Batch Resize & Convert Images on macOS Sierra

This guide shows you how to resize and change the format of many images at once on macOS Sierra (10.12) using the built-in Preview app and Finder. You’ll learn two methods: one for quick resizing without changing format, and one for converting formats (like PNG to JPEG) while optionally resizing. No extra software is needed.

What You’ll Need

A Mac running macOS Sierra (10.12) or later (the steps work on newer versions too).
The images you want to process, stored in a folder on your Mac.
Basic familiarity with Finder and right-clicking (or Control-clicking).

Step-by-Step

Method 1: Resize Images Without Changing Format

Select the images in Finder. Click one, then hold Command and click others to select multiple. To select a contiguous group, click the first, hold Shift, and click the last.
Right-click (or Control-click) on any selected image. From the context menu, choose Open With > Preview. All selected images will open in Preview, each in its own window or as thumbnails in a single window depending on your settings.
In Preview, if you see a sidebar with thumbnails, click the thumbnails to select all (Edit > Select All or Command+A). If you have multiple windows, you’ll need to repeat the next steps for each window.
Go to the menu bar and click Tools > Adjust Size.
In the dialog that appears, you’ll see fields for Width and Height. Enter the new dimensions. By default, the aspect ratio is locked (the chain icon is closed). If you want to distort the image, click the chain to unlock it.
Choose the units: pixels, percent, or inches. For web use, pixels is common.
Click OK. Preview will resize all selected images.
Now save the changes. Go to File > Save All (or press Option+Command+S). This overwrites the original files. If you want to keep the originals, use File > Export Images (see Method 2 step 5) to save copies.

Method 2: Change Format (and Resize) Using Export

Select the images in Finder as described above.
Right-click and choose Open With > Preview.
In Preview, select all thumbnails (Edit > Select All or Command+A) if you have a single window with multiple images. If each image opened in its own window, you’ll need to export each separately.
Go to File > Export Images (not “Export� – that works for a single image).
In the export window, choose a destination folder. Then, in the Format dropdown, pick the desired format: JPEG, PNG, TIFF, etc.
If you also want to resize, check the Size box and enter the new width or height. The aspect ratio is maintained automatically.
Adjust other options as needed (e.g., JPEG quality). Click Choose to export. The images will be saved in the new format with the new size, leaving your originals untouched.

Method 3: Quick Resize via Finder (No Preview)

Select the images in Finder.
Right-click and choose Automator > New Service (or create a Quick Action in Automator). This is a bit more advanced but useful for repeated tasks. However, for a one-off, the Preview method is simpler.

Common Mistakes

Not selecting all images before applying the size change – only the selected ones get resized.
Overwriting originals when you intended to keep them. Always use “Export Images� if you want to preserve the originals.
Ignoring aspect ratio – if you unlock the chain, you may get stretched or squashed images.
Choosing the wrong format – JPEG is good for photos but loses quality; PNG is better for graphics with text or transparency; TIFF is large but lossless.

Tips

For web use, resize to the exact pixel dimensions you need (e.g., 1200px wide for a blog).
If you need to process hundreds of images regularly, consider using Automator to create a folder action or service.
Use File > Save All carefully – it overwrites. You can press Command+Z to undo the resize before saving if you made a mistake.
When exporting to JPEG, set quality to 80% – it’s a good balance between file size and quality for most uses.

Troubleshooting

“Adjust Size� is grayed out. Make sure you have at least one image open in Preview and that you haven’t selected a non-image file.
Only one image resizes even though I selected many. In Preview, you must select all thumbnails in the sidebar before using Adjust Size. If each image is in its own window, you’ll need to apply the change to each window individually.
Export Images is not available. This option appears only when you have multiple images selected in Preview. If you have a single image, use File > Export instead.
The exported images are blurry. You likely resized them up, which cannot add detail. Always resize down from the original resolution.
Preview crashes or behaves oddly. Try closing all windows and reopening the images. If the problem persists, restart your Mac.

Frequently Asked Questions

Can I batch resize images without using Preview?

Yes, you can use Automator or third-party apps, but Preview is built-in and free. For a one-time task, Preview is easiest.

Does resizing reduce file size?

Generally yes, reducing pixel dimensions reduces file size, especially for JPEG. But changing format also affects size – PNG is larger than JPEG for photos.

Will this work on macOS Mojave or Catalina?

Yes, the steps are essentially the same in newer macOS versions. The menu names may differ slightly (e.g., “Export Images� might be under File > Export in some versions).

Can I convert HEIC to JPEG this way?

Yes, if your Mac supports HEIC (macOS Sierra does not natively, but newer versions do). In macOS Sierra, you’d need to convert HEIC to a supported format first using an app like iMazing or a web tool.

How do I resize images to a specific file size (e.g., 100KB)?

Preview does not have a direct “target file size� option. You’ll need to experiment with dimensions and JPEG quality until you get close to the desired size. There are online tools that can do this automatically.

Is there a way to rename the files during export?

Preview’s Export Images dialog doesn’t offer renaming. You can rename them after export in Finder by selecting all and using “Rename Items� from the right-click menu.

General steps for current mainstream versions; menus can differ by device and version. See the disclaimer or report an error.

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