Propane and natural gas are not interchangeable at the burner, and propane is billed by the gallon, not the cubic foot. This calculator converts appliance BTU/h into propane gallons, sizes a storage tank, and shows why the orifice must change when you switch fuels.
How it works
Every figure flows from two energy constants — propane carries far more heat per unit volume than natural gas:
gallons/hour = appliance BTU/h / 91,500 (BTU per gallon propane)
gallons/month = gallons/hour × hours/day × 30
volume ratio = 2,516 / 1,020 ≈ 2.47 (propane needs ~2.5× less volume)
tank size = smallest standard tank where nameplate × 0.80 ≥ monthly useTanks are filled to 80 percent for vapor space, so usable volume is always less than the nameplate. The volume ratio is why a propane orifice is smaller than a natural-gas one for the same heat output.
Example and notes
A 75,000 BTU/h appliance running an effective 8 hours a day burns about 6.6 gallons of propane daily and roughly 197 gallons a month, which a 250-gallon tank covers comfortably at an 80 percent fill on a 30-day cycle. The same load needs about 74 CFH on natural gas but only 30 CFH on propane — the same heat in far less gas volume. Never run a natural-gas-orificed appliance on propane without the proper conversion kit and regulator; the over-firing is dangerous.
Understanding the 91,500 BTU/gallon figure
Propane’s energy density is a physical constant: one US gallon of liquid propane releases approximately 91,500 BTU when burned. This figure comes from propane’s heat of combustion and its liquid density. A common error is confusing this with propane’s heating value per cubic foot of vapor (about 2,516 BTU/ft³). The two measures coexist because propane is stored as a liquid and burned as a vapor after it vaporizes through the regulator. The gallon figure is the right one for sizing a tank and planning a delivery; the BTU/ft³ figure is what appliance manufacturers use when specifying orifice sizes.
How propane delivery and pricing work
Propane is typically delivered by truck and priced per gallon, which fluctuates with wholesale commodity prices. Residential propane prices vary by season (higher in winter), region, and whether you own or rent the tank. Some suppliers offer price-lock programs for the heating season. Knowing your monthly consumption in gallons — which this calculator provides — lets you negotiate delivery frequency, pre-purchase contracts, and whether an automatic delivery schedule makes sense for your usage pattern.
Tank sizing in practice
The standard residential propane tank sizes are 100, 120, 250, 500, and 1,000 gallons (nameplate). Propane suppliers fill tanks only to 80% of capacity to leave a vapor space that prevents pressure buildup as temperature changes. A 250-gallon tank holds usable propane of about 200 gallons. As a rule of thumb:
- A 100-gallon tank suits a single water heater or cooking range
- A 250-gallon tank suits a water heater plus a cooking range
- A 500-gallon tank handles whole-home heating in a mild climate or supplemental heat in a cold one
- A 1,000-gallon tank is needed for high-demand homes in cold climates or for farm and commercial use
The calculator recommends the smallest standard tank that covers one month of estimated use, which generally means two to three deliveries per year — a typical arrangement with most suppliers.
The orifice and regulator: why conversion matters
Natural gas appliances use a two-stage regulator delivering about 3.5 inches of water column pressure, and orifices sized for the lower BTU density of gas. Propane appliances use a regulator set to around 11 inches WC and smaller orifices to compensate for propane’s higher energy density. Connecting a natural-gas appliance directly to propane with no conversion will cause it to fire at roughly 2.5 times its intended heat output, producing incomplete combustion, soot, and carbon monoxide. Always install the manufacturer’s conversion kit — it replaces the orifice and adjusts the regulator — and have a licensed technician verify manifold pressure after conversion.