You drop a photo, move a quality slider, and watch a percent smaller figure update. Most guides stop at "use 70 to 80 percent". We maintain the image compressor on this site, so we can show you what the slider actually sends, which numbers are fixed in code, and the two places where the tool changes your image without asking.
How the Image Compressor Quality Slider Works
Every compression run passes three fixed limits to the library. maxSizeMB is 10, maxWidthOrHeight is 4096, and useWebWorker is true. Your slider value, divided by 100, becomes the encoder quality that rides along with them.
The 4096 limit is the one nobody mentions. Any image with a side longer than 4096 pixels gets scaled down to fit before encoding. The slider cannot turn this off. If you need original dimensions preserved, this compressor is the wrong tool and no quality setting fixes that.
The worker matters for feel. Compression runs off the main thread, so the page stays usable during a run. The slider locks while a run is in progress and unlocks when it finishes.
Moving the slider does not ask you to re-drop anything. Each change reruns compression on the original file held in memory, so every percent smaller figure measures against the same source bytes. Sizes print in Bytes, KB, and MB with two decimal places, which is enough resolution to compare two adjacent slider steps. The drop zone and the file selector feed the same single-file path, so pick whichever input is faster for you.
Your last slider value persists in localStorage under img-compress-quality, with a default of 80. Come back tomorrow in the same browser and the tool remembers your setting. Pressing clear resets the slider to 80.
The Verified Numbers Behind Each Setting
- The slider range is 10 to 100. The default is 80.
- The tool's own FAQ bands are 70 to 80 for web use, 60 to 70 for email attachments, and 80 to 90 for social media posts.
- Files under 10 MB are the recommended input size.
- The percent smaller figure is computed from real output bytes as Math.round of 1 minus compressed over original, so it is a whole number.
- Input is limited to JPEG and PNG. The SEO copy for the page mentions WebP, but the component rejects it with the format alert. The code is the truth here.
Quality 100 deserves a specific warning. It sends an encoder quality of 1.0 through the same pipeline with the same 4096 cap. It is a maximum quality re-encode, not a handoff of your original bytes.
Deterministic Steps to Pick Your Quality
1. Drop one JPEG or PNG file. Anything else triggers the format alert.
2. Read the Original size printed next to the file name.
3. Start at the default 80. Note the Compressed size and the percent smaller line.
4. Move the slider down in steps of 10. Stop when one 10 point step removes less than 5 percent of the original file size. Further drops cost visible detail for marginal bytes.
5. Judge the result outside the tool. The built-in preview is a small square thumbnail with object-cover, which hides softness and cropping artifacts. Open the downloaded file at full size before you ship it.
6. Download. The file name gains a compressed_ prefix, so keep both files until you have compared them.
Condition rules for special cases. If the output is an email attachment and 80 percent still misses your size limit, move into the 60 to 70 band. If the longest side of your source exceeds 4096 pixels, expect downscaling and check the output dimensions. If the pixels must stay identical, skip lossy compression entirely.
Where the Tool Works Against You
- The 4096 cap applies silently. There is no warning, and the file name does not signal a resize.
- Small PNG files can come out larger than the source, and the percent smaller line then shows a negative number. The label reads oddly, but the arithmetic is honest.
- One file at a time. The component holds a single originalFile state, so batches mean repeating the drop and download cycle.
- Settings persist per browser, not per account. Switch machines or clear site data and the 80 default returns.
- The tool has no side-by-side image comparison. You see numbers and a small thumbnail, so the quality verdict happens on your machine after download.
Checklist
- [ ] You confirmed the input is JPEG or PNG before dropping it.
- [ ] You noted the original size and the percent smaller at your chosen quality.
- [ ] You checked the output dimensions when the source exceeded 4096 pixels on a side.
- [ ] You opened the downloaded file at full size before publishing it.
- [ ] You kept the original file until the compressed copy passed your check.
Run the same file at 80, at 70, and at 60 and write down the three output sizes. Those three numbers are your own quality curve and they beat any generic recommendation, including ours. Test it on the [image compressor](/en/image-compressor) page and send us your curve if it disagrees with the bands above.
