How to Resize an Image on Mac with Preview
Preview can change image dimensions without another download. Decide whether you need fewer pixels, fewer bytes, or a different print size before you change its settings.
Change pixel dimensions in Preview
- Open the image in Preview and duplicate it if you want to retain the original.
- Choose Tools → Adjust Size. Choose pixels as the unit.
- Enable resampling to change the number of pixels. Keep proportional scaling enabled, then enter the desired width or height.
- Confirm the change and save a separate output copy.
Choose the right setting
| Your goal | What to change |
|---|---|
| A 1200-pixel-wide website photo | Pixel width, with proportional scaling |
| A smaller attachment | Dimensions and possibly export quality |
| A print size without changing detail | Resolution with resampling disabled |
| An exact 100KB maximum | Use a target-size compressor and verify the result |
An example you can check
A 4000 × 3000 photo resized to 1200 pixels wide becomes 1200 × 900 when proportions are locked. The pixel count falls from 12 million to 1.08 million. That is a predictable geometry change; the resulting file size still depends on format, content and encoding.
If the width changes but the result is distorted, undo and check the proportional-scaling setting. A fixed square needs cropping or padding, not a width and height that squeeze the subject.
Reduce file size after resizing
A resized PNG photograph can still be large. For a normal photograph without transparency, export a JPG copy and compare the result. Keep PNG when clean graphics or transparency matter. For a strict byte limit, use the compressor after resizing and inspect both the reported size and the image.
If you only need one smaller copy, the web resizer is another option. Website processing uploads the image to the server; Preview performs this editing on your Mac.
Resize several images
Preview can work with multiple selected images in a window. Use the sidebar selection carefully so you change the intended files. For a mixed collection, make copies first and check portrait and landscape outputs separately. A shared width is not the same as a shared shape.
Frequently Asked Questions
Why did changing DPI not reduce my image size?
Resolution metadata describes print scale. It does not remove pixels when resampling is disabled.
Should I overwrite the original?
Keep the original when you may need a different size later. Resizing down discards detail that the smaller copy cannot recover.