Where does this Content-Length come from?

I visit the page http://furryfaust.com/waveform and it redirects to http://furyfaust.com/waveform/ with this initial response.

HTTP/1.1 301 Moved Permanently
Location: /waveform/
Date: Sat, 09 Jul 2016 21:32:56 GMT
Content-Length: 45
Content-Type: text/html; charset=utf-8

I found this out using chrome developer tools. I'm curious where the content-length is determined from since there is no response message body.