How Long Will a Propane Tank Last?
Propane delivers about 91,500 BTU per gallon, so runtime is appliance BTU rating ÷ 91,500 = gallons per hour at full fire. A 100,000 BTU/hr furnace burns about 1.1 gal/hr while firing; at a realistic 40–50% winter duty cycle that's 11–13 gallons a day of heating in cold weather.
The same math scales down: a 30,000 BTU fireplace insert is ~0.33 gal/hr; a 60,000 BTU pool heater ~0.66 gal/hr; a 12,000 BTU cooktop burner barely registers. Whole-home usage in heating season commonly lands between 3 and 10 gallons a day depending on climate, insulation and house size.
Worked example: a 500-gallon tank filled to its 80% maximum holds 400 gallons. At 6 gal/day of winter usage that's about nine weeks; at 2 gal/day shoulder-season usage, over six months. A 120-gallon tank (96 usable) feeding only a water heater and range often runs most of a year.
Track your own burn the reliable way: gauge readings a week apart, converted to gallons (gauge % × water capacity — see reading the gauge), divided by days. Then size reorders with the propane charts.
Start with what you actually have
A propane tank's name is its water capacity — how much water the vessel would hold. It never contains that much propane. NFPA 58 limits filling, and a container with a fixed maximum liquid level gauge fills to roughly 80 % of water capacity, because propane expands about six times as much as water for the same temperature rise and the vapour space is what absorbs it.
| Tank | Water capacity | Usable at 80 % fill | Gallons per 10 % of gauge |
|---|---|---|---|
| 120 gal | 120 gal | 96 gal | 12 gal |
| 250 gal | 250 gal | 200 gal | 25 gal |
| 320 gal | 320 gal | 256 gal | 32 gal |
| 500 gal | 500 gal | 400 gal | 50 gal |
| 1000 gal | 1000 gal | 800 gal | 100 gal |
The float gauge reads percent of water capacity, so gallons of propane equals the gauge reading times the nameplate. A 500 reading 35 % holds 175 gallons — not 35 % of the 400 usable gallons. The gauge guide covers the reading itself.
The energy behind a gallon
EIA's thermal conversion factor for propane is 3.841 million Btu per barrel — 91,452 Btu per gallon — estimated from NIST Chemistry WebBook enthalpy-of-combustion data and liquid density at 60 °F. Divide any appliance's input rating by that and you have its consumption at full fire.
gallons per hour at full fire = appliance Btu/hr ÷ 91,452
gallons per day = gal/hr × 24 × duty cycle
days remaining = gallons in tank ÷ gallons per dayDuty cycle is the fraction of the hour the appliance actually fires. A correctly sized furnace runs a low duty cycle in mild weather and approaches continuous operation only at the design temperature.| Appliance | Input rating | Gallons/hour firing | Gallons/day at 40 % duty |
|---|---|---|---|
| Cooktop burner | 12,000 Btu/hr | 0.13 | 1.3 |
| Tankless water heater | 40,000 Btu/hr | 0.44 | — |
| Fireplace insert | 30,000 Btu/hr | 0.33 | 3.2 |
| Pool heater | 60,000 Btu/hr | 0.66 | 6.3 |
| Furnace, small home | 80,000 Btu/hr | 0.87 | 8.4 |
| Furnace, average home | 100,000 Btu/hr | 1.09 | 10.5 |
| Furnace, large home | 150,000 Btu/hr | 1.64 | 15.7 |
The water heater has no daily column because it does not run on a duty cycle — it fires on demand, and a household's hot-water consumption is a better guide than any percentage. Budget roughly one and a half to two gallons a day for a family of four, then check it against your own gauge readings.
How long a tank lasts
Usable gallons divided by daily consumption. Read down your tank and across your burn rate.
| Daily use | 120 gal (96) | 250 gal (200) | 500 gal (400) | 1000 gal (800) |
|---|---|---|---|---|
| 1 gal/day | 96 days | 200 days | 400 days | 800 days |
| 2 gal/day | 48 days | 100 days | 200 days | 400 days |
| 4 gal/day | 24 days | 50 days | 100 days | 200 days |
| 6 gal/day | 16 days | 33 days | 67 days | 133 days |
| 8 gal/day | 12 days | 25 days | 50 days | 100 days |
| 12 gal/day | 8 days | 17 days | 33 days | 67 days |
A 500 through a cold month
- Tank filled to 80 % → 400 gallons
- 100,000 Btu/hr furnace = 1.09 gal/hr at full fire
- Cold-weather duty cycle around 45 % → 11.8 gal/day heating
- Plus about 1.5 gal/day for hot water and cooking → 13.3 gal/day
- 400 ÷ 13.3 = 30 days
About a month of hard winter from a full 500
Which is why a 500 is the smallest tank most suppliers will put on a propane-heated home in a cold climate — a 250 would need refilling fortnightly in January.
Working backwards from two gauge readings
- Jan 6: gauge 70 % on a 500 → 350 gal
- Jan 20: gauge 48 % → 240 gal
- 110 gallons over 14 days = 7.9 gal/day
- At 48 %, 240 ÷ 7.9 = 30 days remaining at that rate
7.9 gal/day — your number, not a table's
Do this once and you never need the estimate tables again. Note that a float gauge is a ±5 % instrument, so a fortnight's gap is about the shortest interval that gives a trustworthy rate.
Reorder points that survive a cold snap
Most suppliers ask you to call at 20–30 %. On a 500 that is 100–150 gallons, which sounds generous until you divide by a January burn rate of 13 gallons a day and get eight to eleven days. In a regional cold snap, that is also exactly when every other customer is calling.
- Below 20 % on a 500 is about a week of hard winter, not a month.
- Below 10–15 % you risk vapour-pressure problems in deep cold: a tank draws heat from its own liquid to vaporise, and both a low level and a low temperature reduce how fast it can supply gas.
- Running out is expensive beyond the fuel. An out-of-gas call normally requires a pressure test and a technician to relight appliances before service resumes.
What throws the estimate off
- Temperature moves the gauge without moving the propane. The same mass reads higher on a hot afternoon than at dawn. Compare readings taken at similar times of day.
- Duty cycle is not a constant. A furnace at 20 % duty in November and 60 % in January is the same appliance with a threefold difference in consumption.
- Float gauges are ±5 % instruments. Over a short interval that error can swamp the consumption you are trying to measure — hence the fortnight.
- Summer load is not zero. Cooking, hot water and a pool heater keep consuming when the furnace does not.
FAQ
How long will a 500 gallon propane tank last?
A 500 holds about 400 gallons of propane at the 80 % filling limit. At a hard-winter rate of 13 gallons a day that is roughly a month; at a shoulder-season 2 gallons a day it is over six months. Work out your own rate from two gauge readings a fortnight apart.
How many BTU are in a gallon of propane?
91,452 Btu per gallon — EIA's thermal conversion factor of 3.841 million Btu per barrel, estimated from NIST enthalpy-of-combustion data and liquid density at 60 °F. Divide an appliance's input rating by that to get gallons per hour at full fire.
Why can I only fill a propane tank to 80 %?
Because liquid propane expands about six times as much as water for the same temperature rise, and the vapour space above the liquid is what absorbs that expansion. NFPA 58 sets the filling limit, and a container with a fixed maximum liquid level gauge fills to roughly 80 % of its water capacity.
How much propane does a furnace use per day?
Input rating divided by 91,452 gives gallons per hour at full fire — 1.09 for a 100,000 Btu/hr furnace. Multiply by 24 and by the duty cycle: at 40 % that is about 10.5 gallons a day, and duty cycle is the number that changes most between November and January.
At what percentage should I reorder propane?
Most suppliers say 20–30 %, but convert it to days before trusting it. On a 500 at a January burn rate, 20 % is about a week — and a regional cold snap is when every other customer is calling too.
Can a propane tank run out of pressure before it runs out of propane?
Effectively yes. Propane has to vaporise to feed appliances, and the rate depends on the wetted surface inside the tank and the outside temperature. A small tank at a low level on a very cold day can fail to meet peak demand while still holding liquid.
Sources and standards
Where a standard genuinely governs what you should do here, it is cited and linked.
- Thermal Conversion Factors — Monthly Energy Review, Appendix A (source documentation)U.S. Energy Information Administrationhttps://www.eia.gov/totalenergy/data/monthly/pdf/mer_a_doc.pdfWhere the Btu-per-gallon figures come from and how they were derived. Distillate fuel oil is 5.770 million Btu/bbl at 15 ppm sulfur and under, 5.817 at 15–500 ppm and 5.825 above 500 ppm — 137,400, 138,500 and 138,700 Btu per gallon respectively. Propane is 3.841 million Btu/bbl, or 91,452 Btu per gallon, estimated by EIA from NIST Chemistry WebBook enthalpy-of-combustion data and liquid density at 60 °F.
- NFPA 58 — Liquefied Petroleum Gas Code (separation distances, Table 6.4.1.1; filling limits, Ch. 7)National Fire Protection AssociationPaid standard — cited by designation; obtain from the publisher.The model code most US states adopt for propane installations, and the one your local inspector is most likely working from. It is not identical to OSHA's Table H-23 — notably for containers of 125 gal water capacity and under — so confirm which is enforced locally. NFPA publishes free read-only access to its codes at nfpa.org.
- ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — Rules for Construction of Pressure VesselsAmerican Society of Mechanical EngineersPaid standard — cited by designation; obtain from the publisher.Stationary propane containers are ASME vessels. The nameplate stamps water capacity, design pressure and the manufacturer's data — and the nameplate, not a tape measure, is the authoritative capacity for a pressure vessel.
- 29 CFR § 1910.110 — Storage and handling of liquefied petroleum gases (Table H-23, minimum distances)U.S. Occupational Safety and Health Administration, via the electronic Code of Federal Regulationshttps://www.ecfr.gov/current/title-29/subtitle-B/chapter-XVII/part-1910/subpart-H/section-1910.110Federal separation distances for LP-gas containers, by water capacity: under 125 gal — 10 ft above ground; 125–250 gal — 10 ft; 251–500 gal — 10 ft and 3 ft between containers; 501–2,000 gal — 25 ft (reducible to 10 ft for a single container of 1,200 gal water capacity or less, if it is at least 25 ft from any other LP-gas container over 125 gal); 2,001–30,000 gal — 50 ft.
- Methodology — the partial-fill formulas and liquid densities used on this sitetankvolumecalc.comhttps://tankvolumecalc.com/methodology/Every chart value here comes from a closed-form partial-fill formula, not interpolation: circular segments for cylinders, a piecewise obround model for 275/330 heating-oil tanks, and scaled spherical caps for dished heads. Densities and their bases are tabulated there.
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cThe legal-metrology reference for U.S. customary units. Fixes the U.S. liquid gallon at exactly 231 cubic inches and tabulates the full liquid-volume chain (minim → fluid dram → fluid ounce → gill → pint → quart → gallon).
Every linked source above was retrieved and checked on 2026-09-02. Factors quoted as exact are exact by definition in the cited document, not rounded by us.