Lossy vs lossless compression.
Every image compression method falls into one of two categories: lossy or lossless. Choosing the right one determines your file size, image quality, and whether you can edit the image later. Here is everything you need to know.
Quick comparison
| Feature | Lossy | Lossless |
|---|---|---|
| Definition | Discards data permanently to reduce file size | Reorganises data efficiently, no data lost |
| File size | Very small (80-90% reduction) | Moderate (10-40% reduction) |
| Quality preservation | Perceptually similar, theoretically lower | Identical to original, pixel-perfect |
| Reversible | No — discarded data is gone forever | Yes — original can be perfectly restored |
| Best for | Photos, web images, social media | Logos, screenshots, text, archival |
| Common formats | JPEG, WebP (lossy), AVIF (lossy) | PNG, GIF, WebP (lossless), AVIF (lossless) |
| Use case examples | Product photos, blog images, thumbnails | Icons, diagrams, medical images, printing |
| Encoding speed | Slower (complex analysis required) | Faster (simpler algorithm) |
| Typical size savings | 500 KB → 50 KB (90% smaller) | 500 KB → 400 KB (20% smaller) |
What is lossy compression?
Lossy compression reduces file size by analysing the image and permanently discarding information that the human eye is less sensitive to — fine colour variations, high-frequency detail in shadows, and subtle texture gradients. The algorithm prioritises preserving the overall visual impression while eliminating redundant or imperceptible data. This is why a 5 MB raw photo can become a 300 KB JPEG with no visible difference at normal viewing size.
The most common lossy format is JPEG, which uses block-based DCT (Discrete Cosine Transform) compression. Modern alternatives like WebP and AVIF achieve even better compression ratios using more advanced prediction and transform techniques. Use our JPEG compressor to apply lossy compression with precise quality control, or the bulk compressor for batch processing.
What is lossless compression?
Lossless compression reduces file size by finding and eliminating redundancy in the data without discarding any pixels. Instead of throwing away information, it stores the same data more efficiently — similar to how ZIP compresses a text file. Every pixel is preserved exactly, so the decompressed image is byte-for-byte identical to the original. This makes lossless compression ideal for images that will be edited, archived, or printed.
PNG is the most widely used lossless format, combining deflate compression with per-row pixel filtering. GIF is another lossless format limited to 256 colours. WebP and AVIF also offer lossless modes that often outperform PNG. Our PNG compressor optimises lossless PNGs by selecting the best filter strategy for each row, typically saving 10-40% with no quality loss whatsoever.
Lossy vs Lossless: file size comparison
A 12-megapixel photograph (4000×3000px): Lossy JPEG at quality 80: 300-600 KB. Lossless PNG: 10-20 MB. Lossy WebP at quality 80: 200-450 KB. Difference: Lossy is 20-50× smaller with no perceptible quality difference at screen resolution.
A 1920×1080 screenshot of a text-heavy page: Lossy JPEG at quality 80: 150-400 KB (visible artefacts around text characters). Lossless PNG: 400-1200 KB (crisp text edges). Lossless WebP: 300-900 KB. Verdict: Lossy saves space but damages text; lossless preserves clarity at a moderate size cost.
A 1024×1024 flat-design logo with transparency: Lossless PNG-32: 8-30 KB (perfect quality, transparent background). Lossy JPEG: Not suitable (no transparency, blurry edges). Lossy WebP: 5-20 KB (supports transparency but quality loss may show on sharp edges). Winner: Lossless PNG remains the gold standard for logos, icons, and UI assets with transparency.
Which should you choose?
Use lossy compression for photographs, product images, social media graphics, and any visual content where small file size matters more than pixel-perfect accuracy. Use lossless compression for logos, screenshots, illustrations, medical images, diagrams, and any file that will be further edited or printed. When in doubt, check whether your image format supports both modes — WebP and AVIF let you switch between lossy and lossless in the same encoder. For batch workflows, the bulk image compressor handles both types automatically.
Frequently asked questions
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What is the difference between lossy and lossless compression?
Lossy compression reduces file size by permanently discarding some image data that the human eye is unlikely to notice, while lossless compression reduces file size by reorganising data more efficiently without discarding any pixels. Lossy achieves much smaller files but the original cannot be perfectly reconstructed. Lossless preserves every pixel exactly — the decompressed image is identical to the original.
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Which compression type produces smaller files?
Lossy compression produces significantly smaller files — typically 80-90% smaller than the original. Lossless compression achieves more modest savings, usually 10-40% depending on image complexity. For a 5 MB photo, lossy JPEG might produce a 300 KB file, while lossless PNG might produce a 15 MB file. The trade-off is quality vs size.
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Is lossy compression bad for image quality?
Not necessarily. At moderate quality settings (quality 75-85), the visual difference between a lossy and lossless image is imperceptible at normal viewing sizes. Lossy artefacts only become visible at very low quality settings or when images are heavily enlarged. Modern encoders like mozJPEG and WebP are extremely efficient at preserving perceived quality while discarding redundant data.
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When should I use lossless compression?
Use lossless compression for images where pixel-perfect accuracy matters — logos, screenshots, line art, medical images, technical diagrams, and text-heavy graphics. Any image that will undergo further editing should also stay lossless to avoid quality degradation from re-compression. Lossless is also preferred for archival and printing where every pixel counts.
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Can I convert a lossy image back to lossless?
No — once data is discarded by lossy compression, it is permanently gone. Saving a lossy JPEG as a lossless PNG preserves the degraded pixels but cannot restore the original quality. Always keep an original lossless master copy if you plan to edit or re-export images later. This is why professional workflows use lossless formats like TIFF or PNG for source files.
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Which image formats use lossy vs lossless compression?
Lossy formats include JPEG, WebP (lossy mode), AVIF (lossy mode), and HEIF. Lossless formats include PNG, GIF, BMP, WebP (lossless mode), and AVIF (lossless mode). Some formats like WebP and AVIF support both modes — you choose lossy or lossless at encoding time. Use our JPEG compressor for lossy and PNG compressor for lossless.
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