To compress a JPEG on a Mac, use the built-in Preview app. Preview gives you two controls: a Quality slider that reduces file size by lowering image fidelity, and an Adjust Size tool that shrinks width and height. Using both in the right order produces a smaller file than lowering fidelity alone. No extra software required.
Open Your Image in Preview and Check the Original Size
Open the JPEG you want to compress in Preview. Double-clicking a JPEG file opens it in Preview by default. Before making changes, confirm the current file size so you can compare later. In Finder, select the file and press Cmd+I to open Get Info. The size appears under the General section. Write it down. This step is optional but useful for verifying the result.
Use Adjust Size to Resize the Image First
Resize a Single Image
Resizing is the most effective way to reduce a JPEG file because you remove pixels that take up space. Preview lets you change width and height with the Adjust Size tool.
- With the image open in Preview, click Tools in the menu bar, then choose Adjust Size.
- A dialog box appears. Enter a new width or height. If Scale proportionally is ticked, the other dimension adjusts automatically to preserve the aspect ratio. Leave it ticked unless you want to distort the image.
- Below the dimensions, the dialog shows the resulting file size before you apply the change. Use this to decide if the new size is small enough.
- Click OK to apply the resize.
Resize Multiple Images at Once
If you have several JPEGs to compress, resize them together. In Preview, open the folder of images. In the sidebar, select all the images you want to resize. Click Tools, then Adjust Size. The same dialog appears. Enter the new dimensions, and Preview applies the resize to every selected image. This saves time when you need to make several JPGs smaller.
Lower Quality with the Export Quality Slider
After resizing, reduce the fidelity using Preview's Export feature. Lowering the fidelity removes image data, which shrinks the file further. The Export dialog shows a Quality slider and an estimated file size so you can see the effect before you save.
- With the resized image still open, click File, then Export.
- In the Export window, choose JPEG as the format if it is not already selected.
- You will see a Quality slider ranging from Least to Best. Drag the slider toward Least. As you move the slider, the estimated file size updates in real time.
- Watch the preview size until it reaches a value you are comfortable with. Lower fidelity means more compression and a smaller file, but the image may look blocky or blurry if you go too far.
- Click Export. A save dialog appears. Choose a location and a file name. The original file is not changed because you used Export instead of Save.
If the estimated size is still too large after moving the slider to the lowest setting, go back and resize the image to smaller dimensions before exporting again. Resizing plus fidelity reduction gives a smaller file than lowering fidelity alone.
Check the Compressed File Size
After you export the compressed JPEG, confirm the new file size. In Finder, locate the exported file. Select it and press Cmd+I to open Get Info. The size shown tells you how much you have reduced the original. Compare it to the size you noted earlier. If the result is still too large, repeat the process: open the original again, resize to smaller dimensions, and export with a lower fidelity setting.
Order of Operations Resize Before Quality
For the smallest possible file, always resize first and then lower the fidelity. Resizing removes pixels, which reduces the amount of data that the fidelity compression has to work on. If you lower fidelity first and then resize, the fidelity reduction is applied to a larger image, and the resize step does not remove as much data as it could. Resize first, and the quality slider becomes more effective.
Keep the Original File Intact
Preview's Export function always leaves the original file unchanged. When you choose Export, you create a new file with the compression settings you selected. The original JPEG stays exactly as it was. Experiment with different resize dimensions and fidelity levels without risking your source image. If you are not satisfied with the result, open the original again and try different settings.
When to Use Other Options
Preview works for basic JPEG compression on a Mac because it is free and already installed. But it only handles JPEG and a few other image formats. If you need to compress PNG or WebP images, or want more control over compression settings, use a dedicated online tool such as compressjpeg.onl. That site shrinks JPEG, PNG, and WebP images for free, with no watermark, and works in a browser. For simple JPEG compression tasks on a Mac, Preview is enough.