# The image quality slider is not a file-size percent

The slider looks like a file-size control. It is not. It sets JPEG quality. A photo can drop to a fraction of its size at 0.8. A screenshot of flat colors may barely change. Read the before-and-after line. Do not trust the slider number alone.

## What the canvas does

One image is loaded. The canvas is set to the image's original width and height. The picture is drawn at those pixels and exported as a JPEG. The tool does not resize. If you need fewer pixels, use the image resizer, then compress if the file is still large.

The slider runs from 0.1 to 1 in steps of 0.05. The default is 0.8. That 0.8 is the JPEG encoder's quality argument. It is not 80 percent of the original kilobytes. The same number means a different size change on a photo, a screenshot, and a flat logo.

The download is always named compressed.jpg, because the canvas export is a JPEG. Rename it after download if you need the original file name. PNG, WebP, and GIF inputs all come out as JPEG.

## Transparency, GIF, and HEIC

JPEG has no transparency. The canvas is not filled with white first, so transparent pixels in a PNG are often black in the result. If the logo has to keep a transparent background, do not use this tool. Convert to a format that supports alpha, or flatten onto a white background in an editor first.

A GIF becomes a still. The browser draws the image it can decode, which for an animation is typically one frame, and that frame is what the JPEG contains. The animation is gone.

HEIC may not preview in the browser's image decoder. Use HEIC to JPG for iPhone photos. This compressor is for files the browser can already display as an image.

## When the new file is larger

The tool does not refuse a larger result. An already-small JPEG, or a flat PNG that becomes a heavy photo-style JPEG, can grow. The line under the button shows both sizes. If the new file is bigger, keep the original.

Raise the slider only when you can see damage you will not accept. Lower it when you are sending a photo by email or chat and the exact pixels matter less than the weight. For a scan that is already a PDF, this page is the wrong tool. Use Compress PDF, and expect the text-layer tradeoff described in that guide.

## Three files, three different outcomes

A 4 MB phone photo at the default 0.8 often drops sharply, because photographic detail is what JPEG quality throws away first. The pixel dimensions stay the same. You are not making a thumbnail. If the photo is still too wide for a form that wants 2000 pixels, resize it first.

A screenshot of a spreadsheet, mostly white and black, may shrink only a little at 0.8 and start to show blocks around the text if you push the slider toward 0.1. For text you still need to read, stay at 0.8 or higher and accept the size. This tool will not keep the PNG you started with.

A logo PNG with a transparent background is the file to refuse. The JPEG comes back with a black or dark rectangle where the transparency was, and the name compressed.jpg does not warn you. Flatten the logo onto the background color you actually want, in an editor, before you compress. Or leave it as a PNG.

## Questions

### Does 0.8 mean 80 percent of the file size?

No. 0.8 is JPEG quality. Compare the before-and-after sizes on the page.

### Why is the download always compressed.jpg?

The export is a JPEG, so the name is fixed. Rename it after download.

### Why did my transparent PNG turn black?

JPEG cannot store transparency, and the canvas is not prefilled with white. Transparent pixels usually become black.

Tool: https://pctoolsonline.com/tools/image-compressor.html