How to compress an image without losing visible quality

2 min read

Most “reduce your image size” advice is about making a number smaller. The useful question is which number, and at what point a person can see the difference. Three levers decide the final size, and they are not equally powerful.

The compressor re-encodes the picture in your browser and shows the original and compressed size side by side, so you can raise the quality until the file is as large as you are willing to accept, or lower it until the picture starts to show.

Open tool →Compress imageRe-encode a JPEG, PNG or WebP at a quality you choose, with a before/after size comparison.

Three levers, in order of effect

1. Dimensions

Halving the width and height removes three quarters of the pixels and is by far the biggest saving available. A 4000-pixel-wide photo displayed at 800 pixels is carrying five times the detail anyone will see. Resize first; compress afterwards.

2. Format

For photographs, JPEG and WebP beat PNG by a wide margin at the same visual quality. PNG is for images with flat colour, transparency or sharp text, not for camera output. If you are compressing a PNG photograph, converting it is a bigger win than any quality slider.

3. Quality

The quality setting controls how aggressively the encoder throws away detail it thinks you will not notice. Around 0.75–0.85 is the usual sweet spot for photographs: the file shrinks dramatically and the difference is hard to see at normal viewing size.

The steps

Compress an image locally

  1. Open the fileDrop the image on the compress tool. It is read by your browser, not uploaded.
  2. Set a maximum width if you have oneFor anything bound for a web page, 1600 px on the long edge is generous.
  3. Move the quality sliderStart at 0.8. The size readout updates so you can see what each step costs.
  4. Compare, do not guessLook at the preview at 100%, then zoom out. Artefacts appear first in smooth gradients and fine texture.
  5. DownloadSave the result and check its file size in your file manager.

What compression cannot fix

  • A blurry photo stays blurry. Compression removes detail; it never adds any.
  • Text in a screenshot suffers quickly. If the image is mostly text, resize less, keep quality high, or convert to PNG.
  • A file that is large because it is genuinely detailed — a map, a texture, a star field — will show artefacts earlier than a portrait will.

Compressing the same file twice does not compound much: the second pass mostly re-encodes what the first one already smoothed. Compress once, from the original.

Tools used in this guide

Questions about this guide

What quality setting should I use?
0.8 for photographs is a safe default. Go lower (0.6–0.7) for thumbnails and previews where nobody inspects pixels, and higher (0.9+) only when the image will be printed or zoomed.
Does compressing remove metadata?
Re-encoding through a canvas drops most metadata, including EXIF. That is usually what you want for a photo you are about to publish, since EXIF can contain GPS coordinates.
Why did my PNG get bigger?
Re-encoding a PNG at low quality does not always help, because the format is lossless. Convert a photographic PNG to JPEG or WebP instead.
Is there a file size limit?
Not from the tool. Nothing is uploaded, so the only limit is your device’s memory. Very large images on a phone may be slow.

All guides

Written and maintained by the FreeOnlineBox project. Every tool named in this guide runs on the reader’s own device; no file described here is uploaded to check it.