A bbq grill propane tank holds 4.7 gallons of propane.
When a propane tank is filled it's never filled to actual full because the propane needs room for expansion otherwise the propane tank could explode.
If your propane tank was or is completely empty then you would buy 4.7 gallons of propane to fill the propane tank.
It's always best to have a propane tank refilled than it is to exchange the propane tank because I once exchanged a propane tank and it was only half full and only lasted a short while.
After I started having the propane tanks filled the propane tanks lasted longer with my bbq grill.