How to Resize an Image on a Mac
Preview resizes the file in place and Photos resizes a copy on export. Here is which to use, how to batch it, and when a browser tool is simpler.
To resize an image on a Mac, open it in Preview, choose Tools > Adjust Size, tick "Scale proportionally", type a new width or height, and click OK. If the picture is in your Photos library, export a resized copy instead with File > Export. For an exact pixel size on any Mac, Resize Image works in your browser, and nothing is uploaded.
How to resize an image in Preview
Preview handles any image file in Finder, a download, or a screenshot. These steps follow Apple's Preview User Guide page Crop, resize, or rotate an image in Preview on Mac:
- Open the image in Preview.
- Click the Markup toolbar button, then the Adjust Size button. Tools > Adjust Size opens the same window.
- Select "Scale proportionally" so that changing the width changes the height to match.
- Type a new width or height, or pick a preset from the "Fit into" menu. To scale by a share of the size, choose "percent" from the menu next to the Width and Height fields.
- Click OK.
Preview resizes the file itself. If you need the full-size version later, duplicate the file in Finder before you start and resize the copy.
How to make the file smaller, not just the picture
Apple's guide gives a separate route for cutting file size: choose Tools > Adjust Size, select "Resample image", and enter a smaller value in the Resolution field. Fewer pixels means fewer bytes. If you need to hit a hard limit on an upload form, our guide to compressing an image to an exact size covers that step.
How to resize several images at once
Preview can batch this. Per the same Apple page, open the images in one Preview window, select them all in that window's sidebar, then choose Tools > Adjust Size. That resizes them together instead of one at a time.
How to resize a photo from the Photos app
Photos doesn't shrink the picture in your library. It exports a resized copy and leaves the original alone. Apple's Photos User Guide page Export photos, videos, slideshows, and memories on Mac gives the steps:
- In Photos, select the photo or photos you want.
- Choose File > Export > Export Photos (the menu shows how many you selected).
- Pick a format from the Photo Kind menu: JPEG, HEIC, TIFF, or PNG.
- Pick a size from the Size menu: Small, Medium, Large, Full Size, or Custom.
- Click Export.
JPEG is the safe pick for a website or an email. Apple describes it as making small files suited to the web and other photo apps.
Preview, Photos, or a browser tool?
| You want to | Use | Why |
|---|---|---|
| Resize a file in Finder, a download, or a screenshot | Preview | Adjust Size works on the file directly |
| Resize a picture from your Photos library | Photos export | Makes a smaller copy and keeps the original |
| See the exact output size before you commit | Resize Image | A live readout shows the new width and height as you type |
| Shrink a folder of mixed sizes by the same share | Resize by Percentage | Each image scales relative to its own size |
How to resize an image on a Mac with Resize Image
- Open Resize Image in Safari or any browser and drop in one image or up to 50.
- Type a width in pixels. With "Lock aspect ratio" ticked (the default), the height fills itself in to keep the proportions.
- Check the "New size" line. It shows the output dimensions next to the original ones.
- Run the resize and download. Two or more files come back together in one zip.
Untick "Lock aspect ratio" only when a template demands an exact width and height that don't match your picture's shape. The tool will then stretch the image to fit. To change the shape without distortion, crop first: our guide on cropping an image on a Mac walks through it.
One thing to know about the output: the resized image comes back as a PNG, while the download keeps your original file name. A PNG of a photo can weigh more than the JPG you started with, so if the file size matters, compress it after resizing.
If you'd rather say "half the size" than pick a number of pixels, Resize by Percentage starts at 50% and scales width and height together. In a batch, each image shrinks by the same share of its own size.
What size should you pick?
That depends on where the image is going, and enlarging a small picture only makes it softer. Our general guide on how to resize an image covers sensible sizes for the web, email, and upload forms, and why shrinking looks clean while enlarging doesn't.
FAQ
How do I resize an image on a Mac without stretching it?
In Preview, select "Scale proportionally" in Adjust Size and change only the width or the height. In Resize Image, leave "Lock aspect ratio" ticked and type just the width.
Can I resize an image by percentage on a Mac?
Yes. In Preview's Adjust Size window, choose "percent" from the menu next to Width and Height. Resize by Percentage does the same in a browser.
Does resizing in Photos change my original?
No. Exporting with a smaller size makes a new file and leaves the photo in your library as it was.