The arithmetic, once
Propane appliances are rated in BTU per hour of input. Multiply that rating by the hours the burner actually runs and you have BTU per day. Divide by 91,500 BTU per gallon and you have gallons. Multiply by 4.24 pounds per gallon and you have pounds, which is the unit bottles are sold in. Those two constants also give 21,580 BTU in a pound of propane, and a 20 pound bottle holding 4.72 gallons and 431,600 BTU.
Run the defaults through it. A 20,000 BTU furnace for four hours is 80,000 BTU. An absorption fridge at 1,500 BTU/hr cycling for twenty hours is 30,000. A water heater at 8,800 for ninety minutes is 13,200, and an hour of cooktop is 7,000. That is 130,200 BTU, 1.42 gallons, 6.03 pounds — a little over three days from a 20 pound bottle and just under seven days from two.
The step people get wrong is hours. An appliance switched on is not an appliance burning. A furnace with the thermostat set cycles; the number you want is burner-on hours, which on a mild night might be two and on a freezing one might be ten. An absorption fridge modulates. A water heater fires to recover and then sits. If you have never watched the cycling, start with a guess and correct it against the bottle you actually emptied.
Why the furnace is the only line that matters in winter
Look at the percentage column in the results. In mild weather the furnace is around sixty percent of the burn. Double its hours for a cold snap and it becomes three quarters, and total consumption goes from six pounds a day to twelve. Two bottles that comfortably covered a week now cover three days.
This is the reason propane estimates made in October never survive January, and it is why the multiplier field exists. It is not a safety factor. It is a second scenario, and on a shoulder-season trip both scenarios are plausible on different nights of the same week. Plan against the cold one and the mild one takes care of itself.
The furnace is also the appliance most affected by things that have nothing to do with propane: how well the coach is insulated, whether the windows are single glazed, whether skirting is fitted, and how low you are willing to set the thermostat. Every one of those changes the hours, and the hours are the whole calculation.
Bottles, gallons and the eighty percent question
| Bottle | Gallons of gas | BTU held | Days at 6 lb/day |
|---|---|---|---|
| 20 lb | 4.72 | 431,600 | 3.3 |
| 30 lb | 7.08 | 647,400 | 5.0 |
| 40 lb | 9.43 | 863,200 | 6.6 |
A frequent confusion is the eighty percent figure. Propane bottles are filled to around eighty percent of their shell volume because liquid propane expands substantially with temperature and the vapour space is not optional. That allowance is already inside the rated pounds — a 20 pound bottle is designed to hold 20 pounds of gas with the vapour space accounted for. So a correctly filled bottle is one hundred percent of its rating, not eighty, and the fill percentage field above is asking how much is left in a partly used bottle rather than applying that allowance twice.
Exchange bottles are a separate matter. Some exchange programmes supply less than the bottle rating, and the figure is printed on the cage. If you exchange rather than fill, put the actual figure into the custom field.
The two hazards, stated plainly
Propane is heavier than air. A leak inside a closed vehicle does not disperse upward and out; it settles into the lowest space it can find and stays there, which makes it both an explosion hazard and an asphyxiation hazard in a volume the size of a camper. Separately, every fuel-burning appliance produces carbon monoxide, which has no smell, causes confusion before it causes alarm, and kills people while they sleep. Both hazards are addressed by working propane and carbon monoxide alarms with in-date sensors, and both are worth the sixty seconds it takes to press the test button before a trip.
What this page will not do is tell you how to find a leak, service an appliance, adjust a regulator, or handle, store or carry a bottle. Those belong with a qualified gas technician, and the rules governing bottle transport and storage differ by state and by park. If you smell gas, the response is to get people out and away first and to sort the cause afterwards, with help.
Related pages
Propane weight counts against your cargo allowance on the RV weight and cargo capacity calculator, and the water heater firing time here is set by the draw on the RV water heater and shower planner. For heating fuel at a fixed address see the heating fuel reserve calculator, and for generator fuel rather than appliance gas the generator fuel runtime calculator.
Questions people ask
How long will a 20 lb propane bottle last in an RV?
It depends almost entirely on furnace hours, which is why the answer people quote ranges from two days to three weeks. A 20 pound bottle holds 4.72 gallons and 431,600 BTU. Fridge, water heater and cooking alone might use 50,000 BTU a day, which is over eight days from one bottle. Add a 20,000 BTU furnace running four hours and the daily figure trebles to 130,000 BTU, giving 3.3 days. Run that furnace eight hours on a freezing night and you are near 210,000 BTU a day, or two days per bottle. Put your own appliances and your own hours in above rather than using anybody's single-number answer.
How many BTU are in a pound of propane?
About 21,580. That comes from the two figures used throughout this calculator: 91,500 BTU per gallon and 4.24 pounds per gallon. Dividing one by the other gives BTU per pound, which is the conversion you need when appliances are rated in BTU per hour and bottles are sold in pounds. Published energy content varies slightly with composition and reference conditions, so treat 91,500 as a working figure rather than a physical constant; the difference is well inside the error in your estimate of running hours.
Does cold weather make propane itself less efficient?
The energy content of the propane does not change, but two other things do. Consumption rises because the furnace runs far more, which is the effect modelled by the multiplier above and by far the larger of the two. Separately, propane vaporises less readily at low temperatures, so a nearly empty bottle in severe cold can struggle to supply a high demand appliance even though there is liquid left in it. That second effect is about vaporisation rate rather than energy content, and it is one more reason not to run a bottle to the bottom in winter.
Should I count the fridge if I am on shore power?
No — set its hours to zero for the days it runs on electricity. Absorption fridges switch between gas and electric heat sources, and on a powered site the propane line for the fridge stops entirely. That single change often halves a mild-weather propane figure, because a fridge cycling twenty hours a day is a large share of the burn when the furnace is idle. If you alternate between powered sites and dry camping, run this calculator twice and use the appropriate number for each stretch.
Can I work out how much is left in a bottle by weighing it?
Yes, and it is the only reliable method available to an owner. Bottles carry a stamped tare weight — the weight of the empty bottle — so the gas remaining is the total weight on a scale minus that tare. A 20 pound bottle with an 18 pound tare reading 27 pounds has 9 pounds of gas left, which at the six pounds a day in the example above is a day and a half. Bathroom scales are accurate enough for this. Gauges that clip to the valve read pressure, and propane vapour pressure barely moves until the bottle is nearly empty, which is why those gauges show full until they abruptly do not.