If you're in level flight, it's less than a 12oz can of soda.
Not that it would be safe to run down that far, because at nose down attitudes you probably would un-port the fuel inlet, but in level flight, it's basically all useable.
Tim Olson - CFI
RV-10 N104CD - Flying 2/2006 - 1200+ hours
RV-14 N14YT - Flying 6/2016 - 100+ hours