PSI to BTU Calculator

Convert a gas pressure in psi to BTU per hour for natural gas or propane, either through a gas pipe of a given size and length or through a burner orifice.

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Calculator Functions

A pressure in psi has no BTU value of its own. The BTU per hour depends on how much gas that pressure pushes through something, so each function asks what the gas flows through: a run of pipe or a burner orifice.

Both functions work in psi, inches of water column or kPa, for natural gas or propane. To size a low-pressure line from a known BTU load, use the Gas Piping Size Calculator. To turn a metered gas flow into BTU per hour, use the Gas Pressure To Btu Calculator, and to pick a propane orifice for a target input, the Propane Orifice Size Calculator.

PSI to BTU Formulas

This is the fuel gas code's high-pressure equation for a supply of 1.5 psi or more [1]. The code writes it as the pipe diameter needed for a flow; solved for the flow, the exponent becomes 1 ÷ 0.381 = 2.625. Natural gas at 2 psi that may lose 1 psi along 50 ft of 1/2 in. Schedule 40 pipe has P₁ = 16.7 psia and P₂ = 15.7 psia, so X = 32.4 × 0.9992 ÷ (0.6094 × 50) = 1.0625 and Q = (18.93 × 0.622 × 1.0126)^2.625 = 669 ft³/h. At 1,000 BTU per cubic foot that is 669,000 BTU/h.

The heating value turns cubic feet into BTU [2]. The calculator uses 1,000 BTU per cubic foot for natural gas, the figure in the code's worked examples [2], and 2,488 for propane [8]. Enter your supplier's figure if you have it.

Low-pressure lines (under 1.5 psi)

Below 1.5 psi the code uses this equation, where ΔH is the pressure drop in inches of water column and 27.7 in. w.c. equal 1 psi [1]. The supply pressure doesn't appear in it: at low pressure the capacity depends on the drop the run is allowed, not on the pressure it starts from. A 7 in. w.c. supply that may lose 0.5 in. w.c. along 10 ft of 1/2 in. pipe carries 173 ft³/h of natural gas, or 173,000 BTU/h. The calculator switches equations at 1.5 psi, as the code does.

How the results compare with the code tables

The code also prints capacity tables for Schedule 40 pipe [1]. The natural gas tables are in cubic feet per hour and the propane tables in BTU per hour.

Most runs agree within 1%. The code's 2 psi and 5 psi natural gas tables follow a slightly different curve and can sit several percent from the equation, about 12% lower for the 1 in. run above. Where a table covers your run and the two differ, the lower figure is the cautious one.

BTU/h through a burner orifice

At appliance pressures the flow through an orifice follows this Bureau of Standards formula, where A is the orifice area in square inches, K the discharge coefficient, H the gas pressure in inches of water column and d the specific gravity of the gas, 0.60 for natural gas and 1.50 for propane [3]. It treats the gas as incompressible, which the Bureau found to be within 1% at city supply pressures [3]. K is the share of the ideal flow that a real orifice passes, and it depends on how the hole is made.

At pressures measured in psi the gas expands as it speeds up through the hole, so the calculator uses the compressible nozzle equations instead [4] [5]:

P₀ is the absolute pressure behind the orifice in psia, M the Mach number at the orifice and γ the ratio of specific heats: 1.30 for natural gas, taken as methane [6], and 1.13 for propane [7]. The constant 1609.8 carries the Bureau's velocity of 66.34 ft/s for 1 in. w.c. [3] up to atmospheric pressure, so at low pressure the two formulas agree. M can't pass 1: the flow goes sonic and chokes [4] once r falls to 0.546 for natural gas, at about 12.2 psi, or to 0.578 for propane, at about 10.7 psi. Above that point BTU/h rises in step with absolute pressure.

Propane at 10 psi behind a 0.035 in. orifice with K = 0.90 has P₀ = 24.7 psia and r = 0.595, which gives Mach 0.97, 17.8 ft³/h and 44,400 BTU/h. At 20 psi the same orifice is choked and passes 62,400 BTU/h. As a check at appliance pressure, a 0.010 in. orifice on propane at 11 in. w.c. comes to 786 BTU/h with K = 0.90, against 812 BTU/h in the National Fuel Gas Code's orifice table [8].

Why psi alone doesn't convert to BTU

Pressure is force on an area, and a BTU is an amount of energy. Pressure becomes energy only when it's multiplied by a volume: 1 psi acting through 1 cubic foot is 0.18505 BTU, so 1 BTU equals 5.404 psi·ft³, and 1 kPa acting through 1 cubic meter is 1 kJ, or 0.9478 BTU [9]. The Pressure Volume Energy Calculator does that conversion. It is a small number next to the heat in the fuel: a cubic foot of natural gas held at 2 psi stores 0.37 BTU as pressure and releases about 1,000 BTU when it burns. That is why a gas pressure turns into BTU per hour only through the flow it drives.

Limits of This Estimate

• The pipe equations assume smooth-walled pipe and the gas properties shown. Fittings, valves and regulators add resistance, so include an allowance for them in the length [2].
• The discharge coefficient is the largest unknown in the orifice result, since measured values run from 0.605 to about 0.91 [3]. The orifice method treats the gas as ideal at 60°F leaving into air at 14.7 psia, and it joins the Bureau of Standards constant to the nozzle equations, a combination no single reference tabulates. These estimates are based on the best available data and estimations.
• The heating values are nominal. Natural gas varies by supplier and propane by blend, and the result is fuel input, not the heat an appliance delivers.
• Gas piping and burner work fall under the local fuel gas code, which limits piping inside buildings to 5 psi unless listed conditions are met [1]. Use the result to check a design, not in place of sizing by a licensed installer.

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