How to Resize an Image on Mac With Preview

Every Mac ships with a perfectly good image resizer: Preview. Its Adjust Size tool changes dimensions in pixels, percentages, or inches, understands DPI for print, and can even resize a whole batch in one pass. Here's how to use it without ending up with blurry results.

Resize a single image

1. Open the image in Preview.
2. Choose Tools → Adjust Size…
3. Pick a unit from the pop-up menu — pixels for screens, percent for quick scaling, inches/cm for print.
4. Type a new width (or height). With Scale proportionally checked, the other dimension updates automatically so nothing gets stretched or squashed out of shape.
5. If you're resizing for a specific purpose — say a 1200px-wide blog image — enter that number in the width field and let the height fall where it may.
6. Click OK, then press Cmd+S to save.

The dialog shows the resulting file size before you commit — useful when you're shrinking a photo to sneak under an email attachment limit. The "Fit into" pop-up menu also offers ready-made sizes like 1280×1024 if you don't care about exact numbers.

What the "Resample image" checkbox actually does

This is the setting everyone ignores and print shops care about:

• Resample checked (default): Preview actually adds or removes pixels. This is what you want for web, email, and screens.
• Resample unchecked: the pixels stay exactly as they are — only the DPI (resolution) value changes, which alters the printed size. A 3000px-wide photo at 300 DPI prints 10 inches wide; the file itself doesn't change.

So: shrinking for a website, leave it checked. Preparing a file to print at a specific physical size without quality loss, uncheck it and set the resolution instead.

Resize a batch, and avoid the blur

Select several images in Finder and open them together in Preview (they load into one window with a sidebar). Press Cmd+A in the sidebar, then choose Tools → Adjust Size… — the change applies to every selected image at once. Percent is the safest unit here since the photos may have different dimensions.

Two rules keep results sharp:

• Shrink, don't stretch. Scaling down always looks fine; scaling up invents pixels and looks soft. If you must enlarge, keep it modest.
• Keep an original. Resizing down throws pixel data away permanently. Cmd+Z undoes it during the session, and File → Revert To → Last Saved works until you close the file — but the safe habit is File → Duplicate before you resize.

Two more resizing tricks

• Resize while converting, from Finder. Right-click images in Finder and choose Quick Actions → Convert Image — alongside the format menu there's an image-size option (Small, Medium, Large, Actual Size). It's less precise than Adjust Size but makes copies automatically, so your originals are never at risk.
• Need an exact width and height? Don't uncheck Scale proportionally and type both numbers — that squashes the photo. Crop to the right shape first (drag a selection, then Cmd+K), and only then use Adjust Size to hit the final dimensions.

Finally, a sanity check that saves re-dos: Tools → Show Inspector (Cmd+I) displays the image's current pixel dimensions and DPI, so you can confirm the result is exactly what the upload form or print shop asked for.

Need the Terminal version?

For scripted or truly huge batches, macOS ships a command-line resizer too: sips can cap hundreds of images to a maximum edge length in one command, and it slots neatly into shell scripts and Automator workflows. See how to resize images with sips for the recipes. For everyday one-offs, though, Adjust Size in Preview remains the fastest tool you already have open.