How to batch compress images and see what each file saved
· 6 min read
To batch compress images, drop the folder into a bulk image compressor, pick a quality level (Light, Balanced or Strong) and an output format (JPG or WebP), and run it once. Each file is re-encoded in your browser and the tool shows the size saved per file, so you can see exactly what you got before downloading the zip.
TL;DR: compression here means re-encoding at lower quality, nothing else. Light is quality 0.8, Balanced 0.6 and Strong 0.4 on the browser's 0 to 1 scale. The pixel dimensions stay the same, so resize first if you need big savings. A file can come out larger than it went in, and the tool shows that rather than hiding it.
What does batch compression do to each image?
The browser draws each image onto a canvas and exports it again as JPG or WebP at the quality you chose. Lossy formats store less detail at lower quality, which is where the savings come from. The width and height are untouched. A 4000 pixel photo compressed at Strong is still 4000 pixels wide, just stored more aggressively, which is why shrinking the dimensions first usually saves more than any quality setting.
How to compress images without losing quality: why resizing before compressing beats a lower quality number.
Which compression level should you pick?
| Level | Quality value | Good for |
|---|---|---|
| Light (best quality) | 0.8 | Photos you want to keep looking right, with a modest saving |
| Balanced | 0.6 | Everyday web and email images, the default |
| Strong (smallest files) | 0.4 | Thumbnails, backgrounds and anything where size beats detail |
These are three fixed intentions rather than a slider. Run a few representative files first, because how much a given image shrinks depends on its content: detailed photographs and flat graphics respond very differently.
Why did a PNG come out bigger after compressing?
PNG is lossless and the browser ignores the quality setting when it exports a PNG, so there is no PNG compression level to turn down. Compressing a PNG here means converting it to JPG or WebP, and the output format is what makes the file smaller. For a small graphic with flat colours, a lossy JPG can occasionally be larger than the PNG it replaced. The tool shows the change per file, with a plus sign when a file grew, so you can discard those results and keep the original.
What happens to transparency?
JPG cannot store transparency, so transparent areas are filled with white before encoding. If you need to keep transparency, choose WebP as the output. Every output is a new file, so the originals on your disk are never changed.
What is the checklist for compressing a folder?
- Resize to the size you will actually display before compressing.
- Keep the originals and treat the compressed copies as disposable.
- Try Balanced first, and move to Light if you can see the difference.
- Choose WebP when you need transparency or the smallest files, and JPG when the destination might not open WebP.
- Check the per-file savings and keep the original wherever a file grew.
- Never compress an already compressed JPG repeatedly, because the loss accumulates.
- More than one file downloads as a single zip named astral_compressed_images.zip.
Re-encoding through a canvas writes a new file made of the pixels only. As a side effect, camera metadata such as EXIF is not carried over, which also removes embedded location data. Do not rely on that as a privacy guarantee for a file you are about to publish; open one result and check it.
- MDN: HTMLCanvasElement.toBlob(), how the browser encodes an image and what the quality argument does
- Google for Developers: WebP, an image format for the web
Batch compress images in your browser: Light, Balanced or Strong, JPG or WebP, with the saving shown per file.
How to batch resize images to exact sizes: set the size first, then compress.
Frequently asked questions
Can I compress images in bulk without uploading them?
Yes. AstralBatch re-encodes each image in your browser with the Canvas API, so the files stay on your device and nothing is sent to a server.
How much smaller will my images get?
It depends on the image and the level, so there is no honest single number. The tool shows the exact saving for each file after it runs, which is the figure to trust.
Which formats can I compress to?
JPG or WebP. You can drop in PNG, JPG or WebP files, and the compressed copies are written as JPG or WebP.
Does compressing change the width and height of my images?
No. Only the stored quality changes. To reduce the dimensions as well, resize the images first with the batch resizer.
Why is my compressed file larger than the original?
Re-encoding a small or already well-compressed file, especially a flat-colour PNG, can produce a bigger JPG or WebP. The tool marks files that grew, and in that case you should keep the original.