Cornelius kegs come in various sizes, but many home brewers use the "five gallon" (nominal) size.
Including the headspace, how much volume is available inside of the standard five gallon Cornelius keg?
I tested one of my pin lock kegs just the other day, and it came out to almost exactly five gallons. There was just a tiny bit of space left at the top after adding five gallons of water, as measured by filling a one-gallon pitcher five times.
I don't know if pin lock and ball lock differ, nor do I know if it might vary by manufacturer. For what it's worth, Wikipedia says soda kegs hold 5 gallons with no mention of variation by type or manufacturer.
Mine seem to hold just under 5.25 gal.