How to compress an image on Mac, without installing anything: use Preview's two built-in tools. Tools, then Adjust Size, shrinks pixel dimensions; File, then Export, cuts bytes directly with a JPEG quality slider. Both apply to many photos at once from one window's sidebar. This guide covers each dialog, when a free app like ImageOptim beats Preview, and what actually happens when you try to compress a PNG.
This is the deepest of the device guides inside the complete guide to compressing image size on any device, because a Mac genuinely ships with more compression tooling than most platforms bother to include. There is no single Compress button anywhere in macOS, not in Preview and not in Finder, so the two real levers, pixels and quality, sit under separate menus you have to already know to look for. I run the browser compressor this site is named for, so where Preview is the faster answer I will say so, and where it quietly cannot do something, I will say that too.
What are your built-in options for compressing an image on a Mac?
A Mac has five built-in routes to compress photos on Mac without downloading anything else, and this table is the whole map; every section below walks one row in detail.
| Goal | Menu path | What it actually does |
|---|---|---|
| Shrink pixel dimensions | Tools, Adjust Size | Fewer pixels, smaller file, new size shown before you save |
| Cut bytes without changing pixels on purpose | Tools, Adjust Size, Resample image, lower Resolution | Same lever as above, entered as a resolution number instead of a width |
| Re-encode at lower quality | File, Export, Format JPEG, Quality slider | Same pixels, fewer bytes |
| Switch to a smaller format | File, Export, Format HEIC | Better compression than JPEG at a similar look |
| Compress many photos at once | Select several in one window's sidebar, then Adjust Size or Export | Applies the same change to the whole batch |
Which row you need depends on whether the file is too big in pixels, in bytes, or both, covered next in order.
How to compress an image on Mac, step by step
File, then Export, not File, then Save, is the dialog that actually re-encodes a photo smaller on a Mac.
Open File, then Export, not File, then Save
Double-click the photo to open it in Preview, the image viewer already on every Mac, then choose File and Export from the menu bar, not File and Save, which only overwrites the file as-is and changes nothing about its size.
Pick a format: JPEG, HEIC, or something else
The Export sheet opens a Format pop-up with JPEG, JPEG-2000, HEIC, PNG, TIFF, PDF, and OpenEXR listed; hold Option while clicking the pop-up to reveal a few more specialized formats that stay hidden otherwise (Apple Support: Convert image file types). JPEG is the safest default because it opens everywhere. HEIC compresses better than JPEG at a similar look, which is worth choosing when the file is staying inside Apple's own ecosystem (Apple Support: Using HEIF or HEVC media).
Drag the quality slider against the live number
Only JPEG and JPEG-2000 show a Quality slider once you pick them; PNG and the rest do not, because there is no quality number to trade away on a lossless format. Drag the slider left to trade quality for a smaller file, save, then check the result in Finder with Get Info. A setting around 75 to 80 percent is a reasonable place to start: it cuts the file meaningfully while staying visually close to the original for most photos. Export again at a lower number if it is still too big.
How do you shrink an image's pixel dimensions instead?
The Adjust Size dialog under Tools is Preview's one resizing tool, and it handles both routes: exact numbers or a percentage.
The Mac Preview resize image dialog, Adjust Size, in detail
Open the photo, show the Markup toolbar if it is hidden, then choose Tools and Adjust Size. Type new Width and Height values directly, or switch the unit pop-up from pixels to percent if you would rather scale by half as big than work out the exact pixel count (Apple Support: Crop, resize, or rotate an image in Preview). Preview shows the resulting file size at the bottom of the dialog before you save, which removes the guesswork: you see the number before you commit, not after.
Scale proportionally and Resample image, the two checkboxes that matter
Keep Scale proportionally on unless you deliberately want to distort the photo; with it on, changing the width updates the height to match automatically. Resample image is the other checkbox, and it decides whether you are changing the file or just how it would print. Selecting it and entering a smaller value removes pixels and shrinks the file. Leaving it unchecked keeps every pixel in place and only changes the physical print size, the move when the goal is a smaller printed photo, not a smaller file.
Where does Preview stop, and when do you need something else?
Preview covers pixels and quality well, and it runs out in three specific spots: format, target sizing, and scale.
It cannot export WebP or AVIF at all, so a form or a CMS that specifically wants one of the newer formats needs another tool. There is no target-size mode, no way to say get this under 200 KB and have the app find the right quality for you; you drag the slider, export, check Finder, and repeat if it is still too big. And a big batch means babysitting one dialog through dozens of files with no summary of what you saved. The browser compressor here fills exactly that gap: JPEG, PNG, WebP, and AVIF up to 25 MB per image and 30 in a batch, no signup, with the after size live on screen so there is no export-check-repeat loop, and EXIF, the hidden metadata block carrying GPS coordinates and camera details, stripped by default on every file.
How much does resizing in Preview actually save?
Resizing saves file size roughly in proportion to the pixel count you remove, and pixel count falls by the square of how much you shrink each side, which is why cutting a photo to half its width keeps only about a quarter of its pixels.
Take a standard iPhone photo, 4032 by 3024 pixels, 12.2 megapixels, and run the arithmetic for a few common resize targets:
| Target width x height | Megapixels | Percent of original pixels |
|---|---|---|
| 4032 x 3024 (original) | 12.2 MP | 100% |
| 2000 x 1500 | 3.0 MP | 24.6% |
| 1600 x 1200 | 1.9 MP | 15.7% |
| 1200 x 900 | 1.1 MP | 8.9% |
| 800 x 600 | 0.5 MP | 3.9% |
That math is exact regardless of format or content, because it is just width times height. What it is not is a promise about the exact byte count Preview will show you, since bytes also depend on the photo's own detail and the format you export to. Treat it as the reason the file-size number at the bottom of Adjust Size drops so fast once you start typing smaller numbers, not as a guarantee of a specific kilobyte figure.
Which lever should you reach for, pixels, quality, or format?
Three different problems call for three different menus, and mixing them up is the single biggest time sink in this whole workflow.
| Lever | Best when | Quality risk | Works on many photos at once |
|---|---|---|---|
| Pixels, Adjust Size | The image has more resolution than the destination will ever show | None, if Resample stays checked and Scale proportionally stays on | Yes, select them all in the sidebar first |
| Quality, Export slider | The pixel count is already right but the file is still too many bytes | Low at 75 to 80 percent, visible below about 50 | Yes, same selection trick, Export applies to the whole batch |
| Format, HEIC | The photo stays inside Apple's own apps and devices | None to the eye, HEIC compresses better than JPEG at a similar look | Yes |
Most oversized photos need pixels first and quality second, because a photo with five times the pixel count a form will ever display cannot be rescued by any quality setting alone.
How do you compress a PNG image on a Mac?
PNG is a lossless format, so Preview's Export dialog never shows a Quality slider for it; the only way to reduce image size on Mac for a PNG is resampling its pixel dimensions in Adjust Size, or converting it to a lossy format like JPEG or WebP.
That single fact trips up more people than anything else in this guide, because it looks like a missing feature rather than how the format works. A PNG stores every pixel exactly, which is exactly why screenshots and logos stay sharp, and exactly why they run large. Two honest options exist. Resample the PNG down in Adjust Size the same way you would a JPEG, which keeps it lossless but with fewer pixels to store. Or export it as JPEG, or on a browser tool as WebP, which trades the lossless guarantee for a real quality slider and a much smaller file. That trade is worth it for a photo and rarely worth it for a screenshot with sharp text or a logo with flat color, where a lossy re-encode tends to blur edges that were meant to stay crisp.
How do you compress many photos on a Mac at once?
Batch compressing on a Mac happens through one trick: open every target photo in the same Preview window, select them together in that window's sidebar, and whatever you do next, Adjust Size or Export, applies to the whole selection in a single pass.
There is no batch progress bar and no running total of bytes saved; Preview just repeats the same operation across everything selected. For a dozen or two photos that beats opening each one, but past a few dozen it turns into a lot of clicking, which is exactly the point where Mac owners discussing this on forums like r/mac tend to switch to a dedicated app instead. ImageOptim is the one named most often: a free, open-source Mac app that applies lossless compression and strips unnecessary metadata from a whole folder dropped onto its window, with no settings to configure (ImageOptim). That is where a dedicated app wins outright: it will batch compress images Mac Photos libraries pile up by the hundred, without you opening a single one by hand.
What common mistakes cost people the most time?
Treating resize and compress as the same button is the single costliest mix-up, because they live under two different menus, and fixing the wrong one wastes an export cycle.
Leaving Resample image checked when you actually wanted a smaller print at the same quality throws away pixels you meant to keep. Leaving it unchecked when you wanted a smaller file changes nothing about the byte count, because unchecking it only affects the printed size. Turning off Scale proportionally and typing mismatched width and height values stretches the photo, often without anyone noticing until it is already sent. Assuming HEIC opens everywhere is another one: it compresses better than JPEG at a similar look, but older software and some non-Apple devices cannot open it directly, so export to JPEG when you are not sure what the recipient's device supports (Apple Support: Using HEIF or HEVC media).
One more, specific to the Mac: right-clicking a photo in Finder and choosing Compress wraps it in a ZIP archive; it does not touch the image itself. A JPEG is already a compressed format, so zipping one rarely saves more than a percent or two, and the recipient now has to unzip it before they can even open the photo. Skip it for images; it is built for folders of mixed files, not a single already-compressed picture.