Lots of folks barbecue their Thanksgiving dinner and, for family and friends, it's a popular cooking option. Here's how to be sure your propane doesn't run out when you're cooking a bird outdoors. A simple test: Bring out the bathroom scale and weigh the tank. Then use this formula to estimate how much cooking time is left. Twenty-pound tanks weigh about 18-pounds empty and 38-pounds when full. The average gas grill burns a pound of propane every 30-minutes, or 10-hours on a full tank. So, if yours weighs 24 pounds, you have 6 pounds of propane left. That's good for three hours of cooking. Recapping: Twenty-four pounds minus the 18-pound tank leaves six pounds of propane and, at 30 minutes per pound, that's three hours of cooking time left.