How to Compress Images on Mac to a Target Size

Phone and camera photos are large because they hold millions of pixels saved at high quality. A 12-megapixel photo, common on phones, is 4032 × 3024 pixels, far more than a web page or an email needs.

Two things set the file size: how many pixels there are and how strongly they're compressed. Lowering the JPEG quality from its maximum cuts the size a lot with little visible change, and reducing the pixel count cuts it further still.

Preview compresses an image through its export sheet: choose File > Export, set Format to JPEG and drag the Quality slider; the sheet shows the resulting file size.

For a hard limit, you nudge the slider until the number drops under it, then move on to the next photo. File > Export Selected Images converts a whole selection, and Tools > Adjust Size reduces the pixel size, but neither aims for a file size. Finder's Convert Image Quick Action handles several photos at once with Small, Medium and Large presets and no quality setting, and Apple Mail can shrink photos as you attach them, with an Image Size menu in the message window, but only to fixed sizes.

File Acture's Compress Images lets you type a size limit, such as 500 KB, and finds the highest quality that fits each image.

Set Compress by to Target size and type any value in KB or MB, or pick 100 KB, 250 KB, 500 KB, 1 MB or 2 MB from Presets. File Acture starts at a high quality and steps down until the photo fits. If even a low quality is too big, it scales the photo down in steps until it fits, so a very small target also reduces the pixel size. When a target is truly out of reach, it tells you the smallest size it can manage.

Format offers Keep original, JPEG and WebP. JPEGs are written with MozJPEG, an encoder built to make smaller files at the same visual quality. Keep metadata is off by default, so location and camera details go. Each file is saved with “-compressed” in its name, and the originals stay as they were.

WebP usually gives the smallest files, JPEG is the safe choice for any upload form, and PNG is only small for simple graphics.

Keep original writes each photo back in its own format, so HEIC stays HEIC and PNG stays PNG. Choose JPEG when a site asks for JPG, and WebP for your own website, where every modern browser shows it.

The format you start with matters more than any slider.

• PNG screenshots: PNG is lossless, so the only way to reach a small target is fewer pixels. Choose JPEG or WebP to keep the dimensions.
• Know the dimensions you need? Resize images in bulk first; fewer pixels save the most space.
• iPhone photos: choose JPEG if the site wants JPG, or see how to convert HEIC to JPG.
• Transparency: JPEG can't store it, so transparent areas turn white. Use WebP or keep PNG for logos.
• Animated GIFs are reduced to their first frame.
• PDFs need their own tool: Compress PDF works the same way with a target in MB.

Online compressors receive your original photos, including any location data stored in them. File Acture compresses on your Mac and removes that data by default. The free version includes 3 runs of up to 5 images on each Mac; see pricing for Pro.