Propane vs Natural Gas: BTU Content, Pressure, and Conversions
Propane vs natural gas for appliances — heating value per cubic foot, specific gravity, delivery pressure, orifice conversions, supply, and cost units.
Updated August 22, 2026
Natural gas and propane burn in the same kinds of appliances, but they are chemically different fuels with numbers far enough apart that nothing sizes interchangeably. A cubic foot of propane vapor carries roughly two and a half times the heat of a cubic foot of natural gas, propane vapor is half again heavier than air while natural gas is lighter, and the two are delivered to appliances at different standard pressures — three differences that between them explain every rule about orifices, regulators, pipe tables, and tank placement.
The supply model is the other half of the decision. Natural gas arrives continuously through a utility main and is billed by the therm; propane is trucked to an on-site tank governed by NFPA 58 and bought by the gallon. Where a main runs past the house the utility usually wins on convenience; where it does not, propane is how a rural building gets gas appliances at all.
The natural-gas property cells below are mapped in code from the NFPA 54 table constants inside the gas pipe sizing calculator; the propane column traces to the PERC Propane Technical Pocket Guide, read directly. A cross-check test holds the mapped cells to those exports.
Side by Side
| Spec | Natural gas | Propane (LP) |
|---|---|---|
| Heating value per cubic foot of vapor | 1000 Btu/ft³ — the NFPA 54 sizing-table default encoded in the gas pipe calculator (the PERC guide lists 1,012 for pure methane) | 2,488 Btu/ft³ (PERC Propane Technical Pocket Guide, Table 1A) — about 2.5× natural gas, which is why every orifice and table differs |
| Specific gravity & behavior in air | 0.6 (air = 1.0), the NFPA 54 table basis — lighter than air, so escaped gas tends to rise and disperse upward | 1.5 vapor (PERC Table 1A; the guide’s sizing tables use 1.52) — heavier than air, so escaped vapor tends to flow downward and collect in low spots, the behavior NFPA 58’s placement rules are written around |
| Typical delivery pressure basis (in. w.c.) | Low-pressure NFPA 54 sizing tables assume inlet under 2 psi with a 0.5 in w.c. allowable drop — the basis the pipe calculator encodes | Undiluted propane is tabled at 11 in w.c. supply with the same 0.5 in w.c. drop (PERC guide table header) — a two-stage regulator train steps tank pressure down to it |
| Appliance conversion requirementsConvert only with the appliance manufacturer’s listed kit and instructions; some appliances are not convertible at all. | An LP appliance converted to natural gas needs larger orifices and the manufacturer’s listed conversion kit — never a drilled-out guess | A natural-gas appliance converted to LP needs smaller orifices, the listed kit, and the right regulator setting; propane also demands far more combustion air per cubic foot of gas (23.68 vs 9.57 ft³ of air, PERC Table 1A), which the kit’s engineering already accounts for |
| Supply & storage | Utility pipeline: continuous supply, no on-site storage, service ends where the distribution network does | On-site tank sized to the load and refilled by delivery truck; storage, placement, and handling fall under NFPA 58, the LP-Gas Code — plan deliveries and monitor tank level in cold snaps |
| Pricing units & comparison basis | Billed by the therm — 100,000 Btu — so dollars per therm is already dollars per unit of heat | Sold by the gallon at 91,502 Btu (PERC Table 1A), so one gallon ≈ 0.92 therm: divide the per-gallon price by 0.915 to compare per-therm — the unit math, not a price claim; local prices decide the winner |
| Sizing-table consequences | Pipe sized from the natural-gas tables (or Spitzglass/Weymouth equations) at 0.60 specific gravity — the gas pipe calculator’s embedded table | Pipe sized from the separate LP tables at LP gravity and 11 in w.c.; the higher Btu content means smaller pipe for the same appliance load, but only the LP tables say how much |
Which One for the Job
Rural new build beyond the reach of the gas main
Propane (LP)
Where no distribution main exists, this is not a comparison — propane is the way a home off the network runs gas heat, cooking, and hot water. A properly sized tank under NFPA 58 siting rules, a two-stage regulator system, and LP-rated appliances (or listed conversions) deliver everything the utility customer gets, with delivery logistics as the ongoing cost of independence.
Standby generator fuel for a house that has a gas main
Natural gas
A generator exists for outages, and the utility main keeps supplying gas through a power failure without any stored fuel to run down — no tank level to remember during a week-long event. Propane’s counterargument is independence from the pipeline itself and indefinite shelf life, which matters where mains are absent or earthquake-prone; but with sound utility service, the main is the simpler fuel. Size the gas line for generator load either way.
Converting a natural-gas range to run on propane
Either
Routine, if done by the book: most ranges ship LP-convertible with a listed kit that swaps orifices and resets the regulator, because propane’s 2,488 Btu/ft³ at 11 in w.c. would badly overfire a burner jetted for 1,000 Btu/ft³ gas. The verdict is "either fuel runs the appliance" — never "either orifice": an unconverted appliance on the wrong fuel is a soot, flame-size, and CO hazard.
Frequently Asked Questions
Why does propane use smaller orifices than natural gas?
Because each cubic foot of propane carries about 2.5 times the heat — 2,488 Btu versus roughly 1,000 for natural gas per the PERC guide and NFPA 54 table basis — and it arrives at a higher standard pressure (11 in w.c. versus the utility’s low-pressure supply). To deliver the same Btu per hour, the burner must pass far less propane volume, so the metering hole shrinks. That is why conversion is a listed-kit job: the orifice, regulator, and air settings all move together.
Is propane more dangerous than natural gas in a leak?
They fail differently. Natural gas at 0.6 specific gravity rises and tends to disperse; propane vapor at 1.5 sinks and can collect in basements, pits, and low corners — the pattern NFPA 58’s placement and appliance-location rules are built around. Both are odorized so leaks announce themselves. Treat any gas odor identically: no switches, no flames, leave, and call from outside. The heavier vapor simply means propane spaces deserve extra respect at floor level.
How do I compare propane and natural gas prices fairly?
Convert both to dollars per unit of heat. Natural gas is billed by the therm — 100,000 Btu by definition — while propane sells by the gallon carrying 91,502 Btu per the PERC guide. So a gallon of propane is about 0.915 therm: divide the propane price per gallon by 0.915 and set it beside the utility’s per-therm rate, remembering to include the utility’s fixed monthly charges and the propane dealer’s tank fees. Which fuel wins is entirely a local-price question.
Can the same pipe system serve either propane or natural gas?
The physical pipe usually can, but the sizing math cannot be reused. NFPA 54 publishes separate capacity tables: natural gas at 0.60 specific gravity on low-pressure supply, undiluted propane at 1.52 gravity and 11 in w.c. — and propane’s higher Btu density generally allows smaller pipe for the same appliance load. Switching fuels on an existing system means re-verifying every run against the correct table, along with regulators and appliance orifices.
Try the Calculators
Sources
- PERC Propane Technical Pocket Guide — Table 1A gas properties (2,488 Btu/ft³, SG 1.5, 91,502 Btu/gal, combustion-air ratios) and the undiluted-propane sizing table at 11 in w.c. / 0.5 in w.c. drop / 1.52 SG (read directly)
- NFPA 54 National Fuel Gas Code — sizing-table bases for natural gas (0.60 SG, low-pressure supply) and undiluted propane, as encoded and provenance-documented in the gas pipe sizing calculator
- NFPA 58 Liquefied Petroleum Gas Code — LP storage, tank placement, and handling jurisdiction (cited for scope; no numeric setbacks quoted)