Two kinds of compression
Lossless compression (PNG, lossless WebP) finds statistical redundancy in pixels — runs of identical colors, repeated patterns — and rewrites them more compactly. Decoding reproduces every original pixel. Great for screenshots, logos, and anything with sharp edges or text.
Lossy compression (JPG, WebP, AVIF, HEIC) first discards information your eyes are bad at noticing, then compresses what remains. Color detail gets downsampled, high-frequency texture gets smoothed, block boundaries get predicted away. The quality slider (0–100) controls how aggressively this happens.
Why "quality 70" means different things for different formats
The number is not standardized across encoders. JPEG quality 80 typically produces a file ~40% smaller than quality 95 with differences invisible at normal viewing size. WebP at the same "80" setting usually produces files 25–35% smaller than JPEG at equivalent visual quality. AVIF goes further still, at the cost of slower encoding.
| Format | Best for | Typical savings vs JPG q85 | Transparency |
|---|---|---|---|
| JPG | Photos, universal compat | baseline | No |
| WebP | Web images, photos | 25–35% smaller | Yes |
| AVIF | Modern web, thumbnails | 40–50% smaller | Yes |
| PNG | Screenshots, graphics | often larger (lossless) | Yes |
How target-size compression works
Forms, marketplaces and government portals often demand "under 2 MB" or "under 200 KB". A quality slider can't promise a byte count, so a target-size tool works differently — and this is exactly what the Bulk Image Compressor does:
- Encode the image at a high quality and measure the resulting byte size.
- If it fits the target — done. If not, binary-search the quality setting: encode at the midpoint, measure, adjust up or down, repeat ~7–8 times.
- If even quality ~10 can't fit (rare, tiny targets), progressively downscale resolution and re-run the search — a smaller image at decent quality always beats a full-size image crushed to mush.
- Return the largest file that fits under your limit.
All of this happens on your CPU, so a 20-image batch takes a few seconds and touches no network.
Practical recipe book
Email attachments (limit ~25 MB, be kind: aim 5 MB)
Compress to 2 MB for photos you want to stay printable, Compress to 500 KB for "just look at this" sharing. WebP output keeps quality highest per byte.
Job applications & government forms (often 100–500 KB)
Use the 200 KB tool with JPG output — forms validators sometimes reject WebP. Check the required pixel dimensions too and pre-resize with the Bulk Image Resizer.
Website speed
Convert photos to WebP at quality 75–85. For hero images, AVIF with a WebP fallback. Photos above 1920 px wide are usually wasted — resize first, then compress.
WhatsApp/document scans
Scans as PNG get huge. Convert to JPG at 200 KB target — text stays crisp and the file drops 90%.
Common mistakes
- Re-compressing repeatedly. Every JPG save loses a little more. Keep an original, compress copies.
- Using PNG for photos. PNG's lossless model is wrong for sensor noise — files balloon. PNG is for graphics, screenshots, and anything with fewer than a few thousand colors.
- Ignoring resolution. A 6000×4000 photo shown at 600 px wide wastes 99% of its bytes. Resize to the display size first — that alone often cuts 80%.