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How to compress an image on Mac

macOS is unusually well equipped here. Preview resizes and re-encodes with a real quality slider, and Finder's Quick Actions handle batches without opening anything.

The short answer

Preview — resize
Tools → Adjust Size. Enter dimensions with the ratio locked.
Preview — compress
File → Export, choose JPEG, and use the Quality slider.
Finder Quick Actions
Right-click → Quick Actions → Convert Image. Batch, with size presets.
Automator
Build a reusable batch workflow for folders.
Exact KB target
Not offered by any built-in tool — Preview shows an estimate as you drag.
Best built-in combination
Adjust Size first, then Export with quality around 80.

Three ways to do it

  1. Step 1: Preview — the complete single-file route

    Open the image in Preview. Choose Tools → Adjust Size to set new dimensions with the aspect ratio locked, then File → Export, pick JPEG as the format, and drag the Quality slider. Preview shows the resulting file size as you drag, which makes finding the right point straightforward.

  2. Step 2: Finder Quick Actions — fastest for a batch

    Select any number of images in Finder, right-click, and choose Quick Actions → Convert Image. Pick a format and one of the size presets. It processes the whole selection at once without opening an application, though the presets are coarse.

  3. Step 3: Preview's batch mode — more control over a batch

    Select several files and open them together in Preview, then select all thumbnails in the sidebar. Tools → Adjust Size now applies to every selected image at once, and File → Export Selected Images exports them together with one quality setting.

Details worth knowing

Preview is genuinely good

It is worth saying plainly, because a great deal of advice pushes Mac users towards downloads they do not need: Preview does everything most people require. It resizes with the ratio locked, re-encodes with a real quality slider, shows the resulting file size live, converts between JPEG, PNG, HEIC and TIFF, and handles batches through multi-selection.

The one thing it will not do is hit a specific number of kilobytes. You can get close by watching the estimate as you drag the slider, but if a form demands "under 100KB" you are guessing rather than targeting.

Quick Actions versus Preview

Quick Actions
No app to open, works on a whole selection, but the size presets are coarse and there is no quality slider.
Preview
Precise dimensions and a real quality control with a live size estimate. Slower for large batches.
Automator
Build once, reuse forever. Worth it if you process folders regularly.

For a handful of files, Preview. For a folder of fifty where precision does not matter, Quick Actions.

Careful with Adjust Size

The Adjust Size dialogue includes a Resolution field measured in pixels per inch. Changing it does not affect how the image looks on screen and does not by itself change the file size — it is a printing instruction stored in metadata.

What matters is the width and height in pixels. Make sure the units are set to pixels rather than inches or centimetres before you type a number, and keep "Scale proportionally" ticked unless you deliberately want to distort the image.

HEIC on a Mac

macOS handles HEIC natively, so photographs from an iPhone open without fuss and Preview can convert them to JPEG through File → Export. This is the easiest way to prepare iPhone photos for someone on Windows, where HEIC support requires an extra codec.

One caveat

Menu names and options move between releases. If a step below does not match what you see, the underlying capability is usually still there under a nearby name — and the browser tool on this page works identically on every version.

Do it here instead

These run in your browser on any device, so nothing is uploaded and there is nothing to install.

  • Compress JPEG

    Shrink JPG and JPEG photographs with a quality slider or an exact target size.

    Open the tool

  • Resize image

    By pixels or percentage, aspect ratio locked, across a whole batch.

    Open the tool

  • Bulk compressor

    One setting across a whole folder, processed in parallel, downloaded as a ZIP.

    Open the tool

Questions

Frequently asked questions

Every answer below is present in the page source, expanded, so it can be read without opening anything.

How do I compress an image without downloading anything on a Mac?

Open it in Preview, use Tools → Adjust Size to reduce the dimensions, then File → Export as JPEG and drag the Quality slider. Preview shows the resulting file size as you drag, so you can stop at a size you are happy with.

Can Preview compress several images at once?

Yes. Select the files and open them together, then select all the thumbnails in Preview's sidebar. Adjust Size and Export Selected Images both apply to the whole selection with a single setting.

Does changing the resolution in Adjust Size make the file smaller?

No. Resolution in pixels per inch is a printing instruction held in metadata — it changes how large the image prints, not how many pixels it contains. Only the width and height in pixels affect the file size.

How do I hit an exact file size on a Mac?

No built-in tool targets a specific size; Preview only shows an estimate as you drag. For a hard limit, use a tool with a target-size mode — the exact-size pages here take a number in kilobytes and search for the best quality that fits under it.

How do I convert HEIC to JPEG on a Mac?

Open the file in Preview and choose File → Export, then select JPEG. For a batch, select the files in Finder and use Quick Actions → Convert Image. macOS reads HEIC natively, so nothing extra is needed.

Which is better, Quick Actions or Preview?

Quick Actions for speed on a large batch where the coarse size presets are good enough. Preview when you want specific dimensions, a real quality slider and a live view of the resulting file size.

From the blog

Related reading

More background from the blog.

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    How to Compress Images for a Website Without Wrecking Them

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  • Image Compression

    JPEG Quality Settings Explained: What the Number Actually Means

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