How Long Will Your Propane Last?

• Energy content: propane delivers 91,502 BTU per gallon; 1 lb of propane ≈ 21,548 BTU. A properly filled "20 lb" cylinder holds 4.6 gal ≈ 420,000 BTU usable (30 lb ≈ 7.0 gal, 40 lb ≈ 9.4 gal).
• Furnace duty cycle: duty = (Tset − Tlow) × k, clamped between 5% and 95%. We use k = 0.0267 tent-like, 0.0183 average, 0.0117 well-insulated — calibrated so a 30°F gap runs an average rig's burner about 55% of each hour (tent-like ≈ 80%, 4-season ≈ 35%). Wind, humidity, and single-pane windows push it higher.
• Furnace propane: gal/day = BTU/hr × duty × hours ÷ 91,502. The overnight low is a conservative stand-in for the average temperature across the hours the furnace runs.
• Water heater: ≈0.1 gal per shower — a full 6-gallon recovery uses roughly 0.07 gal of propane once burner efficiency is counted; we round up to cover standby losses.
• Stove/oven: minutes per day at ≈9,000 BTU/hr. Absorption fridge: ≈1.5 lb/day (≈0.35 gal). Portable heater: rated BTU × hours, straight through — those run continuously at their setting.
• Everything here is an estimate. Regulators ice up, tanks lose pressure below ≈−10°F, and real weather doesn't read forecasts. Carry margin.

A properly filled 20 lb cylinder holds about 4.6 gallons of liquid propane — roughly 420,000 BTU of heat. Tanks are only ever filled to about 80% of their water capacity because liquid propane expands significantly as it warms; that headspace is a safety requirement, not shortchanging. Watch out for exchange-cabinet tanks at gas stations and big-box stores: many are filled to only 15 lb, which is about 3.5 gallons. The actual fill weight is printed on the label, so check it before you assume you're getting a full 20 lb.

Because your furnace almost never runs continuously. The burner cycles on and off with the thermostat, and the fraction of each hour it actually fires is the duty cycle — driven by how much colder it is outside than your setpoint. A 30,000 BTU furnace at a 50% duty cycle burns propane like a 15,000 BTU appliance, which is why the continuous-burn hours quoted on blogs are so misleading. One thing the propane math hides: the furnace blower also draws serious 12 V power, often 5–8 amps whenever it runs, so on a boondocking trip your battery bank can give out before your propane does.

The most accurate method is to weigh the tank: subtract the tare weight (the empty weight, stamped as TW on the collar — typically 17–18 lb for a 20 lb cylinder) from the total, and what's left is your propane. The classic field trick is to pour warm water down the side and feel for where the metal turns cold — the liquid propane inside chills it at the liquid line, and on humid days a condensation or frost line can show up on its own. Stick-on magnetic gauges use the same temperature difference to show the level. One thing that doesn't work: a pressure gauge on the valve, because propane pressure is set by temperature, not by how much liquid is left — it reads nearly the same from full until the tank is almost empty.

It can slow to a trickle. Your appliances burn propane vapor, and that vapor comes from liquid propane boiling off inside the tank — a process that pulls heat from the surrounding air. Propane's boiling point is −44°F, so it will still vaporize in almost any weather you'd camp in, but the colder it gets and the less liquid is left, the slower the boil-off. A nearly-empty tank in deep cold simply can't generate vapor fast enough to feed a big furnace, so the flame starves even though there's propane sloshing inside. The fixes are practical: keep tanks fuller in winter, run larger tanks or twin setups, keep the tank off frozen ground — and never apply open flame to warm one.

A properly working built-in RV furnace, yes — it's a vented appliance that draws combustion air from outside and exhausts outside, and it's designed for unattended overnight use. The non-negotiable condition is working carbon monoxide and propane detectors with fresh batteries, tested before the trip, because a cracked heat exchanger or exhaust leak is exactly the failure a CO alarm exists to catch. Unvented portable heaters (buddy-style catalytic heaters) are a different story: they dump combustion byproducts into your living space, consume oxygen, and require cracked-open ventilation plus a CO detector whenever they run. Never let one run while everyone is asleep — even models with a low-oxygen shutoff, since that sensor does not detect carbon monoxide.

Yes — and it's usually the better deal. Refill stations at propane dealers, U-Haul locations, hardware stores, and many campgrounds fill your own tank to the true 80% level and charge by the gallon, which typically works out well below the effective per-gallon price of a cabinet exchange. Exchanges often hand you a tank filled to only 15 lb, so you pay more for less propane; their real advantages are convenience, all-hours availability, and swapping out a tank whose certification date has expired. A reasonable strategy: refill your own tanks as the routine, and treat exchanges as the roadside fallback. Note that tanks must be recertified 12 years after manufacture — a refill station will refuse an out-of-date tank, while an exchange will take it.