For grills without a built-in gauge, get a scale, like the Grill Gauge (about $11; Ace Hardware
), or use a bathroom scale. Tanks list their tare weight (TW), or weight when empty. Weigh the tank and subtract the TW to gauge the gas inside. A pound of propane produces 21,600 Btu per hour; divide this by your grill's max Btu output to see how many hours of cooking on high you'll have left per pound of gas. It's smart to keep an extra full tank on hand; store it upright and outdoors in the shade.