About Gas Pipe Sizing Calculator (NFPA 54)
The gas pipe sizing calculator selects Schedule 40 steel pipe for low-pressure natural gas systems (inlet below 2 psi, 0.5 in w.c. pressure drop, 0.60 specific gravity) using the longest-length method from NFPA 54 (the National Fuel Gas Code) and the IFGC. Enter the total connected load in BTU/hr - or build it from an appliance list with presets for furnaces, water heaters, ranges, dryers, fireplaces, and tankless heaters - plus the longest run from the meter to the farthest outlet.
The tool converts the load to gas demand in cubic feet per hour (CFH = BTU/hr divided by the heating value, typically 1000 BTU/ft³ for natural gas), rounds the longest run up to the next tabulated length, and picks the smallest pipe size whose Table 402.4(2) capacity meets the demand. It reports the utilization percentage, the next size up, the full capacity list at that length, and a cross-check against the low-pressure Spitzglass equation.
How It Works
- Enter the total connected load in BTU/hr, or add up to six appliances from the presets; the tool sums their input ratings.
- Enter the longest developed run in feet from the gas meter to the farthest appliance; the calculator rounds it up to the next NFPA 54 table length (10 to 200 ft).
- Gas demand is computed as CFH = BTU/hr / heating value (default 1000 BTU/ft³ for natural gas).
- The smallest Schedule 40 size whose table capacity at that length meets the demand is selected, with utilization %, the next size up, and every size's capacity shown alongside the Spitzglass formula value.
Worked Example
A house has a 40,000 BTU/hr water heater, a 100,000 BTU/hr furnace, a 65,000 BTU/hr range, and a 35,000 BTU/hr dryer, for a total connected load of 240,000 BTU/hr. With natural gas at 1000 BTU/ft³ the demand is 240 CFH. The longest run from the meter to the farthest appliance is 30 ft, which is already a table length. At 30 ft the NFPA 54 capacities are 95 CFH for 1/2", 199 CFH for 3/4", and 374 CFH for 1", so the smallest adequate trunk size is 1" pipe at 240 / 374 = 64 % utilization, with 1-1/4" (768 CFH) as the next size up.
Formulas
- Gas demand
Q_demand = load / HV- Longest-length method
L_table = min{ L in table : L >= longest run }; size = smallest with Q_table(L_table) >= Q_demand- Spitzglass low-pressure equation
Q = 3550 * sqrt( dH * d^5 / ( SG * L * (1 + 3.6/d + 0.03*d) ) )- Utilization
utilization % = 100 * Q_demand / Q_capacity
Standards & References
- NFPA 54 / ANSI Z223.1 National Fuel Gas Code, Table 402.4(2)
- IFGC (International Fuel Gas Code) Section 402, longest-length method
- Spitzglass low-pressure flow equation
Frequently Asked Questions
How do I size a natural gas pipe?
Add up the BTU/hr input ratings of every appliance the pipe section serves, convert to cubic feet per hour by dividing by the gas heating value (about 1000 BTU/ft³ for natural gas), and measure the longest developed run from the meter to the farthest outlet. Then read the NFPA 54 / IFGC capacity table at that length - rounding the length up, never down - and pick the smallest pipe size whose capacity meets the demand. This calculator automates all of those steps.
What is the longest-length method?
It is the standard sizing method in NFPA 54 and the IFGC: every section of the system is sized using the table column for the total developed length from the meter to the farthest appliance, even for branches that are physically shorter. This is conservative and guarantees the design pressure drop of 0.5 in w.c. is not exceeded anywhere. Alternative methods (branch-length, hybrid pressure) can give smaller pipe but require more engineering care.
How many BTU can a 3/4 inch gas line carry?
It depends on the run length. Per NFPA 54 Table 402.4(2) for Schedule 40 steel pipe at 0.5 in w.c. drop, 3/4" carries 360 CFH (about 360,000 BTU/hr) at 10 ft, 247 CFH at 20 ft, 151 CFH at 50 ft, and 105 CFH at 100 ft. That is why a tankless water heater at 199,000 BTU/hr often needs a dedicated 3/4" or even 1" line when the run is long.
Does this calculator work for propane (LP gas) or CSST?
The embedded table is specifically for natural gas at 0.60 specific gravity in Schedule 40 steel pipe. Propane has a heating value around 2500 BTU/ft³ and a specific gravity near 1.52, and it is normally distributed at 11 in w.c. with its own NFPA 54 tables, so do not use this table for LP beyond a rough demand estimate via the editable heating value. CSST is also sized from the manufacturer's tables because its flow characteristics differ from rigid pipe by EHD (equivalent hydraulic diameter).
What is the Spitzglass equation and why does it differ from the table?
The Spitzglass equation, Q = 3550 sqrt(dH d^5 / (SG L (1 + 3.6/d + 0.03 d))), is the classical low-pressure flow formula for fuel gas at pressures below 1.5 psi. The code tables are generated from the NFPA 54 low-pressure equation with standardised roundings, so Spitzglass values shown here run somewhat conservative for small diameters. The table values govern for code compliance; the formula is shown as an engineering cross-check.
What pressure drop should I design for?
For typical low-pressure residential and commercial systems delivered at 6 to 7 in w.c., the standard design pressure drop is 0.5 in w.c., which keeps appliance inlet pressures within their rated range. A 0.3 in w.c. drop is used where the supply pressure is marginal, and 2 psi systems with pounds-to-inches regulators use entirely different (much higher capacity) tables. This calculator uses the standard 0.5 in w.c. table.