What BTU Means And Why Your Gauge Isn't The Whole Story
Propane is stored in your tank as a liquid and consumed by your appliances as a vapor. One gallon of liquid propane holds approximately 91,502 BTUs of energy. Your furnace, water heater, range, and absorption refrigerator each draw a rated number of BTUs per hour — and the calculator below uses those numbers to estimate how many days your current supply will last.
Common tank sizes and their approximate gallon capacities: a 20 lb cylinder holds about 4.73 gallons, a 30 lb cylinder about 7.1 gallons, a 40 lb cylinder about 9.5 gallons, and a 100 lb cylinder about 23.6 gallons. A 40 lb tank at full capacity holds roughly 869,000 BTUs of potential energy. How long that lasts depends entirely on what appliances you're running and for how many hours per day.
Why your propane gauge can mislead you: RV propane gauges read vapor pressure above the liquid in the tank, not liquid volume directly. Because propane vaporizes steadily as it's used, the gauge tends to read relatively stable until the tank is nearly empty — then it drops fast. Cold weather compounds this: low temperatures slow the vaporization rate, so appliances may run poorly or shut down before the gauge says you're low. Planning your refill based on a BTU estimate rather than just the gauge gives you a better margin of safety, especially on long trips or in cold weather.
How Long Will Your Propane Last?
Select your tank size, set the gauge level, and turn on the appliances you are running. Your estimated days remaining updates instantly.
- Tank size: select a common size or type your gallons directly.
- Gauge level: drag the slider to match your current propane gauge reading.
- Appliances: toggle on each appliance you are running and enter how many hours per day it runs. Controls expand when toggled on.
- Result: green = 7+ days · yellow = 3–6 days · red = under 3 days. Always carry extra propane on long trips.
1 gallon of propane = 91,502 BTU. Tank capacities: 20 lb ≈ 4.73 gal · 30 lb ≈ 7.1 gal · 40 lb ≈ 9.5 gal · 100 lb ≈ 23.6 gal.
Your Tank
Select a tank size or enter gallons above.
Appliances In Use
Enter your tank size and toggle on the appliances you are running.
Reading The Result And Planning Your Refill
The calculator gives you an estimate — not a guarantee. Use it as a planning tool, not a countdown timer. Actual consumption is affected by temperature, altitude, how often appliances cycle, and the age and condition of your equipment.
- Green result (7+ days): You have a comfortable supply for short to medium trips. On longer trips, plan a refill before you reach the yellow range rather than waiting.
- Yellow result (3–6 days): Start looking for a refill at your next convenient stop. Don't wait until you're in a remote area with no propane available.
- Red result (under 3 days): Refill soon. Propane gauges can drop quickly in cold weather, and running completely empty can introduce air into the lines — which means relighting each appliance and potentially purging the system.
Refill safety reminders: Close all appliance valves and the main tank valve before transporting to a refill station. Tanks manufactured after September 1998 are required to have an overfill prevention device (OPD) — never disable or bypass it. Inspect the regulator cover and hoses for cracks, rust, or brittleness every season, and check the regulator manufacture date — most manufacturers recommend replacement every 5 to 7 years.
Propane Requires A Higher Caution Level
Propane is reliable when the system is maintained. When something is wrong, the signs are usually clear. Take every one of them seriously.
- You smell propane indoors — even faintly. Shut off all appliances, do not touch any switches or create sparks, open windows and doors, exit the RV immediately, and call a certified RV technician before re-entering. Never ignore a propane odor.
- You smell propane outdoors near your tank or regulator. Close the tank valve, do not light anything, and have the system pressure-tested by a technician before continuing use.
- An appliance fails to light after confirming there is propane in the tank. This is often a low-pressure issue from a failing regulator rather than the appliance itself — do not repeatedly attempt to light it before ruling out a system pressure problem.
- Any fitting, hose, or connection on the propane system has been loosened, removed, or repaired. The system must be leak-tested before returning to service.
- Your regulator is more than 5 to 7 years old, shows rust or corrosion on the cover, or has visible physical damage. Regulator failure is a leading cause of propane system problems.
- Your LP gas detector alarms. Treat it as a real leak until a qualified technician confirms otherwise.
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