One gallon of liquid propane contains roughly the same energy as 91 cubic feet of natural gas. That figure assumes natural gas delivers about 1,000 BTU per cubic foot, which is the round number many utilities use for billing. In practice the answer shifts slightly depending on the exact energy content of the natural gas in your area, landing anywhere from about 88 to 92 cubic feet per gallon of propane. The conversion is based entirely on energy equivalence, not on physical volume, and understanding the distinction clears up most of the confusion around switching between these two fuels.
Why the Comparison Is Based on Energy, Not Volume
Propane and natural gas are fundamentally different physical substances stored and delivered in different ways. Propane is a heavier hydrocarbon that liquefies under moderate pressure, so it gets sold by the gallon and delivered in tanks. Natural gas is mostly methane, which stays gaseous under normal conditions and flows through underground pipelines to your meter. Comparing a gallon of liquid to a cubic foot of gas by volume alone would be meaningless. A gallon of liquid propane takes up roughly 231 cubic inches. A cubic foot of gas occupies 1,728 cubic inches. Those numbers tell you nothing useful about how much heat either one will produce.
The only comparison that matters for heating, cooking, or running a generator is energy content, measured in BTU (British Thermal Units). One gallon of liquid propane contains about 91,500 BTU. One cubic foot of pipeline-quality natural gas contains somewhere between 1,000 and 1,050 BTU, depending on the mix of gases in the pipeline. Divide propane’s energy content by natural gas’s energy content per cubic foot, and you get your conversion factor. Using 1,000 BTU per cubic foot gives you 91.5 cubic feet. Using 1,037 BTU per cubic foot, a figure closer to the national average for delivered pipeline gas, gives you about 88 cubic feet. Either way, a single gallon of propane packs roughly the same punch as a bit under one CCF (hundred cubic feet) of natural gas.
Why Natural Gas BTU Content Varies
Natural gas is not a single chemical compound. It is a mixture, primarily methane but also containing small amounts of ethane, propane, butane, carbon dioxide, and nitrogen. The proportions differ by production region, processing facility, and even season. Gas pulled from a well in the Marcellus Shale in Pennsylvania has a different chemical makeup than gas from the Permian Basin in Texas. Before it enters the pipeline system, raw gas goes through processing to remove impurities and heavier hydrocarbons, but the final product still varies in its BTU content from one pipeline to another.
Most utilities in the United States deliver gas that falls between about 1,000 and 1,050 BTU per cubic foot. Some areas receive gas that runs a bit richer, closer to 1,060 or above, especially where the pipeline carries gas with a higher ethane or propane content. Your utility typically publishes a “heating value” or “therm factor” on your bill or on their website, and that number lets you pin down the conversion for your specific area. If your gas runs at 1,050 BTU per cubic foot, then one gallon of propane is equivalent to about 87 cubic feet rather than 91. For rough planning, though, 91 cubic feet is the number most people use, and it gets you close enough for appliance sizing or fuel-cost comparisons.
Turning the Conversion Into a Cost Comparison
The energy equivalence tells you how to compare prices. If propane costs $2.50 per gallon and natural gas costs $1.20 per therm (one therm equals 100,000 BTU, roughly 100 cubic feet), you can figure out which fuel is cheaper per BTU. That gallon of propane delivers about 91,500 BTU for $2.50, so you are paying roughly $2.73 per 100,000 BTU. The natural gas costs $1.20 per 100,000 BTU. In that scenario, natural gas is the cheaper heat source by a wide margin.
In most parts of the country where both fuels are available, natural gas holds a price advantage. Propane prices are more volatile because they are tied to wholesale markets and seasonal demand, especially in cold winters when heating draws down supply. Natural gas prices fluctuate too, but the pipeline infrastructure creates a buffer that keeps price swings smaller at the consumer level. Where the math sometimes tips in propane’s favor is in rural areas where the cost of running a natural gas line to a property is prohibitive. If extending a gas main to your house costs tens of thousands of dollars, the savings from cheaper gas per BTU might take decades to recoup the installation cost. Propane only requires a tank and a delivery truck.
One hidden factor in the cost comparison is appliance efficiency. A high-efficiency natural gas furnace and a high-efficiency propane furnace both convert a high percentage of their fuel’s energy into usable heat, but the exact percentages differ by model. Two furnaces rated at 96% efficiency will each waste about 4% of their fuel’s energy, so the BTU comparison holds up well. But if you are comparing an older propane furnace running at 80% efficiency against a new natural gas condensing furnace at 96%, the real-world gap in heating cost is wider than the raw fuel price suggests.
What Changes When You Convert an Appliance
Switching a home from propane to natural gas, or the other direction, is not as simple as swapping out a hose. The two fuels burn differently. Propane contains more energy per cubic foot of gas (about 2,516 BTU per cubic foot) than methane (about 1,012 BTU per cubic foot). That means propane burners use smaller orifices to let less gas through, because the gas coming through is more energy-dense. Natural gas burners have larger orifices to allow a higher volume of gas to flow and deliver the same heat output.
Most modern furnaces, water heaters, stoves, and dryers are available in both propane and natural gas versions, or they ship as natural gas units with a propane conversion kit in the box. The conversion kit typically includes a set of smaller orifices (sometimes called jets or spuds) and instructions for adjusting the gas valve’s pressure regulator. On a cooktop, you swap out the brass orifices behind each burner. On a furnace, a technician changes the orifice plate at the burner assembly and adjusts the manifold gas pressure. Propane operates at a higher supply pressure (about 11 inches of water column at the appliance) compared to natural gas (about 7 inches of water column), so the regulator needs to be set correctly too.
Getting this wrong is not a minor inconvenience. An appliance running natural gas through propane-sized orifices will starve for fuel and produce a weak, possibly dangerous flame. An appliance running propane through natural gas orifices will get way too much fuel, producing oversized yellow flames, soot, and carbon monoxide. Conversion is straightforward but should be done by someone who knows what they are doing, and many jurisdictions require a licensed technician.
Other Common Propane Conversions
Beyond the natural gas comparison, people often need to convert between propane’s own measurement units. Propane is sold by the gallon at the pump, by the pound in portable cylinders, and occasionally referenced in cubic feet when comparing it to other gases. Here are the most commonly needed conversions for liquid propane:
- Pounds per gallon: Liquid propane weighs about 4.2 pounds per gallon at 60°F. A standard 20-pound BBQ cylinder holds roughly 4.7 gallons.
- Cubic feet of gas per gallon of liquid: When one gallon of liquid propane vaporizes, it expands to about 36.4 cubic feet of propane gas at standard temperature and pressure.
- BTU per pound: Propane delivers about 21,660 BTU per pound, so that 20-pound grill tank holds approximately 433,000 BTU of energy.
- Gallons per therm: One therm equals 100,000 BTU, so one gallon of propane equals about 0.915 therms.
The 36.4-cubic-feet figure sometimes confuses people who conflate it with the natural gas equivalence. If someone says one gallon of propane equals 36 cubic feet, they are talking about 36 cubic feet of propane gas, not natural gas. Those 36 cubic feet of propane gas contain about 91,500 BTU, while 36 cubic feet of natural gas contain only about 36,000 to 37,000 BTU. The two gases are not interchangeable at equal volumes.
Tank Sizes and Real-World Capacity
Residential propane tanks come in a handful of standard sizes, and knowing the gallon-to-natural-gas equivalence helps when sizing a tank for a home that would otherwise use natural gas. A 500-gallon above-ground propane tank, the most common size for a house that uses propane for heating, hot water, and cooking, is typically filled to about 80% capacity for safety. That means you get about 400 usable gallons, which is equivalent in energy to roughly 36,400 cubic feet of natural gas, or about 364 therms. For a home in a moderate climate that uses about 600 therms of natural gas per heating season, a 500-gallon tank would need roughly one and a half fills to get through winter.
Smaller tanks, like the 120-gallon units sometimes used for homes that only run a propane fireplace or a backup generator, hold about 96 usable gallons at 80% fill. That is equivalent to about 8,700 cubic feet of natural gas, or roughly 87 therms. People who switch from natural gas to propane are often surprised at how quickly a small tank empties when it is running a furnace, because they are used to thinking of their fuel supply as essentially unlimited via the pipeline.
How Propane Gets to Your Tank
The infrastructure behind propane delivery is strikingly different from natural gas distribution. Natural gas travels through a vast network of interstate pipelines, local distribution mains, and service lines running directly into each building. Once connected, supply is continuous. Propane, by contrast, relies on a trucking-based supply chain. Large distributors operate hundreds of storage plants and truck depots spread across their service areas, dispatching tanker trucks to refill individual customer tanks on a scheduled or on-demand basis.1European Journal of Operational Research. Integrating purchasing and routing in a propane gas supply chain A major distributor might deliver hundreds of millions of gallons per year to hundreds of thousands of individual sites, all by road.
This logistical reality affects both price and reliability. Propane prices spike during cold snaps partly because the delivery trucks can only move so fast, and storage depots in high-demand regions can run low. The 2014 propane shortage in the American Midwest, driven by a combination of unusually high demand for crop drying and a harsh winter, left some rural customers without fuel for days. Natural gas customers almost never face that scenario, because the pipeline system has enormous storage capacity built in through underground reservoirs and line pack. The tradeoff is that natural gas requires expensive permanent infrastructure, while propane’s truck-based model reaches places no pipeline ever will.
Altitude, Temperature, and Other Quirks
The 91-cubic-feet equivalence assumes standard conditions, usually defined as 60°F and sea-level atmospheric pressure. At higher altitudes, a cubic foot of natural gas contains fewer molecules because the air pressure is lower, which means it delivers slightly less energy per cubic foot. A home in Denver (about 5,280 feet above sea level) receives natural gas that has about 3% to 5% less energy per cubic foot than the same gas measured at sea level. The effect is small but cumulative over a heating season. Propane stored as a liquid in a pressurized tank is less affected by altitude, since the tank maintains its own internal pressure regardless of the atmosphere outside.
Temperature matters more for propane than for natural gas. Liquid propane must vaporize before it can burn, and its ability to vaporize depends on temperature. At around -44°F, propane’s vapor pressure drops to essentially zero, meaning the liquid will not produce gas. In extremely cold climates, propane tanks can struggle to deliver enough vapor to keep appliances running, especially if the tank is nearly empty (less liquid surface area means slower vaporization). Natural gas does not have this problem because it arrives as a gas and remains well above its boiling point of -259°F under any conditions found on Earth’s surface. For homes in very cold regions, propane tanks are sometimes buried underground to take advantage of the earth’s relatively stable temperature, or equipped with vaporizer units that add heat to force the propane into gas form.
Environmental Differences Between the Two Fuels
Both propane and natural gas are fossil fuels that produce carbon dioxide when burned, but their carbon footprints are not identical. Natural gas (methane) produces about 117 pounds of COâ‚‚ per million BTU when combusted. Propane produces about 139 pounds of COâ‚‚ per million BTU. On a per-energy basis, natural gas is roughly 16% cleaner in terms of direct combustion emissions.
That gap narrows or even reverses, however, once you account for methane leakage in the natural gas supply chain. Methane is a far more potent greenhouse gas than COâ‚‚ over short time horizons. Leaks from wells, processing plants, pipelines, and local distribution systems release uncombusted methane directly into the atmosphere. Estimates of the total leak rate vary, but even a few percent of total production leaking as methane can substantially offset natural gas’s combustion advantage over other fossil fuels. Propane’s supply chain leaks are less climatically significant because propane is not a strong greenhouse gas itself, and it is transported as a liquid in sealed containers rather than through miles of aging pipe with countless potential leak points.
For homeowners weighing the environmental angle, the honest answer is that neither fuel is dramatically better or worse than the other once the full supply chain is considered. The real emissions reduction comes from electrification powered by clean electricity, not from choosing between two fossil fuels. But where a fossil fuel is necessary, the choice between propane and natural gas is driven more by infrastructure availability and cost than by any meaningful environmental distinction.
When the “91 Cubic Feet” Rule Breaks Down
The conversion factor of roughly 91 cubic feet of natural gas per gallon of propane is an energy equivalence, not a performance equivalence. In a few real-world scenarios, the two fuels do not behave as interchangeably as the BTU math suggests.
Tankless water heaters are a common example. A high-output tankless water heater designed for natural gas might be rated for an input of 199,000 BTU per hour. To deliver that on propane, the unit needs the same BTU input, but the smaller propane orifices and higher operating pressure create different flow dynamics. Some tankless models run slightly less efficiently on propane than on natural gas, meaning you burn a bit more fuel per gallon of hot water than the BTU equivalence would predict. The gap is usually small, maybe 1% to 3%, but it is real and it compounds over thousands of hours of operation.
Whole-house generators present a similar wrinkle. A standby generator rated at 22 kW on natural gas might only produce 20 kW on propane, even though propane is more energy-dense. The reason is that the engine’s fuel system is tuned for one fuel or the other, and the air-fuel mixture, combustion timing, and exhaust characteristics differ between the two gases. Dual-fuel generators handle this by adjusting automatically, but the rated output on propane is almost always a few kilowatts lower than on natural gas. Manufacturers list both ratings on the spec sheet for exactly this reason.