How to check image dimensions on a Mac and fix upload size mistakes

Open the saved image in Preview, click the Info button to open Inspector, and read its width and height in pixels. The upload rule may require a minimum width and height, an exact pixel size, a particular shape, a file format, or a byte limit. Check the saved file you plan to attach against the wording of the rule. The size of its window or browser preview does not tell you its pixel dimensions.

Check the full image in Preview

Open the image in Preview and use the Info toolbar button to open the Inspector. Apple’s Preview information guide describes where to find image information, including dimensions and resolution. Read the width and height in pixels when the upload asks for image dimensions. You can also open Tools > Adjust Size and choose pixels for the width and height units to read the existing grid. Close that dialog without applying a change if you only want the measurement.

If you have drawn a crop selection, take care to distinguish the selection’s size from the full image’s size. Apple’s crop and resize guide describes the Crop Inspector and selection units. A selected 900 × 900 pixel area does not mean the original file is already 900 × 900 pixels.

After making a copy, crop, or resize, reopen the saved result and check its dimensions. That final file is the one to compare with the upload rule. Attach that saved copy. Apple also documents duplicating and exporting images in Preview, so you can keep the original while preparing a separate upload copy.

Read the full upload rule

Consider a 1600 × 900 pixel PNG. The following are hypothetical upload rules, not results from an upload portal:

Example requirement	What happens with the original file
At least 1200 pixels wide and 800 pixels high	It meets both minimum dimensions.
Exactly 1200 × 800 pixels	It fails: 1600 × 900 is a different size and shape.
Exactly 1000 × 1000 pixels	It fails. A square crop without enlargement can be at most 900 × 900 pixels.
Long edge no more than 1200 pixels	The original fails. A proportional resize to 1200 × 675 pixels meets this dimension rule.
JPEG files only	The PNG fails the format rule regardless of its dimensions.
No more than 20,000 bytes	A 25,887-byte version exceeds the cap by 5,887 bytes.

Read terms such as at least, exactly, and no more than literally. A minimum allows larger dimensions; an exact requirement does not. If a limit is written in KB or KiB, check how that service defines it: decimal KB uses 1,000 bytes, while KiB uses 1,024. An exact byte count is clearer than relying on a rounded file-size label.

Work through an exact-size crop

The 1600 × 900 image has a 16:9 shape. Making its width 1200 pixels while keeping its proportions gives a height of 675 pixels, so proportional resizing alone cannot produce an exact 1200 × 800 file.

One path to 1200 × 800 is to crop the original to the largest 3:2 rectangle its height allows: 1350 × 900 pixels. Then resize proportionally by a factor of 8/9 to reach 1200 × 800 pixels. The crop decides which content stays; the resize changes the remaining pixel grid. In Preview’s resize controls, check the units, proportional setting, and resampling choice before saving, then inspect the saved copy.

A square request has a different constraint. The largest square available within a 1600 × 900 image is 900 × 900 pixels. A central square crop removes 350 pixels from each side. It retains 56.25% of the original pixel positions by area and removes 43.75%; that percentage says nothing about how much important subject matter was lost.

You can download the original 1600 × 900 sample and its 900 × 900 central crop to compare them. The crop is not an exact 1000 × 1000 solution. That would require a higher-resolution original or enlargement, if the upload rules allow it; enlargement cannot recover detail absent from the source.

Separate pixels from print resolution

Two supplied PNGs demonstrate why a resolution number is not a pixel count. The nominal 72 DPI sample and nominal 300 DPI sample both decode to 1600 × 900 pixels, have the same colors, and are 25,887 bytes each. Their resolution metadata differs; the stored values are approximately 72.009 and 299.9994 rather than exactly 72 and 300.

At a nominal 72 pixels per inch, 1600 pixels correspond to about 22.22 inches of print width. At 300 pixels per inch, they correspond to about 5.33 inches. Neither setting adds pixels. With resampling unchecked, changing the resolution or print size preserves the pixel grid. Printer dots per inch and an image’s pixels per inch are related print concepts, but neither replaces an upload rule stated as a pixel width and height.

Do not assume every upload system disregards resolution metadata. If a service explicitly asks for it, check that requirement separately from pixel dimensions.

If the dimensions look right but upload still fails

First, confirm that the file selected for upload is the newly saved copy, instead of an earlier file. Reopen that exact file in Preview and check the full image, especially if you previously measured a crop selection. If the image was rotated, compare the current width and height with the rule’s required orientation.

Then read the rest of the upload message. Correct pixel dimensions will not satisfy a JPEG-only rule when the attachment is a PNG, or a 20,000-byte cap when the attachment is 25,887 bytes. A browser’s displayed image width is another separate measurement. If a service shows a cropped preview, that view alone does not establish what dimensions its server received or accepted.

Source note: Preview steps are based on the linked Apple guides. The sample pixel grids, resolution metadata, crops, and byte counts come from recorded Pillow file inspection; no Mac interface or upload portal was tested.