20 # Bottle

[Hillcountry]

I believe it's general practice to fill propane tanks (large and small) to about 80% capacity. Not sure if this is for safety or that it's a law but my twin 250 gallon tanks are never filled more than 75-80%.
As for the 20 lb. grill tanks, I always have a full one on hand as a backup in case I run out while grilling...never cared about how much propane was inside...