Compress Image

What the quality dial is actually throwing away

Every lossy format works the same way underneath: the encoder discards detail your eye is unlikely to miss, and the quality number decides how aggressively. What goes first is high-frequency detail — fine texture, individual hairs, grain, the crisp edge of small text. What survives longest is smooth areas and large shapes. That is why a landscape holds up at 70 while a screenshot of a spreadsheet already looks dirty at 90.

Two consequences are worth carrying with you. First, the numbers do not transfer between formats: WebP at 80 and JPEG at 80 are not the same amount of damage, and at an equal setting WebP is generally both smaller and cleaner. Do not bring a habit of “always 85” from one to the other without looking at the result. Second, the dial is not linear the way people assume. Going from 100 to 90 often saves 40% or more. Going from 60 to 50 usually saves much less and costs far more in visible artefacts, because by that point the encoder has already discarded everything it could discard cheaply.

What too much compression looks like: blocky stair-stepping along edges against a flat background, banding stripes across gradients like a sky, coloured halos around thin coloured text. If you see those, you went too low for that particular image.

Shrinking pixels and shrinking quality are different tools

These get used interchangeably in conversation and they do very different things, so it pays to be exact. Lowering quality keeps the image the same size on screen and reduces how precisely each pixel is stored. Capping the pixel dimensions throws pixels away outright: halving both width and height leaves a quarter of the pixels, and that is where dramatic savings come from.

The difference decides which one to reach for. If the constraint is “this has to be under 2 MB” and the image will be looked at on a screen, pixels are the safe lever — a 4000-pixel-wide photo is wasted on any display, and cutting it to 1600 usually saves far more than any quality setting could. If the image has to keep its resolution because someone will zoom into it or print it, quality is the only lever you have, and you should stop around 75 rather than pushing lower.

One thing this tool cannot do is make a print file smaller. Print wants roughly 300 pixels per inch, so a full A4 page needs about 3500 pixels across. Compressing a file to slip under an upload limit and then printing it gives you a soft, artefact-ridden print, and nothing at the printing end can put the detail back.

What happens when an image cannot be made smaller

Sometimes there is nothing to win, and the tool says so rather than handing you a bigger file. This happens most often with PNG, and it is worth understanding because the reason is counter-intuitive.

PNG is lossless. There is no quality dial, because there is no lossy step to dial — the pixel data is compressed the way a ZIP file compresses text, and it is either exactly right or broken. When you load a PNG and re-encode it, you are not compressing anything further; you are producing a *different* lossless file, and different can easily mean larger, because the original may have been written by an encoder that chose better filters, or simply by one that had more time.

So when a re-encode comes out no smaller and no pixels were cut, this tool keeps your original file and marks it as such. Returning something bigger than what you handed in, after you clicked a button labelled “compress”, would be worse than doing nothing. If you genuinely need a smaller PNG the honest options are: convert it to WebP, which is typically 25–35% smaller for the same appearance, or reduce the pixel dimensions. And if the PNG is really a photo — a screenshot of a photo, an export from a design tool — converting it to JPG is the real fix, because a lossless format is the wrong container for photographic data.

What is lost besides bytes

Compression is not reversible, and a few side effects are worth knowing before you delete the originals.

• Re-compressing an already-compressed image compounds the damage. Each pass adds artefacts on top of existing artefacts, so a JPEG saved at 70 three times looks markedly worse than one saved at 70 once. Always start from the original.
• Metadata is dropped. That is usually the point: a phone photo carries GPS coordinates, the camera model and a timestamp, and publishing it with those intact tells the internet where you live. The flip side is that the capture date and camera settings are gone too, so keep your own copy if you are archiving.
• Orientation is baked in rather than lost. A photo shot sideways stores “this should be rotated” as metadata; here it is genuinely rotated during decoding, so the result is upright everywhere instead of depending on whether the viewer honours the flag.
• Colour can shift slightly. Browser canvas rendering works in sRGB, so a photo shot in a wide-gamut format such as Display P3 gets converted, and saturated colours can come out a little flatter. If exact colour matters, this is not the right tool.
• Transparency survives, since PNG and WebP both support it and this tool never changes your format. Transparent areas only become a problem when the output cannot store them, which is a job for the format converter.

What savings to actually expect

Claims like “up to 90% smaller” are marketing, because the answer depends entirely on what you brought. Some honest anchors for what these settings typically produce:

• Phone photo at 4000×3000 and 3–5 MB, quality 80, pixels untouched: usually lands between 300 KB and 800 KB.
• The same photo with pixels capped at 1920: usually 100–300 KB, and indistinguishable at any normal viewing size.
• A 1440×900 screenshot: often nothing to gain. Screenshots are flat colour and sharp text, which is the worst case for lossy compression and the best case for PNG.
• Something already optimised by a CMS or an image CDN: expect single-digit percentages, because that pipeline already did this work.
• A scan or a photo of a document: 200–500 KB is plenty, so keep the resolution and lower the quality instead.

Why the per-file readout matters more than a headline number

The list under the result exists precisely because no single ratio applies to every image. Two files from the same camera roll can behave completely differently — one at 4 MB shrinking to 300 KB, another at 800 KB stubbornly refusing to move because whoever exported it already ran it through an optimiser.

That readout is also how you calibrate the dial for your own material. Compress a representative photo at 80 first, look at the number, and decide from there whether to go lower or whether the pixel cap is the control you actually wanted. A tool that promised you a fixed percentage would be lying about half your files, and you would find out after you had already deleted the originals.