Propane Tank Runtime Calculator

Propane runtime is one of the cleaner calculations in this whole area. The energy content is a published constant — 91,452 BTU per gallon — the density is known, and appliance ratings are stamped on data plates. Divide energy by demand and you have hours.

All the uncertainty sits in one input: duty cycle. Almost nothing burns continuously. A furnace fires until the thermostat is satisfied and then stops; a water heater reheats a tank and waits; an absorption fridge cycles on its own control. The rating on the plate is what it draws while burning, not what it averages.

The spread is enormous. A 25,000 BTU/hr furnace at 20% duty averages 5,000 BTU/hr; the same furnace at 70% duty in a hard freeze averages 17,500. On a 20 lb cylinder that is the difference between three and a half days and one. Anyone who has run out of propane at 3am in January has met this personally.

Which means the useful discipline is not more precise arithmetic but better duty-cycle estimates — and the best source for those is your own experience of how long the last cylinder lasted in comparable weather.

Propane is stored as a liquid under modest pressure and used as a vapour. Every time an appliance draws gas, liquid inside the tank must boil to replace it — and boiling absorbs heat, which the tank takes from the air around it through its steel wall.

That heat transfer is the bottleneck. It depends on the wetted surface area — how much tank wall has liquid against it — and on the temperature difference between the propane and the outside air. Both work against you in winter: cold air transfers less heat, and as the tank empties the liquid level falls so less wall is wetted.

The result is a rate limit that has nothing to do with how much fuel remains. A 20 lb cylinder running a 30,000 BTU/hr grill on a July afternoon is untroubled. The same cylinder at 10°F, a quarter full, asked to run a 25,000 BTU/hr furnace, will frost over on the outside, its pressure will sag, and the furnace will starve and shut down. There is propane in the tank. It cannot get out fast enough.

The frost line on the outside of a cylinder is a direct readout of this: it marks the liquid level, because that is where the boiling is drawing heat. A heavily frosted cylinder in cold weather is a cylinder working at its limit.

The remedies are all about surface area and heat. A physically larger tank has more wetted wall. Two cylinders manifolded together share the demand. Reducing simultaneous load — not running the tankless water heater while the furnace is on — keeps peak demand under the limit. What does not work, and is genuinely dangerous, is applying any kind of flame or direct heat to a cylinder to speed vaporisation.

Propane cylinders are filled to 80% of their water capacity, never more, and the reason is thermal expansion. Liquid propane expands roughly seventeen times more than water for the same temperature rise — a cylinder filled cold and then left in the sun can see its liquid volume grow substantially.

The 20% vapour space absorbs that expansion. Without it, a warming cylinder would build pressure until the relief valve opened and vented liquid propane, which is both a fire hazard and an alarming thing to witness.

This is why a "20 lb" cylinder holds 20 pounds of propane but has a water capacity of about 4.7 gallons more than that suggests, and why overfilling — which older filling equipment could do — was a real safety problem. Modern cylinders have an overfill prevention device, a float valve that stops the flow at the 80% level, and this is why cylinders manufactured before 1998 cannot legally be refilled in the US without one fitted.

A related practical point: cylinder exchange programmes typically fill to 15 pounds rather than 20, for weight and liability reasons, while charging a price that often works out at two to three times the per-gallon rate of a refill. Taking your own cylinder to a propane dealer gets the full 20 pounds at a much better rate, and the cylinder stays yours rather than being swapped for whatever is in the cage.

The ranking surprises people, because it is dominated by duty cycle rather than by rating.

Propane is safe when handled properly and unforgiving when it is not, and three of its properties are worth knowing.

It is odourless. The rotten-egg smell is ethyl mercaptan, added deliberately at a concentration chosen so a leak is detectable well below the flammable limit. The additive can fade in a cylinder that has sat for years — a phenomenon called odorant fade — which is one reason not to rely on smell alone.

It is heavier than air. Unlike natural gas, which rises and disperses, leaked propane sinks and pools in low spaces — a bilge, a basement, an RV underfloor compartment. This is why propane lockers on RVs and boats are required to vent at the *bottom* to the outside, and why a propane leak detector belongs a few inches off the floor rather than on the ceiling where a natural gas detector goes.

And it burns in a narrow range. Propane is flammable between roughly 2% and 10% concentration in air, so a leak that produces a smell is already meaningful. The correct response to a strong smell is to leave, from a distance turn off the supply valve if it is safe to reach, and not operate any electrical switch — including a light switch or a phone — until the space is clear.

Any appliance burning propane indoors also produces carbon monoxide if it is not burning cleanly and venting properly. Unvented propane heaters used inside an RV or a cabin are the specific hazard here, and a carbon monoxide alarm is not optional equipment.