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.
I tested mine, (14A) by running all the fuel thru the pump and then pulling the drain plug out. I had about 4 oz (half a coffee cup) drained out of each tank. Left side had maybe about an ounce more but hard to say since I just eyeballed it.