A 100-pound propane tank holds 23.6 gallons of propane. A propane tank is considered full at 80 percent capacity and should be refilled as soon as possible if it drops to 30 percent capacity. Keeping the tank at an appropriate capacity is a safety measure.
Tanks of this size are not required by law to have OPD valves. OPD stands for overflow protection device and prevents the propane tank from being overfilled. This protective valve is required on smaller sizes of propane tanks. Since it is not required on larger tanks, care must be taken not to overfill a 100-pound tank that's not equipped with an OPD.