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 Blue Rhino exchange cylinder of the same size contains 15 lb. Weighing the same cylinder twice measures your use over that interval.

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

Use the cylinder’s own stamped TW (tare weight), not a generic model weight. Subtract it from the total scale reading to find the propane remaining, as Ferrellgas explains. The example below uses TW 17 lb; your cylinder may differ. Do not empty a cylinder to find its tare weight.

Worked example: the same removable cylinder weighs 35 lb, then 32 lb, 24 hours later. Its own collar is stamped TW 17 lb. These are example readings, not your calculator results.

Use TW, not the “20 lb” size

TW is the empty weight of this cylinder. The cylinder’s nominal propane capacity is a different number. Do not assume every 20 lb cylinder weighs 17 lb empty, or empty a cylinder to obtain its tare.

Use the same scale and weighing setup, with the cylinder upright and disconnected. Follow the RV/cylinder instructions for removal, refitting and leak checks.

First weighing: 35 lb total

17 lb cylinder + 18 lb propane

24 hours later: 32 lb total

17 lb cylinder + 15 lb propane

Both weight bars use the same scale. Hatched = cylinder; solid = propane. They show mass, not liquid height.

1. Use per day: (35 − 32) ÷ 24 hours × 24 = 3 lb/day.
2. Propane remaining: 32 − 17 = 15 lb.
3. Estimated time left: 15 ÷ 3 = 5 days, if use stays the same.

No refill or cylinder swap between readings. Water, dirt, ice or extra fittings can change scale weight. Repeat under representative conditions; cold weather, changing appliance use and scale resolution affect the estimate. This is not a fill instruction or a leak test.

Weigh removable cylinders outdoors, away from ignition sources, with the valve closed and the cylinder stable. Never disconnect or lift a permanently mounted RV tank for this method.

Method: Ferrellgas weighing guidance. Handling: PERC portable-cylinder guidance.

1. Record the first total weight and time. Use a suitable scale on a stable, level surface, with the cylinder upright.
2. Weigh the same cylinder again on the same scale. Do not refill or swap it between readings; keep fittings and weighing conditions consistent.
3. Divide the weight lost by elapsed hours, then multiply by 24. A fall from 35 lb to 32 lb over 24 hours is 3 lb/day.
4. Subtract TW from the second reading. With TW 17 lb, that leaves 15 lb; at the measured 3 lb/day rate, the estimate is five more days.

This is for removable cylinders, not permanently mounted RV tanks. Follow your RV’s instructions for shutting off, disconnecting, securing and reconnecting the cylinder, including the required leak check. Work outdoors, keep the valve closed while disconnected, and keep the cylinder upright and away from ignition sources; see PERC’s portable-cylinder guidance. Weighing is not a leak test or a way to determine a safe fill level.

The calculator starts with this example. Repeat the measurement in conditions similar to the trip you are planning: colder weather or different appliance use can change the remaining days substantially.

Exchange or refill: compare the supplied amount

Blue Rhino specifies a 15 lb net fill. The calculator compares that with a nominal 20 lb refill. Other providers and actual refill quantities can differ, so check the label or receipt.

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 colder conditions and a lower liquid level reduce a cylinder’s vaporization rate. A cold, nearly empty cylinder may struggle to supply a furnace even though it still contains propane. Follow the RV’s changeover instructions and plan a reserve for cold conditions; a weight-based days estimate does not guarantee enough vapor flow for every load.

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.

These are published nominal empty weights; use your own cylinder’s TW for weighing. With a nominal full fill, the aluminum 30 lb example weighs 47 lb, versus 37 lb for the steel 20 lb example. The extra propane adds 10 lb to the loaded cylinder even though their empty weights match. Account for loaded cylinders in your vehicle’s measured weight and ratings; our cargo capacity calculator explains the budget. Check bracket dimensions, restraint and hose reach before changing cylinder size.

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. Assuming the nominal capacity is the supplied amount. Blue Rhino specifies 15 lb; check your own provider.
3. Taking 80% off a cylinder’s rating. The rating already is the fill.
4. Treating days remaining as guaranteed vapor flow. Cold and low fill levels can limit supply before the cylinder is empty.
5. Forgetting the water heater. Use your exact model’s burner input and firing time. Our hot water calculator estimates shower duration, not actual burner runtime.
6. Blaming the propane when the fridge stops cooling. An absorption fridge needs to operate within its model-specific leveling limits — and our leveling block calculator works out how far off the rig is and what raises it.

Questions worth answering