Propane Duration Calculator
How many days a propane tank lasts given the weather, with the furnace duty cycle doing the work it actually does.
The temperature sets the furnace hours below. Everything else you can adjust by hand.
Hours of burner time a day
Why a 20 lb tank is not 20 lb of tank
Propane tanks fill to 80% of their water capacity. Liquid propane expands hard as it warms, and the vapour space above the liquid is what absorbs that expansion — fill it to the brim on a cold morning and it has nowhere to go by afternoon. So a "20 lb" tank takes 20 pounds of propane, about 4.7 gallons, in a shell whose volume would hold nearly six.
A gallon of propane carries about 91,502 BTU, so that 20 lb tank is 432 thousand BTU of heat. Divide by what your appliances draw per hour and you have the answer — which is why everything on this page is quoted in BTU per hour.
Typical appliance draws
| Appliance | BTU/hour | Notes |
|---|---|---|
| Furnace | 25,000 | By far the biggest consumer, and the only one whose runtime depends on the weather. Its hours are set by the temperature slider below. |
| Water heater | 10,000 | Burner time, not the time it is switched on — it cycles to hold temperature. Turning it off between uses roughly halves this. |
| Absorption fridge on gas | 1,500 | Small but constant. Over a week it quietly matches a day of furnace running. |
| Cooktop | 7,000 | One burner at a moderate setting. Cooking is a much smaller share of propane than people expect. |
| Oven | 9,000 | Occasional, and it doubles as cabin heat in cold weather, which is not nothing. |
The furnace is the whole answer
Run the numbers at any temperature you would actually camp in and the furnace comes out on top — even at 60 degrees, where it cycles only about five per cent of the hour, it still burns more than a gas fridge running around the clock. Nothing else is close. That means the levers that extend a propane supply are insulation, window covers, a warmer sleeping bag and a lower thermostat. Shorter showers are good practice for the water tanks and make almost no difference here.
Things worth knowing
- A propane tank fills to 80% of its water capacity, because liquid propane expands hard with temperature and the vapour space is what absorbs it. That is why a "20 lb" tank takes 20 pounds and not the 24 its volume suggests.
- The furnace is the whole game in cold weather. Everything else together rarely matches a single cold night.
- Gauges that strap to the outside of a steel tank are guesswork. Pour warm water down the side and feel for the cold line — that is the liquid level, and it is more accurate than most gauges sold for the job.
- Propane vaporises more slowly when the tank is cold, so a nearly empty tank on a freezing morning can fail to keep up with the furnace even though there is gas in it. That is physics, not a leak.
- Never fit a catalytic or unvented heater without cracking a window. It consumes cabin oxygen and produces water vapour, and both of those problems get worse the better sealed the rig is.
How to use
- Pick your tank size and set the overnight low — that drives the furnace hours.
- Adjust the other appliances for how you actually use them.
- Read the days remaining and which appliance is eating the tank.
Frequently asked questions
How long does a 30 lb propane tank last?
Running a furnace six hours a day and nothing else, about four days. In milder weather with the furnace barely cycling it can stretch past two weeks. The weather decides it, not the tank.
Why does a 20 lb tank only hold 4.7 gallons?
Tanks fill to 80 per cent of their water capacity. Liquid propane expands hard as it warms, and the vapour space above it absorbs that expansion. A tank filled to the brim on a cold morning has nowhere to put the pressure by afternoon.
How much energy is in a gallon of propane?
About 91,500 BTU. A 20 lb tank is therefore roughly 432,000 BTU — divide by what your appliances draw per hour and that is your answer.
What uses the most propane?
The furnace, at essentially any temperature you would camp in. Even at 60 degrees, cycling about five per cent of the hour, it still burns more than an absorption fridge running around the clock. Nothing else is close.
How do I make propane last longer?
Insulation, window covers, a warmer sleeping bag and a lower thermostat — everything that reduces furnace runtime. Cooking and showers barely register against it, so economising there feels productive and changes almost nothing.
Why did my furnace stop when there was still gas in the tank?
Propane vaporises more slowly when the tank is cold, so a nearly empty tank on a freezing morning cannot supply gas fast enough to keep up with a furnace. That is physics rather than a fault, and it is worse the emptier the tank gets.
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