How JPG Compression Works
JPG (JPEG) uses lossy compression based on Discrete Cosine Transform (DCT). It divides your image into 8×8 pixel blocks and removes high-frequency visual detail that the human eye can't easily perceive. This allows dramatic file size reductions — typically 60-80% — while keeping the image visually identical.
JPG Compression Quality Guide
| Quality Setting | File Size Reduction | Visual Impact | Best For |
|---|---|---|---|
| 90-100% | 30-50% | Imperceptible | Print, professional photography |
| 75-85% | 60-75% | Indistinguishable at 1x zoom | Web images, social media |
| 60-74% | 75-85% | Minor artifacts at 2x zoom | Thumbnails, email attachments |
| Below 60% | 85%+ | Visible block artifacts | Previews only |
Recommendation: Quality 80% is the sweet spot for most web use. It delivers 65-70% file size reduction with no visible quality difference at normal viewing distance.
When to Use JPG
- Photographs — complex scenes with gradients and many colors
- Web banners — hero images, product photos, blog post images
- Social media — profile pictures, cover photos, posts
- Email attachments — when file size limits matter
When NOT to Use JPG
- Images with text — JPG creates block artifacts around sharp edges. Use PNG instead.
- Screenshots — flat colors and sharp lines degrade with JPG compression.
- Transparency needed — JPG doesn't support alpha channels. Use PNG or WebP.
- Images that will be edited repeatedly — each JPG re-save loses quality (generational loss).