How to Compress Images on Mac

Three things make an image file smaller: lower quality, fewer pixels and a better format. This is how to compress one image or a whole folder on a Mac.

Steps

Download Kompressor for Mac or get it on the Mac App Store
Open the Kompressor app
Drag and drop your image(s) or folders into Kompressor
Select the output format. Keep the format of your images, or select WebP, AVIF or HEIC for smaller files.
Set the quality. The default is 80%.
Optional: set a maximum size, for example a long edge of 2000 pixels
Open the compare view to check the result and its file size
Click on the ▷ (play) button to compress your image(s) offline

Kompressor is free to try: you get 3 conversions, and each conversion can include as many images as you want.

Three ways to make an image file smaller

JPEG, HEIC, WebP and AVIF are lossy formats: they remove detail that the eye does not see easily. Lower quality means a smaller file. Between 100% and 80% the file size falls fast and the image looks almost the same. Below 60% most photos start to show artifacts.

2. Reduce the pixel size

A photo from a current iPhone is 4032 pixels wide or more. A website or an email rarely needs more than 2000 pixels. Half the width and half the height means a quarter of the pixels, and the file size falls by about the same amount. Kompressor can limit the width, the height, the long edge or the short edge, and it never makes an image larger.

3. Use a better format

Newer formats need fewer bytes for the same image. Google reports that lossy WebP files are 25 to 34% smaller than comparable JPEG files, and AVIF files are usually smaller again at the same visual quality. PNG files with photos in them gain the most: in our test, 30 PNG files became 93% smaller as WebP. For photos, see how to convert JPG to WebP and how to convert JPG to AVIF.

Other ways to compress images on Mac

Preview

Open the image, choose File > Export, select JPEG or HEIC and move the Quality slider. Preview shows the estimated file size in the export dialog. To change the pixel size, choose Tools > Adjust Size first. This works well for one image. For many images, Preview applies one setting to all of them without a preview of the results.

Finder Quick Action

Select the images in Finder, right-click and choose Quick Actions > Convert Image. Select a format and one of the sizes: Small, Medium, Large or Actual Size. This is fast, but there is no quality control and you do not see the result first.

Terminal (sips)

mkdir compressed
sips -s format jpeg -s formatOptions 70 -Z 2000 *.jpg --out compressed

-Z 2000 sets the long edge to 2000 pixels, and formatOptions is the quality from 0 to 100. Be careful: sips also enlarges images that are smaller than 2000 pixels, which makes those files larger.

When Kompressor is the better tool

Compression is a trade between size and quality, and the correct setting is different for each set of images. Kompressor shows the original and the result side by side with the real file size before you convert, so you do not need to guess. It handles whole folders, and it works offline: your images are not uploaded to a server, which is the main difference from online image compressors. For details, see Kompressor vs ImageOptim, Kompressor vs Squoosh and Kompressor vs Preview.

Questions

Can I compress images without quality loss?

Yes, with limits. PNG and lossless WebP keep each pixel. A change from PNG to lossless WebP made the files in our test 56% smaller with no loss. Kompressor does not write lossless WebP. Use cwebp -lossless for that. For photos, a lossy format at 80 to 90% quality is the practical choice: the loss is there, but you do not see it at normal size.

Which format gives the smallest files?

At the same visual quality, AVIF usually gives the smallest files for photos, then HEIC and WebP, then JPEG. The same quality number does not give the same visual quality in each format, so compare the results by eye. Use AVIF or WebP for websites, HEIC for Apple devices and JPEG when the file must open everywhere.

How do I compress images for email?

Set the format to JPEG, the quality to 70 or 80% and the long edge to 2000 pixels. A photo of that size is usually below 1 MB.