Make a photo attachment smaller in Mac Preview

To make a smaller file for email, create a separate working copy of the photo before resizing anything. In Preview, reduce the copy’s pixel width with resampling turned on, then export a new JPEG and check its size. Changing only a DPI or resolution label does not reduce the number of pixels.

The right size depends on what the recipient needs to read. A picture meant for a quick look can be smaller than one containing fine print, a receipt, or details someone may need to inspect.

Resize the pixels

Open the photo in Preview, then choose File > Duplicate before making any edits. Save that duplicate with a new name using File > Save, and work on it. Apple documents this duplicate-and-save route.

On the saved working copy, open Tools > Adjust Size. Set the width unit to pixels, turn on Scale proportionally, and turn on Resample image before entering a smaller width. Scaling proportionally keeps the image’s shape; resampling changes its pixel dimensions. Apple documents these controls in its Preview resizing guide.

For a 3600 × 2400 image, entering a width of 1800 pixels produces a proportional 1800 × 1200 image. That is one quarter as many pixels: 2,160,000 instead of 8,640,000. It does not mean the file will be exactly one quarter as large. File size also depends on the picture’s content and how the exported JPEG is encoded.

An 1800-pixel width is a practical starting point when someone may need to view larger details on a screen. You can try 1200 pixels for a casual preview. Treat either number as a starting point, then inspect the result for the detail that matters.

Export a new JPEG

Choose File > Export, select JPEG as the format, adjust the JPEG quality control, and give the copy a descriptive new name and location. Those are the controls in Apple’s Preview export guide. A name such as photo-email-1800.jpg helps you identify the smaller copy when attaching it.

Check the exported file in Finder’s Get Info before composing the message. Use the actual file size for the attachment budget. A DPI setting or the size of a window on screen does not tell you how many bytes will be attached. If the file is still too large, you can try a lower JPEG quality setting or a smaller pixel width, then check the new copy again.

Compare size with readable detail

The files below are versions of a freely reusable synthetic landscape illustration made for practice. It contains fine texture and large and small text on a sign; it is not a camera photo. The measured copies show how much sizes can differ, but the smaller versions changed both pixel dimensions and JPEG encoding settings. These figures do not isolate the effect of resizing, and they do not predict Preview’s output.

Practice file	Pixel dimensions	Measured size
Original illustration	3600 × 2400	5,389,848 bytes
1800-pixel copy	1800 × 1200	146,634 bytes
1200-pixel copy	1200 × 800	52,995 bytes
900-pixel copy	900 × 600	28,527 bytes

Download the copies if you want to compare them at an ordinary reading width and at 100% zoom. Look especially at the small sign text and the edges of its letters. For this illustration, the gate letter and time are the details to compare. For a receipt, check the total and the small date line before deciding which copy to send.

A 900-pixel-wide copy has enough pixels to display 900 pixels wide at 1×. On a 2× display, it supplies native pixels for only 450 CSS pixels of width. That is one reason to check the copy at the size someone is likely to use, and compare it with a small thumbnail. If a receipt, evidence image, or client document contains essential fine print, keep enough detail to read it. When a limit prevents sending that version as an attachment, send a link to the original with access limited to the intended recipient.

DPI is not the attachment size

Preview also has a resolution field, but it serves a different purpose from choosing a smaller pixel width. With Resample image unchecked, changing the physical-resolution relationship does not remove pixels.

The practice illustration includes a 72-DPI copy. It remains 3600 × 2400 pixels and measures 5,391,229 bytes, versus 5,389,848 bytes for the original. The copy was re-encoded as a JPEG with a 72-DPI tag. Its small byte difference should not be attributed to the DPI label alone. For an email attachment that needs fewer bytes, check the exported file’s size after changing pixels and quality.

Count every attachment

For several photos, add the file sizes of the copies you actually plan to attach, along with any other attachments. Ten copies of the measured 1800-pixel illustration total 1,466,340 file bytes; ten originals total 53,898,480 file bytes. That arithmetic describes the files, not the size of an encoded email message or whether a recipient can receive it.

For a personal Gmail account, Google says the total attachment limit is 25 MB, and files over the limit are replaced with a Google Drive link. Work or school Gmail limits are set by an administrator. Those rules should not be assumed for another mail provider.

Check the email draft for the intended named copy. A picture displayed within the message and a file added as an attachment can present differently to the recipient. Confirm that the smaller exported file is the one you selected; a resized image sitting elsewhere on your Mac does not make the original attachment smaller.

The Mac steps follow Apple’s documentation checked Oct. 7, 2026. The practice illustrations were resized and measured with Pillow 12.3, and are not Preview export or email delivery tests.