Bulk image compressor

Files are distributed across a pool of Web Workers — one per available processor core, up to four — so several images are encoded genuinely in parallel rather than one after another. The interface stays responsive throughout, and the progress indicator counts real completed files rather than animating on a timer.

Because everything happens on your machine, throughput is limited by your processor rather than by an upload queue or a rate limit. A hundred photographs on a modern laptop takes a couple of minutes; the same set on a phone takes longer, because the encoder is doing the same work with less silicon.

There is no cap on the number of files. The practical ceiling is the memory available to a browser tab, because each image has to be held uncompressed while it is processed — a 12-megapixel photograph occupies roughly 48 MB as raw pixels regardless of how small the file on disk is.

In practice that means very large batches of very large images can exhaust a tab, particularly on mobile. If a batch stalls, split it in half. Nothing is lost when you do, because nothing was uploaded in the first place.

A single quality value across a varied set is a blunt instrument: the portrait and the foliage shot will not land at the same file size, because detailed images legitimately need more bytes.

If consistency of size matters more than consistency of quality — for a catalogue, say — use target-size mode instead. Every file is then searched independently to the same limit, and the sizes come out even at the cost of slightly uneven quality. Which of those you want depends entirely on the job.

The quality number in a JPEG or WebP encoder is not a percentage of anything. It is an index into a set of quantisation tables that control how aggressively detail is discarded. The relationship to file size is steep and non-linear.

For a typical 12-megapixel phone photograph the curve looks roughly like this:

• Quality 95–100 — near-original appearance, but the file is often only 20–35% smaller. Rarely worth it for the web.
• Quality 80–90 — the sweet spot for photography you care about. Typically 50–70% smaller with no artefact you would notice at normal viewing distance.
• Quality 70–80 — the sweet spot for ordinary web images. Usually 70–85% smaller. Artefacts appear only if you zoom in on flat sky or skin.
• Quality 50–70 — visible softening and blocking in smooth areas, though thumbnails and background images survive it well.
• Below 50 — obvious blocking, ringing and colour banding. Useful only for placeholders.

Diminishing returns set in fast at the top of the range. Going from quality 100 to quality 90 usually halves the file for no perceptible change; going from 90 to 80 saves far less and costs slightly more. If you only remember one number, use 80.