Refilling puts more propane in your cylinder than exchanging it does. Exchange buys you speed and hands the tank's legal upkeep to somebody else. Which one is worth it depends almost entirely on the condition and age of the tank you already own.
The fill level is the whole story
A cylinder sold as a "20 lb" tank is named for how much propane it holds. A refill station sells propane by the gallon and fills your cylinder toward that rated capacity. An exchange cage hands you a tank somebody else already filled, and how much is inside was decided by the company that filled it, not by the size of the cylinder.
That amount got smaller, and the change is documented. In an administrative complaint issued in March 2014, the Federal Trade Commission alleged that in 2008 Blue Rhino and AmeriGas each decided to implement a price increase by reducing the amount of propane in their exchange tanks from 17 pounds to 15 pounds, without a corresponding reduction in the wholesale price. The FTC put the two companies' combined share of the US wholesale market for propane exchange tanks at roughly 80 percent. Both settled in October 2014.
The FTC was not prosecuting the smaller fill. Reducing the amount of propane in a tank is legal. What the agency charged was the agreement between two competitors to coordinate their negotiations with Walmart so the reduction would stick. The fill level is simply a fact the case established along the way.
So the comparison a shopper actually faces is 15 pounds of propane in an exchange tank against a refill of the same cylinder toward its rated capacity. Refill stations price by the gallon, so you pay for what goes in.
Why no tank is ever filled to the top
Propane expands as it warms. The Utah State Fire Marshal's filling guidance puts it plainly: LP-gas pressure increases with temperature, so to allow for expansion, propane containers are to be filled only to 80 percent of their capacity. The vapor space above the liquid is the room that expansion needs.
On cylinders between 4 and 40 pounds this limit is enforced mechanically by an overfill prevention device, the valve with the triangular handle. It closes when the cylinder reaches its permitted level. That is why a refill attendant stopping "early" is not shorting you.
When exchange is the better call
- Your cylinder is out of date. A cylinder that has not been requalified cannot legally be filled. Exchange is the cheapest way out, because you hand the old tank over and walk away with a current one. See when a propane tank expires.
- Your tank is rusted, dented, or has an old valve. Same logic. The exchange company absorbs the tank you no longer want to own.
- It is Sunday evening. Exchange cages sit outside and need no attendant. Refill counters keep staffed hours, and the person qualified to run the pump may have gone home well before the store closed.
- You are in a hurry. Swapping a tank takes about a minute.
When refill wins
- You get more propane, and you pay by the gallon for it.
- You keep your own cylinder. You know its age and how it has been treated. With exchange you do not choose the tank you get, and its history is not yours to know.
- You can top up. A half-empty tank can be filled the rest of the way. Handing a half-full tank to an exchange cage donates the propane still in it.
How to check what you are getting
Exchange tanks state their net propane weight on the wrapper or the cage. The Utah guidance tells buyers to check the posted information where they purchase propane to determine the net weight in the cylinder. It is the single number that makes the two options comparable, and it is the number the 2008 change moved.
Find either one near you
Propane Atlas lists locations by service, so you can filter for one or the other.