How to batch convert image files without uploading them
· 6 min read
To batch convert images, drop the whole folder into a batch image converter, pick the target format (PNG, JPG or WebP) and run it once: every file is converted in the same pass and downloaded together as a zip. With AstralBatch the conversion happens inside your browser tab, so nothing is uploaded to a server.
TL;DR: a batch converter applies one format change to every file in a folder. Use WebP for your own website, JPG for maximum compatibility and PNG when you need transparency. Keep the originals untouched, convert one test file first, and avoid converting lossy files back and forth.
What is a batch image converter?
A batch image converter re-encodes many image files into a different format in a single run, instead of opening each one in an editor and exporting it by hand. The pixels are decoded from the source file and written out again using the rules of the target format: lossy compression for JPG, lossless for PNG, and either for WebP. The picture is meant to stay the same. What changes is the container, the file size and, for lossy targets, a small amount of detail.
Do you need to upload your images to convert them?
No. Browsers can decode and re-encode common image formats on their own through the Canvas API, so a converter can read each file from your disk, draw it to an in-memory canvas and export it in the new format without sending a byte anywhere. Whether a converter uploads your files is a design decision, not a technical requirement. AstralBatch does the whole job locally and works on several files in parallel, which is why a large folder finishes quickly.
Which format should you convert a batch to?
| Your situation | Convert to | Why |
|---|---|---|
| Publishing photos on your own website | WebP | Smaller files than JPG at similar quality, and every current browser opens it |
| Sending files to a system you do not control | JPG | The format almost every editor, printer and upload form accepts |
| Logos, screenshots or graphics with transparency | PNG | Lossless and keeps transparent areas |
| Archiving a master copy | Do not convert | Keep the original and convert copies of it |
WebP, JPEG or PNG: which image format should you use?: the longer comparison of what each format is good at.
Does batch converting lose quality?
It depends on the two formats involved, not on the fact that it is a batch. PNG is lossless, so converting to it adds no loss, but it cannot bring back detail an earlier JPG save already discarded. JPG and lossy WebP discard a little detail every time a file is encoded into them. In AstralBatch the lossy formats are written at a fixed quality of 0.9 on the browser's 0 to 1 scale. Running the same file through a lossy round trip, such as JPG to WebP and back to JPG, is what visibly degrades an image.
What happens to transparency and file names?
JPG has no transparency, so a transparent PNG converted to JPG needs a background. AstralBatch fills it with white, so transparent areas come out white instead of black. If you need transparency to survive, convert to PNG or WebP. Each output keeps the original file name with the new extension. One file downloads directly, several download as a single zip named astral_converted_images.zip.
What is the safe checklist before converting hundreds of files?
- Keep the originals in a separate folder and treat every converted copy as disposable.
- Convert one or two files first and open the results to check the format and the look.
- Check for transparent backgrounds before choosing JPG.
- Do not convert a JPG to another lossy format just to change the extension, because it loses detail twice.
- If the destination software cannot open WebP, convert to JPG or PNG instead.
- Compress after converting if size is the goal, not before.
- 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 convert images in your browser: PNG, JPG and WebP, no account, works on the whole folder at once.
How to compress images without the result looking worse: once the format is right, size is the next lever.
Frequently asked questions
Is it safe to batch convert confidential images online?
It depends on whether the tool uploads them. A converter that runs in your browser using the Canvas API, as AstralBatch does, never sends the files anywhere, so a confidential batch stays on your device.
What is the best format to batch convert to for a website?
WebP in most cases. It supports both lossy and lossless compression and usually gives smaller files than JPG or PNG at comparable quality. Keep JPG as the fallback if the images also go to software that cannot open WebP.
Which formats can AstralBatch convert?
You can drop in PNG, JPG and WebP files and convert them to PNG, JPG or WebP. The tool converts through the browser, so it can only read formats your browser can decode.
Does converting the same file twice make it worse?
Only when a lossy format is involved. Converting a JPG to WebP and back to JPG discards detail twice. Converting to PNG adds no further loss, though it cannot recover what an earlier JPG save already discarded.
Why did my transparent PNG get a white background as a JPG?
JPG cannot store transparency. AstralBatch fills transparent pixels with white before encoding so they do not turn black. Choose PNG or WebP as the target if you want to keep the transparent area.