Gzip Compression Size Estimator

How to use

1. 1Pick a mode: Paste text to measure a JSON, HTML, CSS, or JavaScript snippet, or Choose file to drop a real artifact from your bundle.
2. 2Paste the content into the text box, or drag a file onto the drop zone (the file stays in your browser).
3. 3Read the results table. Each row shows compressed bytes, the bytes saved versus the original, and the ratio bar.
4. 4Use Load sample to try a small JSON payload, or Clear to reset.
5. 5Scroll to the budget panel to see how the gzip result compares to the 14 KB, 100 KB, 170 KB, and 1 MB HTTP transfer budgets.
6. 6Click Copy summary to drop a plain-text report into a ticket, code review, or doc.

About this tool

Gzip Compression Size Estimator measures the on-the-wire transfer size of any text or file after the browser compresses it with gzip, deflate (zlib), and raw deflate. Pick a tab to paste text or drop a file, and the tool reports the original byte length, the compressed byte length, the savings in bytes, the savings as a percentage, and the compression ratio for all three HTTP-relevant formats in one table. A separate budget panel compares the gzip result against four well-known web performance thresholds: 14 KB for the first TCP congestion window (critical HTML in this budget arrives in one round trip), 100 KB for above-the-fold HTML and critical CSS guidance tied to Largest Contentful Paint, 170 KB for Chrome DevTools' yellow warning on a single compressed JavaScript bundle, and 1 MB for general page weight. Each budget shows a coloured bar so it is obvious at a glance whether the payload is within budget, with the exact headroom or overage printed in bytes. The math: original bytes are counted as the UTF-8 encoding of pasted text or the on-disk byte count of a dropped file, and compressed bytes are produced by piping the input through the browser-native CompressionStream API at the default quality setting. CompressionStream ships in Chrome 80 and later, Firefox 113 and later, and Safari 16.4 and later, so the page works on every current modern browser without loading any zlib JavaScript port. Compression is streaming, so a 25 MB file finishes in a few seconds without blocking the UI. Useful for front-end developers asking will my minified bundle fit Chrome's 170 KB hint, will this HTML survive the 14 KB first-packet budget, how much smaller is this JSON over the wire, or whether gzip beats raw deflate for a tiny payload where the gzip header overhead can erase the savings. Useful for SREs and performance engineers shipping API responses, for technical writers comparing markdown payload sizes, and for anyone curious how much weight content-encoding actually removes. Everything runs locally; the text or file you measure never leaves your device.

Free to use. Works in your browser. No signup, no login.