Different gases have different compression ratios. A compression ratio tells you how many cubic meters a liter of liquid yields when released as gas. Propane, in particular, has a very high compression ratio and small amounts of liquid renders a high volume of gas. If you're used to dealing with gallons and feet, you need to do some conversion, as scientific measurements of this kind are usually given in metric.
Take the number of gallons of propane that you want to find the conversion factor. Convert that number to liters by multiplying it by 3.79. For example, suppose you want to convert 30 gallons of propane: 30 * 3.79 = 113.7.
Multiply the metric measure of liquid by the conversion ratio for propane. Since propane has a conversion ratio of 1:270, one unit of liquid propane yields 270 units of vaporous propane: 113.7 * 270 = 30,699.
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Divide the uncompressed measure of propane by 1,000. This yields the number of cubic meters that the given amount of propane fills: 30,699 / 1000 = 30.7 rounded up, so your propane fills 30.7 cubic meters.
Convert from meters back into feet: 1 meter = 3.28 feet, but you're dealing with cubic feet. So, multiply your metric volume measure by 3.28^3: 30.7 * 3.28 * 3.28 * 3.28 = 1,083.32. Hence, 30 gallons of propane fills 1,083.32 cubic feet.
You can get an idea of how large the volume is by comparing it to the volume of a balloon. A standard party balloon has a volume of about 0.5 cubic feet, so your 30 gallons of propane would fill 2,166 party balloons.