How to compress an image on your Mac

Every way to make a picture smaller on disk pulls one of two levers. Fewer pixels: a 4000 pixel wide photo saved at 1600 pixels is a fraction of the size. Lower quality: a JPEG saved at a lower quality setting drops detail you mostly can't see. Your Mac has both levers built in, in Preview, Finder and Photos. What it doesn't have is a box where you type “under 1 MB” and get exactly that.

Koi's Shift wheel does have that box. We make Koi, so weigh that part accordingly. Apple's steps come from its own guides, checked October 21, 2026.

Apple's Preview guide shrinks the pixels:

1. Open the picture in Preview.
2. Choose Tools, then Adjust Size.
3. Make sure Resample image is ticked and Scale proportionally is on.
4. Type a smaller Width, or a smaller value in the Resolution field. The new size shows at the bottom of the window.
5. Click OK, then choose File, Save.

Then pull the second lever. Choose File, Export, set Format to JPEG, and drag the Quality slider to the left. That's in Apple's guide to converting images. Export saves a new file, so the original stays as it is.

Careful with Adjust Size: it changes the open file itself when you save. Duplicate the picture first (File, Duplicate) if you want to keep the big one. To shrink several at once, open them in one Preview window, select them in the sidebar, then choose Tools, Adjust Size.

Finder's Convert Image quick action makes a smaller copy without opening the picture:

1. Select one or more pictures in Finder.
2. Control-click and choose Quick Actions, then Convert Image (under More if you don't see it).
3. Pick JPEG and a smaller size than the actual one.
4. Click the Convert button. The copy is saved beside the original.

That's from Apple's Finder guide. It gives you sizes, not numbers, so you can't aim for a pixel width.

In Photos, choose File, Export, Export [number] Photos, set Photo Kind to JPEG, and open the options beside it for JPEG Quality and Size, as Apple describes. Good for a batch from your library.

1. Start dragging the picture, from Finder, the desktop or another app.
2. While you're still holding it, press and hold Shift. A wheel opens round your pointer, the file's size in its middle.
3. Drop it on Compress. A window opens, the settings on the left and the picture on the right.
4. Pick a Preset: Light, Balanced or Strong. Or pick a smaller Image size: 3840, 2560, 1920, 1280 or 800 pixels on the long side.
5. Check the Result: the size before and after shows before anything is saved. Hold the picture to see the original.
6. Click Save compressed copy. “Photo compressed.jpg” lands beside the original and shows in Finder.

• Light is barely smaller to the eye. Balanced is much smaller with little you'd notice. Strong is as small as it gets, and some detail goes.
• A photo becomes a JPEG, a HEIC stays a HEIC, and a picture with see-through parts stays a PNG.
• The original is never touched. It all runs on your Mac and nothing is uploaded, which matters for a passport scan or a photo of your kids.

Forms love a limit: under 1 MB for a visa photo, under 200 KB for a job portal, under 100 KB for an old CMS. With the built-in tools you guess. Export at one quality, check the size in Finder, try again lower, and if the slider bottoms out, shrink the pixels and start over.

In Koi you type the limit:

1. Drop the picture on Compress as above.
2. Under File size, turn on Compress to a file size.
3. Type the number next to Under and pick KB or MB. For example 1 MB, or 100 KB.
4. Koi tries lower quality first, then fewer pixels, until the copy comes in under it. The Result shows what you'll get.
5. Click Save compressed copy.

What to expect. The tighter the limit, the more has to go. When even the lowest quality is too big, Koi makes the picture smaller in pixels, so a very low limit like 100 KB can mean a much smaller picture. Check the Result before you send it. A PNG with see-through parts only shrinks by pixels, because PNG has no quality setting.

• It's already small. A picture from a messaging app has usually been compressed once, so there's little left to take.
• It's a PNG screenshot. Saved as a JPEG it gets far smaller, but sharp text edges soften a little. Koi keeps a PNG as PNG only when it has see-through parts.
• You saved over the original in Preview. Adjust Size changes the file itself, so keep a duplicate.
• You compressed a JPEG again and again. Each save loses a little. Start from the original.
• The limit counts in KB and the site means 1,000 or 1,024 bytes. Aim a little under to be safe.

• One picture, any size is fine: Preview, Tools, Adjust Size.
• A batch from Finder: the Convert Image quick action.
• A batch from your library: Photos, File, Export.
• A form that says “under 500 KB”, or you do this every week: Koi's Compress. The same wheel converts HEIC to JPG, crops and takes the location out. Shrinking a PDF instead? See how to compress a PDF on Mac.

The Shift wheel comes with Koi on every plan. Pro is $20 a month, or $192 a year, after a 3-day free trial that needs a card. Lifetime is paid once, from $59. See the plans.