A 234 KB photo of the Hubble Deep Field came out at 95.7 KB with the default settings, 59% smaller. That is the usual story for a JPG. The format you choose changes the outcome more than the slider does, as these four files at Quality 75 show:
| Starting file | Same as original | As WebP |
|---|---|---|
| JPG, 1000 × 872, 234 KB | 95.7 KB JPG (−59%) | 95.8 KB (−59%) |
| PNG photo of a coffee cup, 442 KB | 440 KB PNG (0%) | 31.3 KB (−93%) |
| The same coffee photo as HEIC, 134 KB | 33.0 KB JPG (−75%) | 31.4 KB (−77%) |
| WebP, 1000 × 872, 135 KB | 112 KB WebP (−17%) | 112 KB (−17%) |
Look at the PNG row. A photo saved as PNG, which often happens with exports from editing apps, is stored without loss, so keeping it a PNG saves almost nothing. Switching it to WebP removed 93%. At the other end, the WebP file had already been compressed once, and a second pass found only 17% to take out.
Which format to choose
- Same as original keeps JPG, PNG and WebP in their format. HEIC and AVIF are saved as JPG, since the tool does not write those formats.
- JPG is the safe choice for upload forms, email to anyone, and older programs. Transparent areas become white, because JPG has no transparency.
- WebP suits websites and gives the smallest photos when the source was a PNG.
- PNG is for screenshots, logos and graphics with sharp edges. The page says it plainly: “same pixels, smaller file”, and the savings are modest.
What the slider does
Quality runs from 30 to 95 and starts at 75. It decides how much fine detail JPG and WebP discard, and the size next to each file updates a moment after you stop moving it. The hint under the slider says 70–80 is hard to tell from the original on a phone screen, and we’d stay in that range for anything people will look at closely. Go lower only to get under a hard size limit.
Longest side scales big images down to 2560, 1920 or 1280 pixels on the long side, and never enlarges. It is often the stronger lever, because a photo at half the width and height has a quarter of the pixels. For an exact width or height in pixels, Resize image is the better tool.
For the technically minded: one way to put “how close” into a number is PSNR, which compares every pixel with the original, where above about 40 dB differences are hard to see. The Hubble JPG at 75 scored 34.8 dB, and the Hubble WebP, compressed again as WebP, 38.2 dB; star fields are full of tiny points, which is a hard case. The encoders are MozJPEG for JPG, libwebp for WebP and oxipng for PNG.
If a file comes out no smaller in the same format, with Longest side at Original, the tool says “Already well compressed” and the download is your original file, untouched.