How long will your propane last?

The three numbers you need

1. Tank size. Portable tanks are labeled by propane weight. Propane weighs about 4.2 pounds per gallon, so a "20-pound" tank holds roughly 4.7 gallons. Motorhome frame-mounted ASME tanks are labeled in gallons but are filled only to about 80% of listed capacity.

2. Furnace input rating. Printed on the furnace's data plate and in its manual, typically 20,000 to 40,000 BTU per hour for RV furnaces. This is fuel burned per hour of actual run time, not heat delivered — ducted RV furnaces lose a meaningful share of that heat out the exhaust.

3. Duty cycle. The fraction of each hour the furnace actually fires. This is the number weather controls. A mild night might see 10–20%; a cold night with a thin-walled rig can push 40–50% or more. If you do not know yours, time it on a cold evening at home: minutes running per hour ÷ 60.

The math

Daily burn = furnace rating × run hours per day. Days of heat = usable tank BTU ÷ daily burn.

Worked example: a 30,000 BTU/hr furnace holding an RV at comfortable temperatures through a near-freezing night might run around 30% of the time over an 18-hour heating period — call it 5.5 hours of actual burn. That is 30,000 × 5.5 ≈ 165,000 BTU per day. A 20-pound tank holds about 4.7 gallons × 91,500 ≈ 430,000 BTU, so heat alone consumes it in roughly 2½ days. Two 30-pound tanks, by the same math, cover about a week — before the water heater, stove, and refrigerator take their share.

Treat every number here as illustrative. Your rig's insulation, the overnight low, wind, and your thermostat setting swing the duty cycle enormously — which is exactly why the estimate is worth doing with your numbers rather than someone else's.

The furnace drains your battery too

The blower motor runs on 12-volt power the entire time the furnace fires, a draw large enough to matter overnight. When you plan a cold-weather trip, add furnace blower run time as its own appliance in the battery calculator — a propane-rich, battery-dead rig is still a cold rig.

Ways to stretch a tank

• Drop the overnight thermostat setting and sleep warm with bedding instead of BTUs.
• Close off and stop heating spaces you are not using.
• Cut drafts: vent cushions, window coverings, and skirting the underside in extended stays all lower the duty cycle.
• Use the sun — open shades on the south side by day, close everything at dusk.
• Weigh your tanks before departure. A tank's empty (tare) weight is stamped on the collar as "TW"; current weight minus tare, divided by 4.2, is the gallons you actually have.

For the rest of the cold-weather picture — batteries, water protection, and carbon monoxide safety — see winter dry camping.