How to batch resize images on a Mac without stretching them

You can resize several images together in Mac Preview, but choose the dimension according to what the finished images must fit. A 1,200-pixel width produces a different result from a 1,200-pixel longest-edge limit when the batch includes portraits. Keeping proportions intact prevents stretching; it does not, by itself, keep every image within a height limit.

Choose the limit before opening the batch

Suppose a batch contains a 2,400 × 1,600 landscape image and a 1,600 × 2,400 portrait. Setting both to 1,200 pixels wide while preserving proportions gives you:

• Landscape: 1,200 × 800
• Portrait: 1,200 × 1,800

The portrait is unstretched, but its 1,800-pixel height exceeds a 1,200-pixel longest-edge limit. For that limit, the portrait needs to be 1,200 pixels high, which makes it 800 pixels wide.

The calculation is proportional: new height equals original height multiplied by target width divided by original width. For the landscape, the width is halved, so its height falls from 1,600 to 800. For the portrait, changing the width from 1,600 to 1,200 applies a factor of 0.75, leaving the height at 1,800.

If the requirement is instead an exact 1,000 × 1,000 square, resizing alone cannot make a rectangular image fill it while preserving every part of the image and its proportions. That requires a crop or empty space around the image.

Batch resize in Preview

First, make working copies in an output folder so the originals remain available. Apple also documents File > Duplicate, followed by Save, for making a copy of an image.

Open the intended images as a group in Preview. If they open in separate windows, Apple’s setting is Preview > Settings > Images > When opening files > Open groups of files in the same window. In the shared window, choose View > Thumbnails, select only the image thumbnails you intend to change, then choose Tools > Adjust Size. Set the units to pixels and keep Scale proportionally selected so changing one dimension changes the other. Use resampling when you need to change the pixel grid; a resolution-only change is a different operation. Apple also documents Fit into, which can leave one dimension smaller than the stated fit size.

These controls are described in Apple’s guides to opening several files in one window, resizing images, and duplicating images.

For a width requirement, select the relevant images and enter the target width. For a longest-edge requirement, group landscapes and portraits separately: set the width for landscapes and the height for portraits. Check square images and already-small images separately. If an offered Fit into preset suits the requirement, inspect the resulting dimensions; do not assume it creates a particular size for every shape. When the requirement is unclear, save and inspect one working copy before processing the whole batch.

See what the same images become

The downloadable batch practice ZIP contains four synthetic originals, their measured width-based and fit-based outputs, and measurements.json. The fit example uses a 1,200 × 1,200 bounding box with an explicit no-upscale rule. That rule belongs to this example; it is not a guarantee about Preview. Keep images that are already small out of a Preview batch if they must retain their dimensions.

Original	Set width to 1,200	Fit within 1,200 × 1,200, without upscaling
Landscape, 2,400 × 1,600	1,200 × 800	1,200 × 800
Portrait, 1,600 × 2,400	1,200 × 1,800	800 × 1,200
Square, 1,800 × 1,800	1,200 × 1,200	1,200 × 1,200
Small landscape, 600 × 400	1,200 × 800	600 × 400

The dimension label printed inside the card describes its 1,600 × 2,400-pixel source. The saved example shown here is 800 × 1,200 pixels.

Both portrait results preserve its shape, but only the 800 × 1,200 result meets a 1,200-pixel longest-edge limit. In the no-upscale example, the 600 × 400 image stays as it is. Setting its width to 1,200 instead creates interpolated pixels; it cannot recover detail that was absent from the original.

Plan a mixed batch with a 1,200-pixel cap

For the four practice originals, put the landscape and square into a width group. Their target width is 1,200; both finish within the cap. Put the portrait in a height group and enter 1,200 for its height. Leave the small landscape out of both resizing groups. It already has a longest edge of 600 pixels.

Name the finished copies so their role remains obvious, such as a folder named “Gallery 1200” beside the originals. The expected set contains four files: 1,200 × 800, 800 × 1,200, 1,200 × 1,200, and 600 × 400. Compare the saved copies with that list before uploading them. A missing portrait means the batch is incomplete; a 1,200 × 1,800 portrait means the width setting was used where a height setting was needed.

Check the saved results

If an image did not change, check whether its thumbnail was included in the selection. If circles or other shapes look stretched, check Scale proportionally and compare the saved width-to-height ratio with the original. If a file still has the old pixel dimensions, check whether you changed print resolution rather than resampling its pixels.

Finally, check file size separately from dimensions. A smaller pixel grid does not promise a smaller PNG: the synthetic landscape in this example went from 35,159 bytes at 2,400 × 1,600 to 57,565 bytes at 1,200 × 800. Its flat-color original compressed well, while resampling introduced additional colors. Use the saved file’s actual dimensions and bytes for any delivery requirement.

Source note: Preview steps above come from the linked Apple documentation. The downloadable geometry outputs were produced and measured with Pillow 12.3.0 on Windows; they are not Preview exports or a Mac runtime test.