Most propane tanks that are 100-gallon capacity or more will have a percentage gauge. This is normally located under the tank lid and embedded in the tank. The gauge is round and is usually calibrated in increments of 10.
These gauges are only accurate to ± 5 percent. For example: If you have a 500-gallon tank, it will be considered full when it is between 80 and 85 percent. If the gauge reads 40, then you have about 200 gallons in the tank (.40 x 500 = 200 gallons).