There are two ways to avoid running out of fuel during a barbecue. The first is to always have a spare tank on hand; the second is to keep track of how much fuel you have left.
The most accurate method is to grab the bathroom scale and weigh the tank, then calculate how much cooking time remains.A normal-size tank with a 20-lb. capacity weighs about 17 to 18 lbs. empty and about 37 to 38 lbs. when full of propane. A full-size gas grill (35,000 Btu) will cook for 30 minutes per pound of propane.
My buddy's tank (shown) tipped the scale at 21 lbs., so it contains about 4 lbs.—or approximately two hours of grilling time (4 lbs. x 30 min.). Fortunately for him, my rib eye will only take him about 13 minutes.