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What's the Difference Between Lossless and Lossy Compression?

Lossless keeps every bit of the original; lossy discards imperceptible detail to save space. Here's when to use each, with real file-size numbers.

Zro7 compresses images, audio, and video entirely in your browser with WASM codecs. Files stay on your device.

Lossless compression rewrites data in a smaller form without discarding anything — decompress and you get the exact original bytes. Lossy compression throws away perceptually unimportant detail to shrink far more aggressively. Use lossless for archives, code, and text; lossy for photos, audio, and video meant for playback.

How each works

  • Lossless — Deflate, Brotli, Zstd, PNG, FLAC, ZIP. Finds statistical redundancy (repeated bytes, common patterns) and codes them shorter.
  • Lossy — JPEG, WebP, H.264, MP3, Opus. Transforms data into a domain where human perception is dull (DCT for images, psychoacoustic model for audio), quantizes there, then losslessly encodes what remains.

Real numbers on the same photo

  • Original 24-MP JPEG from camera: 8.2 MB.
  • PNG (lossless re-encode): 41 MB.
  • PNG optimized (OxiPNG): 32 MB.
  • WebP lossless: 12 MB.
  • JPEG q90 (visually indistinguishable): 4.1 MB.
  • WebP q80: 1.8 MB.
  • AVIF q60: 0.9 MB.

When to pick which

  • Text, code, logs, CSV → lossless (always; lossy would destroy data).
  • Screenshots / diagrams → lossless PNG / WebP-lossless.
  • Photos for the web → lossy WebP or AVIF.
  • Master recordings / medical scans → lossless (FLAC, DICOM lossless).
  • Music / podcast / video for playback → lossy (Opus, AAC, H.265).

Reversibility

Once you save lossy, the discarded detail is gone. Always keep a lossless master somewhere; export lossy copies for distribution.

Steps to try it

  1. Open Zro7 Compress Image.
  2. Drop a photo.
  3. Toggle Lossless (PNG/WebP-lossless) vs Lossy (JPEG/WebP/AVIF).
  4. Compare output sizes for the same quality target.

Updated December 25, 2026 · Zro7 editorial team.

Frequently asked questions

Is ZIP lossy or lossless?

Lossless — you get every byte back.

Can lossy compression be undone?

No. The discarded information is gone. Keep a master.

Which is smaller for photos?

Lossy — often 5–20× smaller than lossless at visually indistinguishable quality.

Does lossless mean no visible difference?

It means no difference at all, byte-for-byte. Lossy at high quality is also visually identical to most viewers.

Is FLAC really as good as WAV?

Yes — FLAC is lossless. WAV → FLAC → WAV yields the original WAV bit-for-bit.

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