It seems like it should work: a photo is already a JPEG, JPEGs are compressed, so running it through a compressor once more should shrink it further. In practice this rarely does much, and pushing it hard enough to see a real difference usually makes the image look noticeably worse for a surprisingly small size saving. Understanding why clarifies what actually helps instead.
What JPEG compression actually removes
JPEG compression works by identifying detail the human eye is least sensitive to — subtle colour variations, fine texture — and simplifying or discarding it to save space. The first time a photo is compressed, there is a great deal of this kind of redundant detail to remove, which is why the first pass typically achieves a large size reduction with little visible cost. By the time that first pass is done, most of that easily-removable detail is already gone. A second compression pass is working on an image that has already had its redundancy stripped out, so there is simply much less left to save.
Why it can make quality worse without saving much size
Each JPEG compression pass introduces small artefacts — subtle blockiness, colour banding, slight blurring around edges. These are usually invisible after a single pass at a reasonable quality setting, but they compound with each additional pass. A second or third compression does not meaningfully identify new redundancy to remove; instead, it re-processes the artefacts from the previous pass, degrading the image further while barely moving the file size, since there is little genuine redundancy left for it to find.
What actually works instead
- Start from the original, uncompressed source if you still have it, rather than compressing an already-compressed copy. A single well-chosen compression pass from the original almost always beats two passes on a derivative copy.
- Resize the pixel dimensions with the Image Compressor rather than compressing harder. A smaller image, compressed once at a reasonable setting, is smaller and often looks better than a full-size image compressed twice.
- Crop out anything unnecessary before compressing, since every pixel of irrelevant background is data the compressor has to spend bytes describing regardless of how many passes you run.
How to tell if an image has already been compressed hard
Zoom in on a photo, particularly around sharp edges or areas of flat colour, and look for small blocky squares or a slightly smeared texture — both are telltale signs of JPEG compression that has already been applied at a noticeable strength. If you see this and the file is still too large for your needs, resizing the dimensions is a far more productive next step than compressing the same image again, since there is little further compression headroom left to exploit.
A comparison that makes the effect concrete
Imagine an original photo at 8 MB. A single compression pass at a reasonable setting might bring it to 800 KB — a 90% reduction, with quality that looks essentially unchanged at normal viewing size. Compressing that 800 KB result again at the same setting might bring it to 650–700 KB, a comparatively small further reduction, and by this point some quality loss is usually starting to become visible if you look closely. A third pass typically achieves even less, while continuing to add cumulative degradation. The pattern is consistent: each additional pass yields diminishing size returns and increasing visible cost.
When starting over from the original is not possible
If the original, uncompressed source is genuinely gone and all you have is the already-compressed copy, resizing the pixel dimensions is still your best lever, since it reduces the total amount of data the image has to represent, which a compression pass alone cannot do. Accept that the image is unlikely to shrink dramatically further through compression alone at this point, and plan around that reality rather than expecting a second pass to work the same magic the first one did.
Applying this thinking to PDFs, not just standalone photos
The same principle applies to a PDF that has already been through the PDF Compressor once — a second compression pass on the same file tends to offer only a modest further reduction, for reasons that mirror exactly what happens with a JPEG, since the embedded images inside the PDF are subject to the same diminishing returns. If a compressed PDF still needs to be smaller, reducing the page count or lowering the original scan resolution before recompressing is usually more effective than repeating the compression step on the same file.
Why this trips people up so often
The confusion is understandable, because compression really does work the way it feels like it should the first time — a large file becomes noticeably smaller, and the natural instinct is to assume repeating the same action will keep producing similar results. Most other size-reduction actions in daily life scale roughly linearly (removing more items from a suitcase keeps freeing up space at a fairly steady rate), which makes JPEG compression's sharply diminishing curve feel counterintuitive the first time someone runs into it. Once you know the curve flattens out fast after the first pass, it stops being a surprise and starts being something you plan around from the start.
A quick way to check before you waste a second pass
Before compressing an image a second time, it is worth a quick sanity check: note the file size before the extra pass, run it, and compare. If the reduction is small — a few percent rather than a meaningful chunk — that confirms you have already hit the point of diminishing returns, and the better move is resizing the dimensions or, if possible, going back to an earlier, less-compressed version of the file rather than compressing the same copy a third time hoping for a different result.
Frequently asked questions
Is there ever a benefit to compressing an image twice?
A very small further reduction is sometimes possible if the first pass used a conservative setting, but it is rarely significant, and the risk of visible quality loss increases with each additional pass.
Does this apply to PNG the same way it applies to JPEG?
PNG is lossless, so it does not accumulate the same kind of compounding artefacts, though re-saving it repeatedly still offers little further size benefit once it has already been optimised once.
How can I tell if I am working from the original or an already-compressed copy?
If you are unsure, check the file's history or where it came from — a screenshot, a download from a website, or a file that has passed through several apps is more likely to already be compressed than a fresh photo straight off a camera.
Is resizing always better than compressing again?
For an already-compressed image that still needs to shrink further, yes — resizing reduces the underlying amount of data, while another compression pass mostly just re-processes existing artefacts.
Will I be able to see the quality loss from a second compression pass with the naked eye?
At a light second pass, often not immediately; but zoom in and the artefacts usually become apparent, and repeated passes make the effect progressively more visible even at normal viewing size.
Does converting to a different format between compression passes help?
Not meaningfully — converting a JPEG to PNG and back does not recover any of the detail the first JPEG compression pass removed; it just adds an extra, unnecessary step.
Is the tool free and private?
Yes. No sign-up, no watermark, and files are removed from the server automatically about an hour after processing.