How can I estimate the size of my gzipped script?

7-zip supports compressing to the GZIP format.

I often use this to approximate and compare file sizes.

When creating an archive, look for Archive Format, and gzip is the 3rd option.

Update:

In the comments, we discussed that there might be a difference between 7-zip's GZIP compression, versus an actual server's GZIP compression. So, I compared using just the homepage of http://www.google.com/.
Google's GZIP'd payload was 36,678 bytes. 7-zip, with "gzip Normal" setting, was 35,559 (3% smaller). With "gzip Fastest" setting, it was 37,673 (3% larger).

So, long story short: 7-zip had results that were about 97% accurate.

If you're on unix - gzip -c filename.min.js | wc -c will give you a byte count of the gzipped file

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.

Directly from the terminal,

gzip -9 -c path/to/file.js | wc -c | numfmt --to=iec-i --suffix=B --padding=10

If you need the original size for comprison,

cat path/to/file.js | wc -c | numfmt --to=iec-i --suffix=B --padding=10

To get it programatically there are utilities like gzip-size. It's a node package but you can install it globally as a general tool.