Which app compresses images most on a Mac, and when is a free one enough?
On screenshots, Levati at half size saved 84.6% and Preview at half size as JPEG 80.7%, Caesium at quality 80 75.6%, and ImageOptim 28.9% losslessly. A free option is enough if you shrink the odd image by hand and JPEG or lossless compression will do; Levati pays off if new images should get smaller on their own and photos should keep their metadata.
How much does each app compress screenshots?
Between 28.9% and 84.6% at the median, and image quality varies widely along the way. Caesium at quality 80 saved 75.6% at the median but only reached a SSIMULACRA 2 score of 76.1, and 52.9 on a page full of photos; Levati at full size saved 63.5% at 88.4.
The table lists size, saving and SSIMULACRA 2 for each method; ImageOptim comes from the earlier measurement with the same script, the other values from a measurement of the same pages.
| App and method | Median size | Median saving | SSIMULACRA 2 median (lowest) | Price |
|---|---|---|---|---|
| Original (PNG) | 1.09 MB | 0% | 100 | – |
| Preview, export as JPEG | 536 KB | 44.5% | 84.3 (83.0) | part of macOS |
| Preview, half size as JPEG | 201 KB | 80.7% | 84.8 (83.0) | part of macOS |
| ImageOptim, lossless | 0.90 MB | 28.9% | 100 (100) | free |
| Caesium, lossless | 806 KB | 29.5% | 100 (100) | free |
| Caesium, quality 80 | 264 KB | 75.6% | 76.1 (52.9) | free |
| Levati, full size | 354 KB | 63.5% | 88.4 (80.0) | free for 30 days, Pro $19 once |
| Levati, half size | 156 KB | 84.6% | 89.7 (83.8) | free for 30 days, Pro $19 once |
SSIMULACRA 2 compares an image with its original and returns a score up to 100; 100 means identical pixels, 90 visually lossless, 85 excellent, 80 very high quality (SSIMULACRA 2 on GitHub, checked October 28, 2026).
How do the apps do with photos?
On iPhone photos, Levati saved 17.8% at the median and kept gain map, Display P3 and EXIF without location; ImageOptim saved 14.2% losslessly without that metadata. Preview and Caesium were not measured on photos.
The details are in What’s left of an iPhone photo after Levati and ImageOptim?; how the two differ otherwise, in What is a good ImageOptim alternative for Mac?.
What does Preview do?
Preview resizes with Tools > Adjust Size and saves as JPEG with File > Export; at half size, screenshots got 80.7% smaller at the median this way, at a SSIMULACRA 2 score of 84.8. Without half size, JPEG saved 44.5%, and pages with a lot of text on a light background got larger than the PNG.
Apple describes both in the Preview User Guide, for resizing and for converting (both checked October 30, 2026).
What does Caesium do?
Caesium compresses losslessly or at a quality from 0 to 100; lossless, it saved 29.5% on screenshots without changing a pixel, at quality 80 it saved 75.6%. The command-line tool caesiumclt installs with brew install caesiumclt or cargo install caesiumclt and handles JPEG, PNG, WebP and GIF (caesium-clt on GitHub, checked October 30, 2026).
When is a free option enough?
When images only need to get smaller now and then, by hand: Preview at half size as JPEG saves 80.7% on screenshots at the median and comes with macOS, ImageOptim and Caesium save about 29% losslessly without losing a pixel. A Folder Action with sips even does the same automatically, as How do you compress screenshots automatically on a Mac? shows.
An app like Levati pays off if screenshots should stay PNG and still get small, if photos should keep gain map, Display P3 and EXIF, and if new files should get smaller on their own without a script of your own. After 30 days, 30 images a month by hand stay free.
How was this measured?
The measurement covers 10 screenshots of public web pages taken on October 30, 2026 on a MacBook Pro with M5 Pro running macOS 27.2, without a window and without the Levati app; each method got its own copies of the same material.
- Screenshots: captured headless with Chromium via Playwright 1.64.1 at Retina resolution, 2880 × 1800 pixels at 144 dpi (script
docs/messung/bildschirmfotos.mjsin the website’s repository). - Preview: reproduced with
sips, which uses the same macOS image library; export as JPEG without a quality setting, half size withsips -z. Where the slider in the export dialog sits was not read, because no window was opened. - Caesium:
caesiumclt1.5.0 from Homebrew, once with--lossless, once with-q 80. - ImageOptim: values from the measurement of October 29, 2026 with ImageOptim 1.9.3 on 10 screenshots from the same script; the pages had changed slightly since. ImageOptim was not run again.
- Levati 1.0.6: the function behind Levati’s command-line tool with the app’s default settings and once without half size. Photos: values from the measurement of October 29, 2026 on 31 iPhone JPEGs; the originals were no longer available for a new measurement.
- Quality: SSIMULACRA 2 from jpeg-xl 0.12.0 against the original at the same resolution, half-size results against the original scaled to the same size with
sips. Analysis: file size in bytes, median; 1 KB is 1,000 bytes, 1 MB is 1,000,000 bytes. Script and raw data:docs/messung/bildschirmfoto-wege.shandrohdaten-wege.csv.
Not measured: Preview and Caesium on photos, ImageOptim’s lossy setting and HEIC. The images themselves are not published.