There's a popular belief that opening and re-saving a JPEG repeatedly will eventually turn it into an unrecognizable mess. You might have seen those YouTube videos showing a JPEG after being re-saved 1,000 times — blocky, color-shifted, barely recognizable.
I wanted to know what actually happens under realistic conditions. So I ran the experiment.
The Setup
I took a 2400x1600 photo (a landscape shot with sky, trees, and a building — mix of smooth gradients, organic textures, and sharp edges). Then I compressed it to JPEG at quality 85 and saved the result. Then I took that result, opened it, and saved it again at quality 85. And again. And again. Fifty times total.
At each step, I measured the SSIM (structural similarity) against the original and saved the file for visual comparison.
The Results
| Re-save Count | SSIM vs Original | File Size | Visual Assessment |
|---|---|---|---|
| 1 (first save) | 0.982 | 187KB | Looks great. No visible artifacts. |
| 2 | 0.978 | 186KB | Identical to step 1 to my eye. |
| 5 | 0.971 | 185KB | Still can't tell the difference. |
| 10 | 0.963 | 184KB | Slight softening visible at 300% zoom. |
| 20 | 0.948 | 183KB | Subtle softening at 200% zoom. Fine at 100%. |
| 50 | 0.921 | 182KB | Noticeable softening. Edges less defined. |
What I Learned
The degradation is real but slow. After 50 re-saves at quality 85, the image has noticeably softer edges and some smoothing of fine texture. But it's not the apocalyptic destruction those viral videos suggest. It's more like a slight blur filter.
The file size barely changes. After the first compression, each re-save at the same quality produces nearly the same file size. The JPEG algorithm is essentially finding the same compression solution each time, with only minor variations due to rounding.
The quality setting matters enormously. I repeated the test at quality 50, and the degradation was catastrophic. By 10 re-saves, the image had visible blocking artifacts everywhere. By 50, it looked like a Minecraft screenshot.
Why Those Viral Videos Look So Bad
The dramatic JPEG degradation videos typically do something sneaky: they change the quality setting each time, resize the image, or use quality 20-40. Under those conditions, yes, you get rapid degradation. But that's not how most people use JPEG files.
In normal use — saving at a consistent quality of 75 or above — re-saving a file 2-3 times (which is the realistic maximum for most workflows) produces no visible degradation.
The Real-World Takeaway
Don't stress about JPEG re-compression in normal use. If you're editing a JPEG, saving it, then editing and saving again — totally fine at quality 80+.
But there are two situations where it does matter:
1. Repeated social media re-uploads. Download from Instagram, re-upload to Facebook, someone saves it and uploads to Twitter. Each platform recompresses at its own quality settings. After 4-5 platforms, the image looks awful. Always share from the original file.
2. Low quality settings. If you're compressing at quality 50-60, every re-save amplifies the artifacts. At those levels, convert to PNG for editing (lossless) and only export to JPEG once at the end.
Pro tip from building the compress tool: If you need to make multiple edits to a JPEG, keep a lossless copy (PNG or the original) as your working file. Only export to JPEG as the final step. This avoids generation loss entirely.
For the curious: I noticed during development that different JPEG encoders handle this differently. The MozJPEG encoder (which imagemochi uses) produces more consistent results on re-saves than the standard libjpeg encoder. It's a small thing, but it's one reason I chose it.