Key takeaways
- One appliance empties the tank, and in the cold it's the furnace. It burns more than cooking, hot water, and the fridge combined.
- Tank size isn't the answer. The burn rate is. The same propane lasts weeks in summer and a couple of days when the furnace runs all night.
- The furnace math is brutal. A 20,000 to 30,000 BTU furnace burns a quarter to a third of a gallon for every hour it actually runs.
- Everything else is small. Cooking, the water heater, and a propane fridge together are often under a gallon a day.
- The cheap days come from heating smarter, not a bigger tank. Turn it down at night, insulate, and a catalytic heater on its own bottle takes the load off your main supply.
The appliances on propane, and why one of them empties the tank
Four things in a typical rig run on propane. The furnace heats the air. The water heater heats your water. The stove and oven cook your food. And the absorption fridge, if you have one, runs on gas when you're not plugged in. Off-grid, they all pull from the same tank, and the question that decides your trip is which one is doing most of the pulling. In warm weather the answer is cooking or the fridge, and the numbers are so small that propane is rarely what ends a trip. The moment it gets cold, the furnace takes over and the math changes completely. As the propane overviews put it, the furnace is the single biggest gas consumer in most RVs, and how long your propane lasts depends almost entirely on how much you run it.
The appliance-by-appliance breakdowns all land in the same place: the furnace dwarfs everything else when it's running. So the honest way to plan propane isn't to look at your tank size. It's to figure out how hard the furnace is going to work, because that's the number that sets your days.
How much propane the furnace actually burns
Start with the energy. A gallon of propane holds about 91,500 BTU. An RV furnace is rated in BTU per hour, usually somewhere between 20,000 and 40,000. The furnace math from The RV Geeks does the division cleanly: a 30,000 BTU furnace running flat out for an hour burns 30,000 of those 91,500 BTU, which is about a third of a gallon. A smaller 20,000 BTU unit burns about a quarter of a gallon an hour. The catch is that word "running." The furnace cycles. It kicks on, brings the rig up to temperature, and shuts off, so what matters is how many hours it actually fires across a day.
That's where the cold comes in. On a mild night the furnace might run a little in the morning and evening. Below freezing it cycles hard all night long. The furnace-usage guides put the extreme case plainly: a furnace running continuously can burn around eight gallons in a day, and at a realistic overnight duty cycle you land somewhere in the one-to-four-gallons-a-day range depending on how cold it is. That single number swamps the rest of your propane budget.
| Nights | Furnace propane, per day | What it looks like |
|---|---|---|
| No heat | 0 gallons | Warm enough to leave it off |
| Mild | ~0.6 gallons | A little morning and evening |
| Cold | ~1.8 gallons | Real furnace use overnight |
| Freezing | ~3.2 gallons | Cycling hard all night |
Why everything else is a rounding error in the cold
Put the other three appliances next to those furnace numbers and they nearly disappear. Cooking is the most visible because you do it every day, but a stove burner runs around 7,000 to 9,000 BTU, and an hour of cooking spread across a day is well under a tenth of a gallon. The water heater feels like it should be a hog, but it only fires to reheat the tank, a few minutes at a time. The water-heater numbers work out to a small daily figure unless you're taking long hot showers every day. And the absorption fridge, running continuously on gas, uses around a pound and a half of propane a day, which sounds like a lot until you remember a furnace can burn that much in under an hour when it's freezing.
Add them up on a cold trip and cooking, hot water, and the fridge together might be three-quarters of a gallon a day. The furnace alone is two or three times that. This is the whole reason the tank-size question misleads people. If the furnace is the limit, a second tank just buys you a little more of the same problem. The lever is the furnace.
What's actually in your tank, in gallons not pounds
Propane is sold by the pound on portable tanks and by the gallon on the fixed tanks in motorhomes, which makes the numbers confusing. Here's the translation. Tanks fill to 80 percent for safety, so a "20 pound" tank holds about 4.5 usable gallons, a 30 pound tank about 7, and a 40 pound tank about 9.4. Most travel trailers carry two 30 pound tanks, so call it 14 usable gallons. Motorhomes run larger fixed ASME tanks. The RV propane tank sizing guides lay out the common cylinders if you want to confirm yours. The calculator below starts you at a realistic capacity for your rig type, and you can set the exact number if you know it.
Stretching propane when it's cold
Once you know the furnace is the problem, the moves that add days are obvious, and most of them are about needing the furnace less. Turn it down at night and sleep under good bedding instead of heating the whole rig to 68 degrees. Put Reflectix in the windows, where most of your heat escapes. Close any slides you can spare so there's less air to warm. And the big one: a standalone catalytic or radiant heater that runs off its own small bottle takes the entire overnight load off your main tank, which is exactly why so many cold-weather boondockers carry one. The staying-warm guide from RV With Tito and the winter-boondocking tips from Escapees cover the rest. None of it requires a bigger tank. It requires running the furnace less.
One boondocker tracking a winter stay watched the furnace turn a tank that normally lasts weeks into a two-or-three-day problem. The fix wasn't more propane. It was a small catalytic heater run off a separate bottle overnight, plus turning the built-in furnace down once they were under the covers.
The point they kept coming back to is the one this page is built around: in the cold, the furnace is the whole propane story, so the thing to manage is the furnace, not the tank. Heat smarter and a single fill stretches a long way. Run the furnace like you're hooked up and no tank is big enough.
Run your numbers
Figure your days in about 60 seconds
Tell us your rig, how cold your nights are, and what runs on propane. We'll tell you how many days your propane lasts on one fill, which appliance is burning most of it, and where the cheap extra days are hiding.
The mistakes that leave you cold or out of gas
Sizing propane by the tank. The tank is a fixed number; the furnace decides how fast you spend it, so the burn rate is the real budget. Forgetting that warm-weather propane lies to you. A tank that lasted three weeks in October can go in three days in January, and the rig didn't change. The furnace did. Heating the whole rig to room temperature overnight. The furnace runs far longer than it needs to when you could sleep warm under bedding instead. Skipping window insulation. Most of your heat leaves through the glass, so the furnace works harder to replace it. Buying a second tank to fix a furnace problem. If the furnace is the limit, more propane just delays the same outcome. Manage the burn first.
RV propane, frequently asked
How long does a propane tank last in an RV boondocking?
How much propane does an RV furnace use per day?
What uses the most propane in an RV?
How do I make my propane last longer boondocking in winter?
How long will a 20 lb propane tank last in an RV?
Does the RV fridge use a lot of propane?
Can I run my RV furnace all night while boondocking?
Next: size the battery the furnace leans on
Propane makes the heat, but the furnace blower runs off your house battery, and a cold night is the hardest test your bank ever faces. The Battery calculator sizes the bank for the loads you actually run, the same way this one sized your propane. About 90 seconds.
Start the Battery calculator →Sources: furnace BTU-to-gallon math from The RV Geeks and the furnace-usage guide at Do It Yourself RV; the propane-system and how-long-it-lasts overview from Camping World; the appliance-by-appliance breakdown from The Roving Foley's; water-heater usage from Camper FAQs; tank sizing from etrailer; cold-weather conservation from RV With Tito and Escapees.