How many BTU are in a gallon of propane?

A US gallon of liquid propane holds about 91,500 BTU. Dividing an appliance's BTU/h rating by 91,500 gives the gallons consumed per hour at full fire, which is the basis for daily and monthly use estimates.

Why does propane need a smaller orifice than natural gas?

Propane has roughly 2.5 times the heating value per cubic foot of natural gas, so far less volume must flow for the same BTU/h. Burner orifices are sized smaller and the regulator runs higher pressure, which is why an appliance must be converted, not just connected, when switching fuels.

How is tank size chosen?

Propane tanks are filled to about 80 percent to leave vapor space, so usable volume is 0.8 times the nameplate. The tool picks the smallest standard tank — 100 up to 1000 gallon — whose usable volume covers one month of estimated use for a roughly 30-day refill cycle.

What run-time figure should I enter?

Use an effective full-load-equivalent, not clock hours, because appliances cycle and rarely run at full input. For a whole house, an estimate of a few full-load hours per day across heating season is typical; size conservatively if you cannot get a clear figure.

Can I run a natural gas appliance on propane unchanged?

No. Without changing the orifice and regulator, a natural-gas appliance fed propane will over-fire badly, producing soot, carbon monoxide, and a fire hazard. Always install the manufacturer's propane conversion kit and verify the manifold pressure.

LPG / Propane Conversion & Sizing Calculator

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 use

Tanks 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.