File size and dimensions are different things
People say image size for two different things. The file size is how much space the file takes: 3.4 MB or 120 KB. The dimensions are how many pixels wide and tall the picture is: 4,000 by 3,000. A website that rejects your picture is looking at the file size, and a picture that looks too small on a screen has too few pixels.
The two are linked. A picture with more pixels has more to store, so shrinking the dimensions is a strong way to reduce the file size. But you can also cut the file size without touching the pixels, by saving them more cleverly, and that costs less in looks.
Three ways to reduce an image, and when to use each
- Compress (the level setting). Keeps the pixels, removes detail you cannot see. Use it first, whenever the picture will be seen on a screen at about its own size.
- Shrink the dimensions (largest side). Use it when the picture is far larger than it will ever be shown. A photo shown 800 pixels wide does not need 4,000, and shrinking it saves the most of all.
- Change the format (WebP, AVIF, JPG). A photo saved as PNG is several times bigger than the same photo as JPG or WebP. Change it when the receiving place accepts the other format. Not every change helps: a HEIC photo is already very small, so a JPG made from it is usually larger.
These add up: shrinking the dimensions, then compressing, then switching a PNG photo to WebP can turn a file of several megabytes into one under 200 KB.
A limit in KB or MB
If a form says 100 KB, there is no need to experiment. Switch on Stay under a file size and enter the limit. The tool tries qualities for the highest one that fits and makes the picture smaller in pixels only when it has to, then tells you what it did. For specific limits there are pages for 20 KB, 50 KB, 100 KB, 200 KB, 500 KB and 1 MB: 20 KB, 50 KB, 100 KB, 200 KB, 500 KB and 1 MB.
A limit counts a kilobyte as 1,000 bytes and the list shows sizes the same way, so a file that says 99.5 KB really is under 100,000 bytes. The field takes 5 to 50,000 KB; below 5 KB a picture has too little left to be of use.
How much to expect
In our tests at the Balanced level, camera style JPG photos came out 40 to 70 percent smaller, PNG photos 60 to 80 percent smaller, and PNG screenshots and logos 50 to 65 percent smaller. A WebP that an editor exported at quality 90 shrank by 40 to 56 percent. Pictures that were already compressed gain little or nothing: a JPG saved at a low quality, a WebP saved at 75 to 80, or a PNG that already has few colours save a few percent at most, and come back unchanged when that is all there is to gain. Making the pixels fewer with the largest side setting saves more than any of these when the picture is much bigger than it needs to be.
What the list tells you about each file
Under each finished file the tool says what it did and, when something is unexpected, why. A file that is already as small as it can get is returned unchanged and keeps its own name. A picture that could not reach your limit shows the smallest size it got to and the pixel size. A conversion that came out larger says so and suggests a better format. Animated pictures are left exactly as they are, since reducing them would keep only the first frame.