Images
Reduce an Image’s File Size in Preview on Mac
In Preview, choose Tools > Adjust Size and lower the width or height, then choose File > Export, pick JPEG and drag the Quality slider left. Fewer pixels and lower quality both shrink the file. Do both for the biggest drop.
This page is about images. For PDFs, Preview uses a different filter, covered in reduce PDF file size in Preview.
How do I reduce an image’s file size in Preview?
- Open the image in Preview (double-click it, or right-click > Open With > Preview).
- Choose Tools > Adjust Size.
- Keep Scale proportionally ticked and Resample image ticked. Set the unit to pixels and type a smaller width. Preview shows the resulting size in the dialog. Click OK.
- Choose File > Export.
- Set Format to JPEG. Drag Quality toward Least. Preview shows an estimated file size under the slider.
- Name the copy and click Save.
Use Export, not Save. Save would overwrite the original, and reducing a photo twice stacks the quality loss.
What does Adjust Size actually change?
Width and height set the pixel dimensions. If Resample image is on, Preview throws away pixels, which is what shrinks the file. If Resample is off, changing the numbers only changes the print size in inches and the file size does not drop. The Resolution field alone is a print setting, not a compression setting.
Pixel count is the biggest lever. On our 5.2 MB test photo (4032 x 3024), capping the long edge with
sips -Z| Long edge | File size |
|---|---|
| 4032 px (original) | 5.2 MB |
| 2048 px | 598 KB |
| 1600 px | 362 KB |
| 1024 px | 158 KB |
How does the Preview export quality slider work?
The slider only appears for JPEG, HEIC and some other lossy formats. Left is smaller and softer, right is larger and cleaner. It does not show numbers, but the size estimate underneath updates as you drag, so use that as your guide.
The relationship is not a straight line. Using sips quality values on the same photo, with no resize:
| sips JPEG quality | File size |
|---|---|
| 30 | 729 KB |
| 50 | 1.9 MB |
| 60 | 4.8 MB |
| 80 | 6.1 MB |
The test image is dense synthetic noise, which exaggerates the jumps. On a normal photo the curve is gentler, but the lesson holds: a small move on the slider can change the size a lot, so watch the estimate. Also note that exporting a JPEG at a high setting can make a file larger than the original. The same photo saved as PNG was 24.6 MB, so never export photos as PNG.
Why is Preview not enough sometimes?
Preview is fine for one image. It is slow for twenty: you open, adjust, export and rename each one. You can select several images in the sidebar and use Adjust Size once, but you then have to export them separately and there is no way to set a target file size. Preview also gives no control over which metadata survives, which matters before you post photos. See compress images on Mac: methods compared and the sips command guide for scripted alternatives.
When is Smol the wrong tool for this?
For one image, Preview is enough. Do not buy anything to shrink a single photo.
Smol earns its $29 (once, one Mac) when you have a folder: drop it in, set quality and a maximum size once, and get a smaller copy of every file beside the original, in mixed formats if you like. There is also a Finder right-click action, and nothing is uploaded. By default Smol resizes images to fit inside 2000 x 2000 px and strips metadata, so raise or switch off the resize if you need full resolution. It is not as fast as a sips loop. See the best image compressor for Mac or buy Smol.
How we checked this
Measured file sizes with sips 316 on macOS 27.0.1 on a synthetic 4032 x 3024 JPEG. Preview menu steps are from the app; its slider values were not measured, sips values stand in. Facts last checked . We make Smol, so treat our claims about it with the same care as any vendor’s, and see our editorial policy.
Preview and image size: common questions
How do I change the file size of an image in Preview?
Use Tools > Adjust Size to reduce the pixel dimensions, then File > Export, choose JPEG and lower the Quality slider. Preview estimates the resulting size under the slider. Save as a new file so the original stays intact.
Why is my Preview export larger than the original?
The export quality was set higher than the original used, or you exported to a lossless format such as PNG. Drag the Quality slider left, choose JPEG, and check the estimated size before saving. Resizing as well usually fixes it.
Does Preview reduce quality when I resize?
Resizing with Resample on removes pixels, so fine detail is lost. Exporting as JPEG then compresses again. Both steps are lossy. Keep your original and work on a copy, and do not repeat the export on the same file.
Can I set a target size like 500 KB in Preview?
No. Preview has no target file size field. You adjust pixels and the Quality slider, then read the estimate. It takes a couple of tries. A tool with a target-size mode, or a batch tool, saves the trial and error.
Can Preview shrink many images at once?
Partly. You can select multiple images in the sidebar and apply Adjust Size to all of them, then export. It is still manual. For a folder, a sips loop in Terminal or a batch tool is faster and easier to repeat.
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