The fastest way to compress an image on Mac

The built-in Preview app on your Mac can compress an image in under a minute. Open the image in Preview, click Tools in the menu bar, select Adjust Size, and lower the width or height — the other dimension scales automatically. Then go to File, click Export As, choose your format (JPEG usually compresses smaller than PNG), and drag the Quality slider left before saving. That single action handles most compression needs without installing anything.

If you need to compress many images at once, Automator — also built into your Mac — can batch-process them in seconds. But for a single image or a handful, Preview is faster than learning Automator's interface.

Key Takeaways

  • Preview's Export As dialog lets you lower image dimensions and quality in one place, making it the fastest option for most people.
  • JPEG format compresses to smaller file sizes than PNG, but PNG preserves detail better if you need transparency or text clarity.
  • Lowering width and height by 10 to 20 percent usually cuts file size in half without visible quality loss on screens.
  • Automator can compress dozens of images in one batch if you rename them consistently first.

When to resize versus when to compress quality

Resizing (making the image smaller in pixels) and lowering quality (reducing color information) are two different levers. Resizing shrinks the image on screen; lowering quality makes the file smaller without changing how big it appears. Most people need both, but which one matters depends on what you're doing with the image.

If you're uploading to a website, email, or messaging app, resize first. An image that's 3000 pixels wide takes up far more space than one that's 1200 pixels wide, even at the same quality setting. If you're printing or need the image to look sharp when zoomed in, keep the original size and lower quality instead — you'll see the difference less on a printed page than on screen.

For social media, most platforms compress images anyway, so resize to their recommended dimensions (Instagram is 1080 pixels wide for feed posts, for example) and use JPEG at 75 to 80 percent quality. You'll save space and the platform won't re-compress your work.

Using Preview to compress a single image

Open the image in Preview by right-clicking it, selecting Open With, and choosing Preview. If Preview is already your default, just double-click the image.

To resize, go to Tools in the menu bar and click Adjust Size. A dialog opens showing the current width, height, and resolution. Change the width or height to a smaller number — the other adjusts automatically if the chain link icon is connected. For a web image, 1200 to 1500 pixels wide is usually enough. Click OK.

To lower quality, go to File and click Export As. Choose JPEG from the Format dropdown (it compresses smaller than PNG). A Quality slider appears below — drag it left to lower quality. Start at 75 and look at the preview; if it looks acceptable, go lower. Most people can't see the difference between 70 and 85 percent quality on screen. Type a new filename if you want to keep the original, then click Save.

Batch compressing images with Automator

If you have 20 or more images to compress, Automator saves time. Open Automator (search for it in Spotlight), click File, and select New. Choose Workflow from the template options.

In the left sidebar, search for "Image" and drag "Scale Images" into the workflow area on the right. Set the size you want (1200 pixels wide is common). Then search for "Change Type of Images" and drag that below Scale Images. Select JPEG and set quality to 75 percent.

Finally, search for "Ask for Folder" and drag it to the top. This makes Automator ask you which folder contains the images. Save the workflow with a name like "Compress Images to Web Size" and you can reuse it anytime. Click Run, select your folder, and Automator processes every image in it.

Choosing between JPEG, PNG, and WebP

JPEG is the smallest file size and works everywhere, but it blurs fine details and doesn't support transparency (the see-through parts of an image). Use JPEG for photographs and images with lots of colors.

PNG keeps sharp edges and supports transparency, but the file is usually two to three times larger than JPEG. Use PNG for logos, screenshots, and images with text or solid colors. If file size matters more than sharpness, lower the PNG quality in Preview's export dialog instead of switching to JPEG.

WebP is newer and compresses better than both, but not all browsers or apps support it yet. If you're uploading to a website you control, WebP is worth trying — it can be 25 percent smaller than JPEG at the same quality. Most online image converters can turn a JPEG into WebP if Preview doesn't offer it.

What file size is actually small enough

For email, aim for under 5 MB per image — most email providers reject anything larger. For websites, 200 to 500 KB per image is typical; anything under 1 MB loads quickly on most connections. For messaging apps like iMessage or WhatsApp, the app compresses for you, so anything under 10 MB will work.

The easiest way to check file size is to right-click the image, select Get Info, and look at the "Size" line. If it's larger than your target, go back to Preview, lower the dimensions by 10 to 20 percent, and export again at a lower quality setting. Repeat until the file size drops to where you need it.

When to use online tools instead of Preview

Preview handles most compression tasks, but online tools like TinyPNG, ImageOptim, or Squoosh are worth trying if Preview's results aren't small enough. These tools use different compression algorithms that sometimes squeeze out another 10 to 20 percent of file size without visible quality loss.

The downside is uploading to a website and waiting for the result. If you compress one or two images a month, Preview is faster. If you compress dozens regularly, read ImageOptim (free, Mac-only) and drag images onto it — it compresses in the background without opening a dialog.

Frequently Asked Questions

Can I compress an image without making it smaller on screen?

Yes. In Preview's Export As dialog, skip the Adjust Size step and only lower the Quality slider. The image stays the same pixel dimensions but the file gets smaller. This works best with JPEG; PNG quality reduction is less noticeable.

What quality setting should I use?

Start at 75 percent and look at the preview. If it looks acceptable, try 70. Most people can't see the difference between 70 and 85 percent on screen. For printing, stay above 80. For web, 70 to 75 is usually fine.

Does compressing an image damage the original?

No, as long as you export with a new filename. Preview doesn't change the original file unless you explicitly save over it. Always use Export As and give it a new name like "image-compressed.jpg".

Why is my PNG still huge after lowering quality?

PNG compression works differently than JPEG — lowering quality in Preview has less effect. Switch to JPEG format instead, or try an online tool like TinyPNG that specializes in PNG compression.

Can I compress video files the same way?

Preview doesn't compress video. Use QuickTime Player (built into Mac) to export video at a lower resolution, or use free tools like HandBrake for more control over file size and quality.