Propane weighs 4.24 lb/gallon, so a "20 pound tank" would hold 4.7 gallons if you could fill it all the way. However, tanks cannot be filled to more than 80% by law, to leave some room for expansion. This limit is enforced by a device called an OPD (overfill protection device) which is built into the tank, and cuts off the flow when the tank reaches 80% full. And 80% of 4.7 gallons is 3.77. So you are getting what you are supposed to get. By the way, it is a "20 pound tank", not a "20 gallon tank".
It is worth noting that there are 3 ways the seller can compute the cost of a fill.
1. By weight. How many pounds of propane were put into the tank? A scale is used.
2. By volume. How many gallons were put into the tank?
3. Flat rate. You pay the same amount, no matter how much propane was added.
Obviously #3 is not a good choice unless your tank is completely empty.
I have filled at several Ace stores, and they always use #1 or #2, so no matter how many gallons or pounds go in, I am charged for that amount and no more.
It is also worth noting that most (all?) propane exchange racks don't even give you 3.7 gallons. If you look at the top of the tank, you will find a tag stating how much propane is in it.
Bill
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