How Much Propane (LP) is Left?
There's usually* no "gauge" on a propane tank (and even if there were, they're not terribly reliable. The accurate way to calculate remaining gas is found at Wikipedia:
 Propane Tank Remainder Measurement
The only accurate way to measure the propane left in a propane tank is to weigh it. Engraved on the side of the tank should be the letters TW then a number. This number is the empty tank weight. For example, the typical 5 gallon propane tank might say TW 17.2. The weight of the empty tank in the example is 17.2 lbs. Next, weigh the tank on a bathroom scale to find the total current weight. Let's say that the current weight of the tank is 24.2 lbs. To find the weight of propane simply subtract the tank weight from the total weight.
24.2 lbs - 17.2 lbs = 7 lbs
Each gallon of liquid propane weighs 4.23 lbs. Divide 7 by 4.23 to get the number of gallons currently in the tank.
7 ÷ 4.23 ≈ 1.66 gallons of propane
We can go a step further and find out how long 1.66 gallons of propane will take to burn. Each gallon of propane contains 91,690 BTUs. In the example above we were left with approximately 1.66 gallons of propane in the tank. Simply multiply the number of gallons by the number of BTUs contained in each.
1.66 × 91,690 = 152,205.4 BTUs
Next, find the BTU output of the appliance in question. These can usually be found on the manufactures website. Let's use 12,000 BTUs for our example. Divide the number of BTUs left in the tank by the number of BTUs that your appliance consumes per hour to get the total running time left for propane in the tank.
152,205 ÷ 12,000 ≈ 12.68 hours of burn time.
*An exception: The Tank claims to measure the amount of liquid propane remaining, which should produce a reading that is an accurate measure of what's left.