How to resize an image on a Mac with Preview

2 min read · 7 steps · Updated 2 October 2026

Preview, the app that opens pictures on every Mac, can make an image smaller in pixels and in file size. That is enough for email attachments, application portals and websites. No extra software is needed.

These steps are for recent versions of macOS.

Step by step

1. Work on a copy

   Preview saves changes into the file automatically. If you want to keep the original, select the file in Finder first and press Command + D to duplicate it.

   Done

2. Open the picture in Preview

   Double-click the file. If it opens in another app, Control-click it and choose Open With, Preview.

   Done

3. Choose Tools, Adjust Size

   The same command is behind the Adjust Size button in the Markup toolbar.

   Done

4. Enter the new size

   Type a width or a height, or choose a ready-made size from the Fit into menu. To shrink by a percentage, change the unit to percent.

   Done

5. Keep the proportions

   Leave Scale proportionally ticked. Then you only type one value and Preview works out the other.

   Done

6. Check the resulting size and click OK

   The bottom of the window shows the new file size. Click OK, and the picture is resized.

   Done

7. Resize several pictures at once

   Select all the files in Finder and open them together, so that they appear in one Preview window. Select them all in the sidebar with Command + A, then choose Tools, Adjust Size.

   Done

Good to know

To make a file smaller without changing how large it prints, tick Resample image and lower the Resolution value instead.

Enlarging a small picture does not add detail. It only becomes blurry.

You can do the same in a browser, on any computer, with our Resize image and Compress image tools. The pictures stay on your device.

iPhone photos in HEIC format: how to convert HEIC photos to JPG on a Mac.

In short: Duplicate the file, open it in Preview, Tools, Adjust Size, type the width, keep Scale proportionally, OK.