Propane Tank Duration Calculator: How long will a 20 lb propane tank last?
In summer a 20 lb propane tank often lasts two to three weeks with cooking and the fridge. As soon as the furnace runs, it's only three to five days in freezing weather. This calculator estimates the duration of your propane tank from heating demand, cooking time, fridge and hot water – or directly from your known daily consumption.
Also enter the planned trip length, and you'll learn how much propane you need in total and how many tanks to bring.
Result
Propane tank duration
6.1 days
6.1 days at 3.25 lb per day
- Daily consumption
- 3.25 lb/day
- Available propane
- 20 lb
- ≙ 117 kWh of energy
- The heating consumption is a rule of thumb. Insulation, vehicle size, outside temperature and desired temperature can easily double or halve it.
Show calculation
- 1Furnace (rule of thumb “Medium”): 2.2 lb/day
- 2Cooking: 7,140 BTU/h × 30 min = 3 oz/day
- 3Fridge: 12 oz/day
- 4Hot water: 1 × 2 oz = 2 oz/day
- 5Total daily consumption: 3.25 lb/day
- 6Available propane: 20 lb × 100% = 20 lb
- 7Duration: 20 lb ÷ 3.25 lb/day = 6.1 days
How it's calculated
The duration is the available amount of propane divided by daily consumption. The available amount follows from the tank's fill weight (20 lb, 30 lb or 40 lb of propane – not the tank weight) and the current fill level.
Duration [days] = fill weight × fill level ÷ daily consumptionEstimating consumption from appliances
The propane consumption of the furnace can be calculated physically from heat output, burner running time and efficiency. Propane contains about 12.87 kWh (43,900 BTU) of energy per kilogram, i.e. about 21,600 BTU per pound:
Furnace propane [kg/day] = heat output [kW] × burner running time [h/day] ÷ (12.87 kWh/kg × efficiency)Because hardly anyone knows the burner running time, the calculator also offers rules of thumb by weather – from 0.5 kg (1.1 lb) per day on cool nights to 3 kg (6.6 lb) per day in freezing weather with continuous operation. These values are rough averages for a typical mid-size motorhome and can differ considerably depending on insulation and vehicle size.
- Cooking: stove consumption × cooking time. A large burner uses up to 160 g/h (7,600 BTU/h) at full flame; in everyday use 100–150 g/h is a good average.
- Absorption refrigerator: manufacturer figures are 14–18 g/h (670–860 BTU/h), i.e. 330–440 g (0.7–1 lb) per day – more in heat and with frequent opening.
- Hot water: a 10 L (2.6 gal) water heater (e.g. Truma B10, 120 g/h) needs about 25 minutes per heat-up, i.e. about 50 g (1.8 oz).
For trip planning the daily consumption is multiplied by the trip length and rounded up to whole tanks.
Worked example
Fall trip with a full 20 lb tank and a full spare: the furnace runs at about 41 °F (5 °C) (“medium”), cooking 45 minutes a day, the absorption fridge runs on propane, water is heated once a day. The trip lasts 14 days.
Inputs
- Propane tank: 20 lb
- Current fill level: 100 %
- Tanks on board: 2 pcs
- Specify consumption: Estimate from furnace, stove, fridge
- Heating demand: Medium – fall, about 41 °F / 5 °C (≈ 2.2 lb/day)
- Cooking per day: 45 min
- Stove consumption: 7,140 BTU/h
- Absorption fridge runs on propane: yes
- Fridge propane consumption: 12.3 oz/day
- Water heater heat-ups per day: 1 ×
- Other consumption: 0 oz/day
- Planned trip length (optional): 14 days
Result
6 days
Duration of the tank in use
- Daily consumption
- 3.33 lb/day
- Available propane
- 20 lb
- Duration incl. spare tanks
- 12 days
- Propane needed for 14 days
- 46.7 lb
- Tanks needed
- 3 × 20 lb
Calculation
- Furnace (rule of thumb “Medium”): 2.2 lb/day
- Cooking: 7,140 BTU/h × 45 min = 4 oz/day
- Fridge: 12 oz/day
- Hot water: 1 × 2 oz = 2 oz/day
- Total daily consumption: 3.33 lb/day
- Available propane: 20 lb × 100% = 20 lb
- Duration: 20 lb ÷ 3.33 lb/day = 6 days
- With 1 full spare tank(s): (20 lb + 20 lb) ÷ 3.33 lb/day = 12 days
- Trip: 14 days × 3.33 lb/day = 46.7 lb → 3 tank(s) of 20 lb
The variables explained
- Fill weight
- Propane weight of the full tank: 20 lb, 30 lb or 40 lb (9.1, 13.6, 18.1 kg). The empty weight of the steel tank (tare, stamped “TW”) is added on top.
- Fill level (%)
- Weighing is the most reliable method: (total weight − tare) ÷ fill weight. Gauges and ultrasonic sensors are approximations.
- Daily consumption
- Sum of all propane appliances per day – entered directly or estimated from the appliances.
- Rated heat output (kW)
- Heat output of the furnace per the data sheet (e.g. Combi 4: 4 kW ≈ 13,650 BTU/h, Combi 6: 6 kW ≈ 20,500 BTU/h; 1 kW = 3,412 BTU/h).
- Burner running time (h/day)
- Time the burner actually works at this output. A thermostat-controlled furnace only runs part of the time.
- Efficiency (%)
- Share of the propane's energy that reaches the interior as heat. Modern forced-air furnaces: about 90% and more.
Common mistakes
- Confusing tank weight with fill weight: a full 20 lb steel tank weighs about 37 lb – “20 lb” is only the propane.
- Driving into winter with the summer consumption: the furnace dominates consumption; in winter it's easily five to ten times higher.
- Counting the fridge while driving: on propane an absorption fridge usually only runs when parked; while driving often on 12 V.
- Forgetting vaporization capacity in freezing weather: nearly empty tanks deliver too little propane below freezing – a spare tank is a must in winter.
- Butane in winter: butane no longer vaporizes below 32 °F (0 °C); use propane only for winter camping (the standard in North America).
Assumptions and limits
- Heating value of propane 12.87 kWh/kg (46.3 MJ/kg, ≈ 21,600 BTU/lb).
- The rules of thumb for heating demand are averages for a typical mid-size motorhome; poorly insulated vans or large Class A motorhomes deviate.
- Stove, fridge and water heater consumption are typical manufacturer figures; your own data sheet takes precedence.
- Spare tanks are assumed to be full.
- The result is a planning figure, not a guarantee – always plan a reserve for winter camping.
Frequently asked questions
How long does a 20 lb propane tank last when heating in winter?
In freezing weather with the furnace running continuously, 4–6.6 lb (2–3 kg) per day is realistic – a 20 lb tank then lasts only 3–5 days. At mild temperatures around 50 °F (10 °C) consumption is more like 1.1 lb (0.5 kg) per day, i.e. a good two and a half weeks.
How long does a 1 lb propane cylinder last?
On a camping stove at about 100 g/h (a medium flame) roughly 4–5 hours of cooking. For an RV furnace or fridge, 1 lb cylinders are impractical – use a 20 lb tank.
How do I find out how much propane is left in the tank?
Most accurately by weighing: total weight minus the stamped tare weight (TW) gives the propane content. Alternatively, ultrasonic level sensors help, or check the condensation on the tank after operation (it's cooler below the liquid level).
Sources and background
- promobil: propane in the motorhome – monitoring consumption (consumption figures for furnace, stove, fridge, water heater)
- Truma Combi 4 / Combi 6 – technical data (propane consumption 160–335 g/h and 160–480 g/h)
- Propane properties: heating value 46.3 MJ/kg ≙ 12.87 kWh/kg ≙ 21,600 BTU/lb