RV Propane Calculator: How Long Will a 20 lb Tank Last?

A 20 lb propane cylinder lasts 20 lb divided by what you burn in a day — about 6.7 days at 3 lb a day, under five with a furnace running a few hours. But an exchange cylinder of the same size holds 15 lb, not 20, and the only way to know your daily figure is to weigh the cylinder.

How long does a 20 lb propane tank last?

Every calculator that ranks for this does the same sum: add up each appliance’s Btu rating, multiply by the hours it runs, and divide by the energy in propane. The sum is right. The inputs are the problem.

A furnace’s rating is what the burner draws while it fires, and nobody knows how many hours it fired last night. It cycles, so eight hours on the thermostat is some smaller number of hours of burner — how much smaller depends on the cold and the rig, and nothing on the furnace tells you. Guess the hours and the answer is a guess, however precise it looks.

Weigh it instead

A bathroom or luggage scale does what no Btu table can: it measures what you actually used.

1. Weigh the whole cylinder and write it down.
2. Weigh it again after a day of normal use, on the same scale.
3. The difference is your daily use — say 35 lb to 32 lb is 3 lb a day.
4. Take the empty weight off the second figure for what is left. Flame King lists its 20 lb steel cylinder at 17 lb empty, so 32 lb is 15 lb of propane: five days at 3 lb a day.

That is the calculator’s default, and it is the mode worth using. Do it on a cold night if cold nights are what you are planning for.

Exchange or refill: the 5 lb nobody counts

Blue Rhino states it plainly: its “propane grill tanks contain a net weight of 15 lbs. of propane.” A 20 lb cylinder refilled holds 20.

And one thing not to do with a cylinder’s rating: take another 80% off it. Flame King describes its 30 lb aluminum cylinder as “30 lb / 7 gallons (at 80% capacity)” — the pound rating already is the filled amount.

Why a cylinder can run short before it is empty

The Propane Technical Pocket Guide explains that “when the outside air temperature is lower or the container has less liquid in it, the vaporization rate of the container is a lower value.” Both happen together on the last cold nights of a cylinder: the tank is cold and nearly empty, so it turns liquid into gas more slowly just when the furnace wants the most.

The practical upshot is to switch to the full cylinder before the old one is completely empty on a cold night, rather than waiting for the furnace to go out.

Which cylinder lasts the days you want

Enter how many days you want between fills and the calculator shows the smallest cylinder that gets there — not the biggest. The three in our records:

• 20 lb steel — 17 lb empty, 17.8 in tall.
• 30 lb steel — 25 lb empty, 23.7 in tall.
• 30 lb aluminum — 17 lb empty, 26.5 in tall.

The aluminum 30 lb weighs the same empty as the steel 20 lb. That is 10 lb more propane for no more weight on the tongue before it is filled — worth knowing if your trailer is close to its limits, which our cargo capacity calculator works out. It is also the tallest, so measure the bracket first.

The cylinders compared

Model	Propane	Empty weight	Size	Material	Buy
Flame King 20 lb steel	20 lb (Flame King also lists it as "5 gal")	17 lb	12 x 12 x 17.8 in	Steel	Check price(opens in a new tab)
Flame King 30 lb steel	30 lb / 7 gallons	25 lb	12.3 x 12.3 x 23.7 in	High-grade welded steel, powder coat finish	Check price(opens in a new tab)
Flame King 30 lb aluminum	30 lb / 7 gallons "at 80% capacity" (also listed as "8 gal")	17 lb	12 x 12 x 26.5 in	High-grade aluminum	Check price(opens in a new tab)

Where this goes wrong

1. Guessing burner hours. The commonest error, and the reason to weigh.
2. Counting an exchange cylinder as 20 lb. It holds 15, by Blue Rhino’s own figure.
3. Taking 80% off a cylinder’s rating. The rating already is the fill.
4. Letting a cylinder run dry on a cold night. Switch before it is empty.
5. Forgetting the water heater. Suburban’s 6 to 16 gallon tank heaters burn 12,000 Btu/h while they fire; our hot water calculator shows how long each shower keeps it firing.
6. Blaming the propane when the fridge stops cooling. An absorption fridge has to be near level to work at all — Norcold publishes 3° side to side — and our leveling block calculator works out how far off the rig is and what raises it.

Questions worth answering