How to Compress an Image on Mac (4 Free Ways)

Quick answer: To compress an image on a Mac, open it in Preview, choose File > Export, set the format to JPEG, and lower the Quality slider until the file size shown is small enough. For several images, select them in Finder and use Quick Actions > Convert Image with a smaller image size. To hit an exact KB limit, use a target-size compressor in your browser.

macOS has no button labelled “compress image”, but it includes several ways to make photos smaller. Which one is best depends on whether you are handling one image or a batch, and whether you need a specific file size. For a strict form cap, skip the guesswork and use Image Compressor or Compress Image to 100KB in Safari—files stay on your Mac.

First: what actually makes an image file smaller?

Two things control file size:

1. Pixel dimensions. A 4032×3024 iPhone photo has about 12 million pixels. Shrinking it to 1600 pixels wide removes most of them and usually cuts the file size the most.

2. Compression quality. Saving as JPEG at a lower quality throws away detail you are unlikely to notice.

Combining the two gives the best results: resize to the size you actually need, then compress. For the reasoning, see How to Compress Images Without Turning Them Blurry.

Method 1: Preview (best for one image)

Preview comes with every Mac and shows the resulting file size before you save.

To compress with the quality slider

1. Open the image in Preview.

2. Choose File > Export.

3. Set Format to JPEG.

4. Drag the Quality slider left. Watch the File Size readout under the slider.

5. Click Save. Use a new name if you want to keep the original.

To reduce pixel dimensions first

1. In Preview, choose Tools > Adjust Size.

2. Make sure Scale proportionally is ticked.

3. Enter a smaller width, for example 1600 pixels.

4. Check that Resample image is ticked. The window shows the new file size.

5. Click OK, then File > Export as above.

Exporting a PNG as JPEG often makes the file much smaller, but JPEG does not support transparency, so transparent areas become solid.

Method 2: Finder Quick Actions (best for batches)

On macOS Monterey and later, Finder can convert and shrink several images at once:

1. Select the images in Finder.

2. Right-click and choose Quick Actions > Convert Image.

3. Choose JPEG as the format.

4. Pick an Image Size: Small, Medium, Large or Actual Size. Smaller sizes reduce the pixel dimensions.

5. Optionally untick Preserve Metadata to remove location and camera data.

6. Click Convert. The new files are saved next to the originals.

This is the quickest built-in way to prepare a folder of photos for email or a website. The drawback is that you cannot set an exact quality or file size.

Method 3: The Photos app (for photos in your library)

If the images are in Photos:

1. Select them.

2. Choose File > Export > Export [number] Photos.

3. Set Photo Kind to JPEG.

4. Set JPEG Quality (Medium is a sensible starting point) and Size (Small, Medium, Large, Full Size, or Custom with a maximum width).

5. Click Export and choose a folder.

Method 4: A browser compressor (best for exact size limits)

Built-in tools cannot aim for a specific number like “under 100KB”. Upload forms for jobs, visas and exams often require exactly that, so a target-size tool is faster than repeated trial and error in Preview:

• Image Compressor: choose High Quality, Balanced or Small Size, or use the slider, for up to 20 JPG, PNG or WebP images at once.

• Compress Image to 100KB: for forms with a 100KB cap.

• Compress Image to 50KB: for stricter portals.

These run in Safari, Chrome or Firefox on your Mac. Processing happens in the browser, so images are not uploaded to our servers, and there is no account.

Which method should you use?

Tips for good results

• Resize before compressing. Most websites do not display images wider than 1600–2000 px.

• Do not keep re-saving the same JPEG. Start from the original each time.

• Keep the original. Export to a new file name.

• Check at 100% zoom. If skin looks blocky or edges show halos, raise the quality a little.

• Email from Mail? When you attach images, Mail can offer an Image Size menu in the message header (Small, Medium, Large, Actual Size) that shrinks them before sending.

FAQ

How do I reduce the file size of a photo on a Mac?

Open it in Preview, choose File > Export, select JPEG and lower the Quality slider until the displayed file size is small enough. Reducing the pixel dimensions with Tools > Adjust Size first gives a bigger reduction.

How do I compress multiple images at once on a Mac?

Select the images in Finder, right-click, choose Quick Actions > Convert Image, then pick JPEG and a smaller image size. A browser compressor can also process up to 20 images at once with more control over quality.

Can Preview compress a PNG?

Preview can export a PNG as JPEG, which usually makes it much smaller, but transparency is lost. To keep transparency, reduce the pixel dimensions instead, or convert to WebP.

How do I compress an image to 100KB on a Mac?

Preview’s slider shows the file size, but reaching an exact number takes trial and error. A target-size tool like Compress Image to 100KB finds the right quality for you in the browser.

Does compressing an image on a Mac reduce quality?

JPEG compression always removes some detail, but at moderate settings the change is hard to see. Resizing to the size you actually need keeps quality higher than heavy compression of a full-size photo.