GZIP / DEFLATE / Brotli Size Estimator

What actually happens during compression

Strip away the branding and every format here is the same two-stage recipe. Stage one, LZ77, slides a window across your bytes and replaces repeats with a tiny pointer: "go back N and copy M." Stage two, Huffman coding, looks at what is left and gives the frequent symbols short codes and the rare ones long ones. GZIP is just that stream plus a header and a checksum; Brotli is that stream plus a giant built-in dictionary of web phrases. Understanding those two steps is enough to predict why your JSON shrinks and your ZIP does not.

The formal specs are worth a look if you want the bit-level detail: RFC 1951 defines DEFLATE, RFC 1952 defines GZIP, and the pako project is a readable JavaScript take on the same zlib logic. If you also package files, our zip creator uses DEFLATE under the hood too.

Picking a level without the guesswork

The slider is the whole game. GZIP level 6 and Brotli level 4 or 5 are the sensible defaults for anything served on the fly, because they compress fast enough to sit in the request path without hurting latency. Brotli level 11 is the trophy hunter: best ratio, brutal CPU cost, and only worth it when you compress once during a build and serve the result forever. The duration column in this tool makes that cost visible on your own machine instead of someone else's benchmark. When privacy is the reason you are here, the client side processing guide explains why keeping bytes on the device matters under GDPR Article 28.

If your work spans more than compression, the same local-first idea shows up across the site. Our PDF compressor shrinks documents without uploading them, and the file checksum tool lets you verify integrity after a transfer, all on device.

Build a private compression workflow

An estimator is one step in a larger routine. You might measure your bundle here, pre-compress static assets with Brotli at build time, serve GZIP as a fallback, then verify the delivered file with a checksum to catch corruption. Each step can run locally, so regulated or confidential payloads never leave your control. The facts behind this page draw on public references: RFC 1951 (DEFLATE), RFC 1952 (GZIP), the pako zlib port, and Google's Brotli compression research summarized in the Gzip vs Brotli writeup. For the privacy angle, the GDPR Article 28 text is the canonical source.