How this is calculated
Vaporization is a heat-transfer problem. Every pound of propane that turns to gas must absorb its latent heat, and the only source is the air outside, through the wall, into the liquid touching it. Wall with vapor behind it barely counts, so what matters is wetted surface area.
For an upright bottle the wetted wall is four times the liquid volume divided by the diameter, so halving the liquid halves the surface. For a horizontal tank it follows the chord of the liquid line, which is why a tank at 10 percent keeps 47 percent of the surface it had at 60.
Heat crossing that wall scales with the gap between the air and the boiling liquid, which sits at the fuel’s atmospheric boiling point.
This page uses 250 Btu/hr per square foot of wetted wall per degree Fahrenheit above propane’s minus 44 °F, which is 1.42 kW per square meter per kelvin above minus 42 °C. That constant is what two manufacturer tables imply when their figures are divided by the wetted area of the vessel described.
Worked example A 100 lb cylinder is 14.9 inches across and holds 28.6 gallons of water. At 40 percent liquid that is 1.53 cubic feet, so the wetted wall is 4 × 1.53 ÷ 1.24 = 4.9 square feet. At 20 °F the flux is 250 × (20 + 44) = 16,000, so the ceiling is 78,000 Btu/hr — against 100,000 for a furnace and water heater firing together.
Worked example A 13 kg bottle is 315 mm across and holds 31 liters of water. At 40 percent liquid that is 12.4 liters, so the wetted wall is 4 × 0.0124 ÷ 0.315 = 0.157 square meters. At minus 7 °C the flux is 1.42 × (−7 + 42) = 49.7 kW per square meter, so the ceiling is 7.8 kW — against 18 kW for a boiler alone.
Demand is the sum of the plate inputs of everything that can fire at once. A generator is counted at 4.8 kW of propane per kW of electricity: the 20 kW Guardian burns 130 cubic feet an hour at full load, and the same sheet says to multiply cubic feet by 2,500 for Btu, so 325,000 Btu/hr buys 20 kW.
Design to about 75 percent of the ceiling. The ceiling falls all winter as the vessel empties, and the number the calculator returns is the best case for a clean, unshaded, wind-exposed tank.
Quick answers
Why did my propane stop flowing when the tank is not empty?
The liquid could not boil fast enough. Low liquid plus cold air leaves very little boiling surface, so pressure sags and burners starve while the gauge reads a quarter.
How much can a 20 lb cylinder deliver in the cold?
On the wetted-wall arithmetic here, about 62,000 Btu/hr full at 60 °F, 38,000 full at 20 °F, and 19,000 at 20 °F once down to 40 percent. Treat those as a floor for small cylinders. One 30,000 Btu/hr heater still outruns a half-empty barbecue cylinder on a cold day.
Why does frost form on a propane tank?
Boiling pulls latent heat through the steel, so the wetted wall runs below air temperature and water freezes on it. The frost line marks the liquid level, and a wide band means the vessel is near its limit.
Will a butane bottle work in winter?
No. Butane boils at essentially 0 °C, so at freezing it holds almost no pressure above atmospheric and delivers nothing however heavy the bottle feels. Propane holds 3.9 bar gauge there.
Wetted surface, the frost line and the 80 percent rule
Two bottles holding the same liquid have wetted areas in inverse proportion to their diameters, so a tall narrow bottle beats a squat wide one at every level. Shape matters as much as size.
Frost appears at the liquid line because that is where heat is leaving the steel, and a band creeping up the wall is a vessel working at its ceiling. A frosted regulator instead means liquid is carrying over, a different fault.
The vapor space above the liquid is what the 80 percent rule protects. The US limit in OSHA 29 CFR 1910.110 Table H-27 is written by weight rather than by volume: 42 percent of the container’s water capacity for an aboveground container of 1,200 gallons or less, at propane’s specific gravity band of 0.504 to 0.510. Divide 42 by a liquid gravity of 0.51 and you get 82 percent of the container by volume, which is where the 80 percent rule comes from.
Maximum continuous draw by vessel and temperature
Everything below is at 40 percent liquid, the trade’s reference level, with a full vessel in the last column.
| Vessel | 0 °F | 20 °F | 40 °F | 60 °F | Full, 20 °F |
|---|---|---|---|---|---|
| 20 lb cylinder | 13,100 | 19,100 | 25,100 | 31,100 | 38,200 |
| 30 lb cylinder | 19,700 | 28,700 | 37,700 | 46,700 | 57,400 |
| 40 lb cylinder | 26,200 | 38,200 | 50,300 | 62,300 | 76,500 |
| 100 lb cylinder | 53,700 | 78,300 | 103,000 | 128,000 | 157,000 |
| 120 gal tank | 175,000 | 255,000 | 335,000 | 415,000 | 376,000 |
| 250 gal tank | 302,000 | 440,000 | 578,000 | 716,000 | 649,000 |
| 500 gal tank | 471,000 | 687,000 | 903,000 | 1,120,000 | 1,010,000 |
| 1,000 gal tank | 842,000 | 1,230,000 | 1,610,000 | 2,000,000 | 1,810,000 |
Btu per hour of propane vapor, continuous. Cylinder diameters and water capacities from Manchester Tank’s DOT cylinder catalog 49500 — 12.2 inches across at 20, 30 and 40 lb, 14.9 inches at 100 lb. Tank diameters and lengths from Quality Steel Corporation chart CT002048.
| Vessel | −20 °C | −10 °C | 0 °C | 10 °C | Full, −10 °C |
|---|---|---|---|---|---|
| 6 kg bottle | 2.8 | 4.1 | 5.3 | 6.6 | 8.1 |
| 11 kg bottle | 4.4 | 6.3 | 8.3 | 10.3 | 12.7 |
| 13 kg bottle | 4.9 | 7.2 | 9.4 | 11.6 | 14.3 |
| 35 kg bottle | 13.9 | 20.2 | 26.5 | 32.8 | 40.4 |
| 454 L tank | 46.5 | 67.6 | 88.7 | 110 | 99.8 |
| 946 L tank | 80.3 | 117 | 153 | 190 | 172 |
| 1,893 L tank | 125 | 182 | 239 | 296 | 269 |
| 3,785 L tank | 224 | 326 | 428 | 529 | 481 |
kW of propane vapor, continuous. Tank diameters and lengths from Quality Steel Corporation chart CT002048, which prints them in liters as well as gallons. Bottle diameters are the nominal steel shells this calculator assumes — 256 mm at 6 kg, 300 mm at 11 and 35 kg, 315 mm at 13 kg — and they vary by supplier. Measure your own: boiling surface scales inversely with diameter, so a shell 10 percent wider gives 10 percent less.
What the published tables say about this model
Two manufacturer tables anchor the model. Quality Steel Corporation’s card (chart CT002048) gives 235,008 Btu/hr for a 120 gallon tank and 634,032 for a 500 gallon tank, both at 40 percent liquid and 20 °F air. The model returns 255,000 and 687,000.
Against that card the model sits within about 1 percent at 60 °F across every tank size, and runs 8 percent high at 20 °F for the 120, 250 and 500 gallon rows. At 1,000 gallons it is 13 percent high, and there the card is the odd one out: the trade worksheet below, working from the same 41 by 190 inch shell, lands within 2 percent of the model.
Flame Engineering’s table for a 100 lb cylinder gives 113,000 Btu/hr at 0 °F and 277,000 at 60 °F with the cylinder full. The model returns 107,000 and 255,000, so 5 to 8 percent low. Divide their figures by the wetted area and the implied flux is 251 to 262 Btu/hr per square foot per degree F, which is where the 250 in the model comes from.
Where the cylinder table disagrees
Flame’s table also runs down the fill, and it does not fall as fast as pure geometry says it should. At 20 lb of propane left in that 100 lb cylinder they publish 51,000 Btu/hr at 20 °F; the model returns 32,000.
The gap runs one way. This model counts only wall the liquid is touching, while a real cylinder also pulls heat down the dry wall and through the base. For cylinders at low fill, read the answer as a floor.
The trade rule of thumb runs out sooner
An older sizing rule sits on wholesalers’ worksheets: outside diameter in inches, times overall length in inches, times a constant K for the percentage full, times a temperature multiplier T. Its K values are the wetted-perimeter geometry of a horizontal cylinder to within about 3 percent from 20 to 60 percent full, and 4 percent at 10 percent full.
Its multiplier is where it parts company. T runs 0.25 at minus 15 °F, 1.0 at 0 °F and 2.0 at plus 20 °F, a straight line that reaches zero at minus 20 °F. That is exactly where propane vapor pressure falls to about 10 psig and a first-stage regulator has nothing left to work with.
| 500 gallon tank, 40 percent full | Trade rule | This model | Manufacturer card |
|---|---|---|---|
| 60 °F | — | 1,119,000 | 1,127,168 |
| 20 °F | 710,400 | 687,000 | 634,032 |
| 10 °F | 532,800 | 579,000 | — |
| 0 °F | 355,200 | 471,000 | — |
| −10 °F | 177,600 | 363,000 | — |
Btu/hr. Trade-rule column from the tank vaporization worksheet published by F.W. Webb (D × L × K × T, K = 80 at 40 percent). Manufacturer column from Quality Steel Corporation chart CT002048, which publishes only 20 °F and 60 °F. Model column computed by the calculator above. The three sources publish only in Btu/hr on a Fahrenheit grid, so the table keeps their own steps rather than inventing metric ones: divide by 3,412 for kW, and 60 °F is 15.6 °C, 20 °F is minus 6.7 °C, 0 °F is minus 17.8 °C.
Before you size anything for a cold climate The three columns agree near 20 °F and diverge hard below it. At 0 °F the trade rule is a third lower than the model; at minus 10 °F it is half. Published measurements back the model, since Flame’s 0 °F column implies the same flux as its 60 °F column. But the trade rule carries margin on purpose, and it collapses toward minus 20 °F because a starving regulator ends the job whatever the steel is doing. Below about 10 °F, size to the trade-rule column.
Buried tanks are the one place the rule gets generous. The worksheet says to use 20 °F or better for any underground or mounded tank with at least nine inches of cover, whatever the air is doing, because soil holds heat that air does not.
Butane, propane and the bottle that goes quiet
This part only bites in Europe, where much bottled gas is butane or a butane-propane mix and the bottle by the back door may be either. NIST’s Chemistry WebBook puts n-butane’s boiling point at 273 K, essentially 0 °C, against 231 K for propane. Energy per pound is almost the same — 21,591 Btu for propane against 21,221 for butane on Quality Steel’s card — so the fuel is not the issue. Pressure is.
| Air temperature | Propane, bar gauge | Propane, psig | Butane, bar gauge | Butane, psig |
|---|---|---|---|---|
| −29 °C / −20 °F | 0.8 | 11 | none | none |
| −20 °C / −4 °F | 1.5 | 22 | none | none |
| −10 °C / 14 °F | 2.5 | 37 | none | none |
| −5 °C / 23 °F | 3.2 | 46 | none | none |
| 0 °C / 32 °F | 3.9 | 56 | 0.03 | 0.5 |
| 5 °C / 41 °F | 4.7 | 68 | 0.24 | 3.5 |
| 10 °C / 50 °F | 5.5 | 80 | 0.48 | 6.9 |
| 20 °C / 68 °F | 7.6 | 110 | 1.06 | 15.4 |
Vapor pressure from the Antoine coefficients in the NIST Chemistry WebBook (Rips 1963 for propane; Aston and Messerly 1940 and Das and Reed 1973 for n-butane). “None” means the pressure sits below atmospheric, so the bottle cannot push gas out at all. Quality Steel’s card gives 40 psig at 20 °F and 10 psig at minus 20 °F, within 3 psi of the Antoine figures at both.
The same arithmetic runs for butane with its own boiling point. A 13 kg bottle at 40 percent liquid gives 9.4 kW of propane at 0 °C; the same shell filled with butane gives 0.1 kW at 0 °C, 1.4 kW at 5 °C and 3.8 kW at 15 °C.
The label names the gas and is the only reliable check, because color coding is set nationally and does not travel. Calor’s specifier guide has British propane cylinders red and butane blue; another country’s scheme will differ. A mixed bottle gets worse as it empties, since the propane boils off first.
What actually fixes it
In rough order of cost:
- Keep the fill higher through winter. Going from 15 percent to 40 lifts the ceiling by half on a horizontal tank and by two and a half times on an upright bottle, and costs nothing but an earlier delivery.
- Manifold cylinders so they draw together. Wetted surface adds, so four 100 lb cylinders deliver four times one.
- Move up a size. A 500 gallon tank buys about 56 percent more continuous draw than a 250 at the same fill level.
- Bury or mound the vessel if the site allows, for the reason given above: soil holds heat that air does not.
- Fit a vaporizer for a large continuous load. It draws liquid off the tank and boils it with added heat, decoupling the appliance from the weather. Never apply a flame to a container.
Start with the load in the backup power calculator, turn it into fuel with the generator fuel consumption chart, size the store with the propane storage sizing chart, then come back here. Storage and vaporization are separate limits, and a store that passes one can fail the other.
Sources
Quality Steel Corporation, approximate vaporization capacities and standard domestic tank specifications, chart CT002048. Flame Engineering, vaporization rate for a 100 lb propane cylinder. Manchester Tank, DOT cylinders 5 lb to 100 lb, catalog 49500. The tank vaporization rule-of-thumb worksheet published by F.W. Webb, sheet dated January 2023. Calor Gas specifier guide, on cylinder color coding. NIST Chemistry WebBook, phase change data and Antoine coefficients for propane (Rips 1963) and n-butane (Aston and Messerly 1940; Das and Reed 1973). OSHA 29 CFR 1910.110 Table H-27. Generac 20–24 kW Guardian specification sheet, part A0000937814 Rev. F, 28 September 2022. Every figure above was re-checked against these documents in September 2026.
Go deeper
Size the electrical side in the backup power calculator, then read wood against propane, heat pump and pellet for what the fuel costs once it keeps up, and whether off-grid solar needs a generator at all. For cooking and cold storage, off-grid refrigeration compared covers the loads that run all year.