if you resize down (e.g., from 4000px to 1600px), the loss is usually invisible on screen. For print, keep the resolution above 300 DPI at the final print size.

Will this work for HEIC or RAW files?

Preview can resize HEIC files (iPhone photos) easily. For RAW files, Preview can open and resize them, but the result will be a JPEG or other format when exported. The resizing itself works the same.

How do I resize to a specific file size (like 500 KB)?

Preview doesn’t have a “target file size” option. You’ll need to experiment: resize to a smaller dimension, check the file size, and repeat until it’s under your limit. Alternatively, use the Export function and lower the JPEG quality slider.

Can I resize photos in a folder without opening Preview?

Yes, you can use a Finder shortcut: select the photos, right-click, and choose Quick Actions > Resize Image if you’ve enabled it in System Settings > Privacy & Security > Extensions > Finder. This uses a similar built-in tool but may not offer as many options as Preview.

What’s the difference between resizing and compressing?

Resizing changes the pixel dimensions (width/height). Compressing reduces the file size by lowering image quality (JPEG compression) without changing dimensions. Preview’s Export lets you adjust quality, which is compression. You can do both: resize then compress.

Will resizing delete my original photos?

Only if you use File > Save All. If you use File > Export Images, the originals remain untouched. Always export to a new folder to be safe.

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