http://closure-compiler.appspot.com/home lets you paste in code, and it will give you compression ratios for a particular file before and after GZIP.

Original Size:    90 bytes (100 bytes gzipped)

Compiled Size:    55 bytes (68 bytes gzipped)

Saved 38.89% off the original size (32.00% off the gzipped size)

You can use the pretty-print and white-space only options to estimate the compression of non-minified content.

If you need an estimate:

• Start with 100 JS files that have gone through the same minification pipeline.


• For each file, compute the ratio in sizes between gzip -c $f | wc -c and wc -c $f


• The average of those ratios is an approximation of the compression you should expect for a similar JS file.

Cygwin contains command line implementations of gzip and wc for Windows.