Images
How to Resize an Image on a Mac (Preview, Photos and sips)
To resize an image on a Mac, open it in Preview, choose Tools > Adjust Size, type a new width in pixels with Scale proportionally on, and click OK. Then save with File > Export. For a command-line option, use sips -Z 1200 photo.jpg.
Resizing changes pixel dimensions. It is not the same as compressing, though it usually shrinks the file too. Here are all the built-in methods, and which one to pick.
How do I resize an image in Preview?
- Open the image in Preview (double-click for JPG or PNG, or right-click > Open With > Preview).
- Choose Tools > Adjust Size.
- Set the unit to pixels. Keep Scale proportionally on, or the picture will stretch.
- Check Resample image. If it is off, only the print size changes and the pixels do not.
- Type a Width, press Tab so Height updates, then click OK.
- Choose File > Save to overwrite, or File > Export to keep the original.
Preview saves over the original on File > Save. If you want the big version later, use Export or duplicate the file first (File > Duplicate).
How do I resize an image from Terminal with sips?
sips comes with macOS and needs no install. Write to a new file with --out so the original stays intact:
sips -Z 1200 photo.jpg --out photo-1200.jpg # longest side 1200 px
sips --resampleWidth 800 photo.png --out small.png # width 800, height follows
sips --resampleHeight 600 photo.png --out small.png # height 600, width follows
sips -z 600 800 photo.png --out exact.png # exactly 600 tall by 800 wideThe last form can distort the image if the ratio differs, since it forces both sides. Use -Z when in doubt. More commands in the sips command guide. For folders, see batch resizing images with Automator.
Can I resize an image in the Photos app?
Photos has no resize tool, but its export dialog has a Size setting. Select the photo, choose File > Export > Export 1 Photo, set Photo Kind to JPEG or PNG, then pick a Size from the menu (Small, Medium, Large, Full Size or Custom) and a JPEG Quality. This works well for photos already in your library and avoids the library-wide changes of editing.
Does resizing make the file smaller?
Yes, because fewer pixels means less data. I resized one synthetic 2880 x 1800 PNG (3,878,872 bytes) with sips and saved a JPEG for comparison.
| Version | Pixels | Bytes |
|---|---|---|
| Original PNG | 2880 x 1800 | 3,878,872 |
| PNG, sips -Z 1000 | 1000 x 625 | 202,692 |
| JPEG quality 70, sips -Z 1000 | 1000 x 625 | 45,521 |
Dimensions matter most for PNG, and format matters most for photos. If the aim is a smaller file rather than a particular size, compare options in compressing images on Mac, methods compared. Do not enlarge a small image to make it bigger. Stretching adds no detail, and that is a job for an upscaler.
What is the difference between pixels, DPI and file size?
Pixels are the width and height of the image. DPI is a label for printing: how many pixels go on one inch of paper. Changing DPI with Resample off does not change the pixels or the file size. File size depends on pixels, format and quality. If a form says it needs a photo at 600 x 600, set pixels. If it says 300 DPI, you need enough pixels for the print size, which is width in inches times 300.
When is Smol the wrong tool for this?
For one image, Preview or sips is the right tool. They are free, instant and already installed. Smol is overkill for a single resize.
Smol helps when you resize and compress a whole folder at once, including mixed JPG, PNG and HEIC files. It offers fit, fill, width and height modes. Fill crops to the box and you cannot control what is cut. It makes one size per run, so several sizes of the same image need separate runs. By default, Smol resizes anything larger than 2000 x 2000 pixels, strips metadata, and writes a copy with a suffix, so check those settings before a print job. It is also slower than a sips loop. Compare tools in the best image compressor for Mac, or see Smol at $29 once.
How we checked this
Ran sips -Z on a synthetic 2880 x 1800 PNG on macOS 27.0.1, checked the -z, --resampleWidth and --resampleHeight options in sips help, and read Smol 1.0.35 defaults via its MCP. 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.
Resizing images on Mac: common questions
How do I resize a JPG on a Mac without losing quality?
Any resize re-encodes a JPG, which costs a little quality. Resize once from the original, not from a copy that was already resized, and export at a high quality setting. In Preview, use Tools > Adjust Size, then File > Export and drag Quality toward the right.
Why is Adjust Size greyed out in Preview?
Usually the file is a PDF or the Markup toolbar is in a mode that disables it. For an image, make sure it opened as a single picture, not inside a PDF. If the item is in a locked folder or a read-only file, duplicate it and open the copy.
How do I resize an image to an exact size on Mac?
In Preview, untick Scale proportionally in Adjust Size and type both numbers, which may stretch the picture. To avoid distortion, crop to the right ratio first with Tools > Rectangular Selection and Crop. In Terminal, sips -z height width forces an exact size.
Does resizing reduce file size?
Yes. In my test a 2880 x 1800 PNG of 3,878,872 bytes became 202,692 bytes at 1000 pixels on the long side. Saving that as a JPEG at quality 70 gave 45,521 bytes. Results vary with the image, so test your own.
Can I resize many images at once on a Mac?
Yes. A sips loop in Terminal or an Automator Quick Action handles a folder for free. Apps such as Smol also take whole folders with resize modes and compression in one pass. Preview can resize several images at once too: select them in its sidebar, then use Adjust Size.
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