How can I estimate the size of my gzipped script?

How can I estimate the size of my JavaScript file after it is gzipped? Are there online tools for this? Or is it similar to using winzip for example?

Solution

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.