Workflow
Does Compressing a File Reduce Its Quality?
It depends on the kind of compression. Lossy compression (JPEG, H.264, MP3) throws away detail you are unlikely to notice, so quality drops a little. Lossless compression (PNG, FLAC, zip) brings back every bit exactly.
Below is what we measured when we compressed and re-compressed real files on a Mac.
Does compressing a photo reduce quality?
Yes, if you save it as a JPEG, WebP, AVIF or HEIC at a lower quality setting. The encoder permanently removes fine detail to save bytes. If you save as PNG, no. PNG is lossless.
We took a 2000 px JPEG saved at quality 95 (524,458 bytes) and re-saved it at quality 75 with
sipsTwo caveats. The photo is a smooth macOS wallpaper, which hides JPEG damage better than a busy scene would. And ten saves is a stress test, not a normal workflow. The practical lesson: compress once, from the original, and keep the original.
Does compressing a video reduce quality?
Almost always, because video compression is lossy. The question is how much you can see. We encoded an 8 second 720p test clip at near-transparent quality (H.264, CRF 12, 6.9 MB), then re-encoded it as H.265 at CRF 28. The new file was 2.1 MB, about 30% of the size, with an SSIM score of 0.992 against the original. SSIM tops out at 1.0 for identical frames.
That clip is synthetic test video, so real footage will behave differently. Lower CRF means higher quality and a bigger file. Our guide to H.265 vs H.264 covers the trade, and compress video on a Mac has the steps.
Which kinds of compression lose nothing?
We checked three lossless cases by comparing decoded data with an MD5 hash:
- PNG: we saved a PNG ten times in a row with sips. The pixel hash after ten saves matched the first save exactly.
- FLAC: a 10 second WAV clip (882,078 bytes) became a 677,098-byte FLAC. Decoded audio matched the WAV hash exactly.
- Zip and similar archives: the file comes back byte for byte when you unzip it.
The price is size. Lossless saves far less than lossy. Our FLAC was about 77% of the WAV, while a 128 kbps MP3 of the same clip was 160,958 bytes, about 18%. Photos and video barely shrink in a zip because they are already compressed.
What changes for each file type?
| File type | Typical compression | Quality lost? | What we measured |
|---|---|---|---|
| JPEG, WebP, AVIF, HEIC | Lossy | Yes, adjustable | JPEG q95 to q75: 30% smaller |
| PNG | Lossless | No | 10 re-saves, identical pixels |
| MP3, AAC, M4A | Lossy | Yes, by bitrate | 128 kbps MP3: about 18% of WAV |
| FLAC, ALAC | Lossless | No | FLAC decoded identical to WAV |
| H.264, H.265 video | Lossy | Yes, by CRF | CRF 28 H.265: SSIM 0.992, 30% of size |
| Zip | Lossless | No | Unzips byte for byte |
How do I compress without losing quality I can see?
- Start from the original file, never from an already compressed copy.
- Lower the quality setting in steps and look at the result at full size. Most people stop seeing a difference well before the file gets small.
- Change the format when it helps. A modern format like AVIF or HEIC holds the same look in fewer bytes than JPEG. See the best image compressor for Mac.
- Keep your resolution unless the destination needs less. Shrinking pixels is also a quality loss.
- Strip metadata. It cuts bytes without touching a single pixel; see what compressing removes.
For PDFs the same idea applies: text stays sharp while embedded images get re-encoded. Details in compressing a PDF without losing quality.
When is Smol the wrong tool for this?
If you need pixel-identical results, Smol is not it. It has no lossless mode for images and always re-encodes them. ImageOptim does lossless optimization for free; see Smol vs ImageOptim. For one photo, Preview’s File > Export slider or
sipsafconvertSmol earns $29 when you want a lossy result you chose on purpose: one quality setting across a mixed folder of images, PDFs, video and audio, with a side-by-side size per file, on your Mac with nothing uploaded. Watch its default for images, which resizes to fit 2000 × 2000 px unless you change it. Originals stay untouched by default. See pricing.
How we checked this
On macOS 27.0.1 we re-saved a 2000 px JPEG ten times with sips and measured PSNR with ffmpeg, compared PNG and FLAC output by MD5, and encoded a synthetic 720p clip to H.265 and measured SSIM. Facts last checked . We make Smol, so treat our claims about it with the same care as any vendor’s, and see our editorial policy.
Compression and quality: common questions
Can you compress a file without losing any quality?
Yes, with lossless methods: PNG for images, FLAC or Apple Lossless for audio, zip for anything. We decoded PNG and FLAC and found the data identical to the source. The trade-off is that lossless files shrink much less than lossy ones, and zip does almost nothing for photos or video.
Does compressing a JPEG again make it worse?
Usually a little each time. In our test, ten re-saves at quality 75 moved PSNR from 52.1 dB to 49.7 dB on a smooth photo. Busy images degrade faster. Always compress from the original, and avoid saving a JPEG over and over.
Does zipping a file reduce its quality?
No. Zip is lossless, so unzipping returns the exact original. It also saves little on files that are already compressed, such as JPEG photos, MP4 video and PDFs full of images. Zip helps most on text, spreadsheets and uncompressed formats like WAV or BMP.
Does compressing a video reduce its quality?
Yes, because every common video format is lossy. The loss can be invisible at moderate settings. Our synthetic 720p test went from 6.9 MB to 2.1 MB with an SSIM of 0.992. Real footage varies, so preview the result before you delete the original.
Is HEIC lower quality than JPEG?
No. HEIC is also lossy, but it usually holds the same visual quality in a smaller file. The difference shows up when you convert it. Converting HEIC to JPEG can enlarge the file, because JPEG needs more bytes for the same look.
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