How to Compare Image Quality Before and After Compression
2026-11-08
See how much quality a photo loses when compressed, using a side-by-side comparison, and check how much the file size shrank.
How much quality does compression cost?
Lowering the file size lowers the quality a little. The real question is how much, and that is easiest to judge by comparing the two side by side. There are two ways to do it.
Method 1: Before and after in the compressor
1. Add photos to the image compressor and set your compression options.
2. In the results list, press "Compare" on a photo.
3. Drag the divider left and right to compare the original with the compressed version. Press "Close compare" when you are done.
Method 2: Compare two images directly
1. In the image compare tool, add the original to the left slot and the result to the right slot.
2. Move the "Position" slider. The left and right arrow keys also work.
3. Read "File size" to see how much smaller the result is.
4. Press "Save this position as one image (PNG)" to keep the comparison picture.
What to look for
- Zoom in on text edges and smooth gradients. Smeared or blocky patches mean the compression is too strong.
- For email or web pages, a long edge of 1600 to 2000 pixels at quality 75 to 85 is a common starting point. Check each photo yourself, since results vary.
Notes
- If the two images differ in size, the right image is stretched to match, so the comparison is misleading. Match the sizes first.
- A photo that is already well compressed may not get smaller. In that case keeping the original is normal.