How to Compress an Image to 100KB
At 100KB you can keep a decent picture, but only if the pixel size is sensible first. Choose the dimensions, then let the quality fill the rest.
The trade-off at 100KB
Two things decide whether a picture fits in 100 KB: how many pixels it has, and how hard the JPEG encoder has to squeeze each one. You can only choose the first one deliberately. At 100 KB (800,000 bits):
| Pixel size | Pixels | Bits per pixel |
|---|---|---|
| 600 × 600 | 360,000 | 2.2 |
| 800 × 1000 | 800,000 | 1.0 |
| 1080 × 1350 | 1,458,000 | 0.55 |
| 2000 × 2500 | 5,000,000 | 0.16 |
| 4000 × 3000 | 12,000,000 | 0.067 |
Around one bit per pixel or more is a comfortable range for an ordinary photo. Much below that, the encoder has to discard so much detail that faces and text turn blocky. That’s why a straight “compress to 100KB” on a 12 MP phone photo gives a poor result, and why the same photo scaled to roughly 800 × 1000 first gives a good one.
A repeatable way to do it
- Does the form fix the pixel size? If yes, use KB + Dimensions: enter the width, height and a 100 KB maximum, and the tool finds the best quality inside both.
- If not, choose a size that suits the use. A photo that will be viewed on a screen rarely needs more than about 1000 px on the long side. Resize Image can set that exactly.
- Then compress. In Compress Image, pick Target size and a 100 KB maximum. Because the pixels are already sensible, the tool should land at a good quality without touching them.
- Read the quality number. It is an encoder setting out of 100, not the percentage of quality kept. The tool warns when it has to go below 60/100, because that is where the picture starts to look heavily compressed.
A measured example
We ran a generated 3000 × 4000 test image (2.1 MB) through Target size mode with a 100 KB maximum and the default settings. Chromium gave 86.7 KB at 2168 × 2890 pixels, quality 51 / 100. Safari’s engine gave 89.1 KB at 1843 × 2457, quality 28 / 100. Both fit, both flagged heavy compression, and they got there by different routes: the encoders map the quality number differently, so the pixel size the tool settled on differs too. That is why a quality number from one run says little about another. Size, pixels and quality move together, so read all three, and look at the picture. Real photos will land differently from a generated one.
When it still doesn’t fit
- The tool shrank the pixels. In Target size mode it keeps the highest quality that fits and only shrinks pixels when quality alone can’t reach the limit. If you’d rather keep the dimensions, turn on “Keep the pixel size” under Advanced and accept a lower quality, or raise the limit if the portal allows.
- The result is a PNG over 100KB. PNG is lossless. Save as JPG; see PNG vs JPG for online forms.
- Minimum size problems. Some forms set a minimum as well as a maximum, such as 50–100 KB. A plain image can come out lighter than the minimum. KB + Dimensions has a way to raise a too-small file and tells you which method it used.
Files that already meet the limit are returned untouched, whatever their format.