What Is Lossless Data Compression?

By Lucian — builder & engineer, LK Forge

Lossless compression makes a file smaller and then gives every original byte back — decompress it and you have an exact copy, not an approximation. That is the whole promise, and it is testable: compress, decompress, and check the result matches the source bit-for-bit. We ran that test on a standard set of files with five real compressors, and measured how small each one goes. Below is what lossless actually means, a live demo you can run on your own text, and the numbers.

Try it: lossless in your browser

Type or paste anything below. Your browser compresses it with gzip (the DEFLATE algorithm, via the built-in CompressionStream API), then immediately decompresses it and checks the result against what you typed. Nothing leaves your device. Watch the size shrink on repetitive text — and notice the output always comes back identical.

How small can lossless go?

There is a naive limit called the order-0 entropy — the best you can do if you only look at how often each byte appears, ignoring order. For this corpus that floor is 45% of the original. Real compressors blow straight through it by modeling context and repetition: gzip reaches 26%, and modern codecs (xz, brotli) reach about 17.5%. Bigger bar = smaller file.

Compressibility depends on the data

The same codec behaves completely differently on different files. The standout is kennedy.xls: a spreadsheet full of structure and repeated zeros, it drops to just 5% of its size with xz. A fax image (ptt5) compresses almost as hard. English prose sits around 30%, and the already-dense SPARC binary (sum) resists the most. Each row is one file; every number below survived a byte-for-byte round-trip check.

Each cell is the compressed size as a percent of the original (smaller is better). Already-compressed data — a JPEG, an MP3, a ZIP — has little redundancy left, so lossless tools barely shrink it; that is expected, not a failure.

When to use lossless vs lossy

Use lossless whenever every byte matters: text, source code, spreadsheets, logs, executables, and archives — anything where a changed bit is a bug. Use lossy when the data is headed for human eyes or ears and a small, well-chosen loss is invisible: photos, music, streaming video. The reason lossy wins so much more size is exactly that it is allowed to discard information — see how that trade plays out for photos in image compression: quality vs file size, and for documents in how PDF compression works.

Want to try it on real files? Our Image Compressor and PDF Compressor run entirely in your browser — nothing is uploaded.

Reproduce this

No cherry-picking — every figure is the whole-corpus total on a standard public file set, using each tool's stock maximum setting. Run it yourself:

# one file, every codec, at max — sizes in bytes
for f in corpus/*; do
  o=$(wc -c < "$f")
  g=$(gzip  -9  -c "$f" | wc -c)
  b=$(bzip2 -9  -c "$f" | wc -c)
  x=$(xz    -9e -c "$f" | wc -c)
  z=$(zstd  -19 -c "$f" | wc -c)
  r=$(brotli -q 11 -c "$f" | wc -c)
  echo "$f  orig=$o gz=$g bz=$b xz=$x zstd=$z br=$r"
done