Gas pipe sizing
Draw a tree of gas pipe sections and appliances and get a size for every section — natural gas or propane, schedule 40 steel, CSST, copper or polyethylene, at the usual supply pressures, by the longest-length or branch-length method — computed from the IFGC/NFPA 54 sizing equations and held at or under the code's tables, with the pressure left at each appliance and a capacity table laid out like the code's.
Added
Works with
- Plumbing sizing Building tradessizes the water and drain side of the same house
- Heat loss and HVAC sizing Home automation and devicesthe heating load that decides the furnace input you put on the gas line
- Pipe offsets and takeoffs Building tradesthe offsets and cut lengths for the runs you size here
How to use it
A learning and estimating aid, not a design. Gas piping must follow the code your jurisdiction has adopted (usually the International Fuel Gas Code or NFPA 54) and its amendments, the CSST maker’s installation instructions and the appliance rating plates — and in most places it needs a permit, a pressure test and a licensed gas fitter. Use this to understand the method and to check a layout, then have it done properly.
How to use it
At the top choose the gas and pressure (natural gas under 2 psi with a 0.5″ w.c. drop is the usual house system; propane after the second-stage regulator is 11″ w.c.), the pipe or tubing, the method and, if your supplier gives one, the heating value.
Piping — the tree. Every section starts at the meter (or regulator) or at the end of another section, and has a length in feet. Put an appliance at the end of a section with its input from the rating plate, or leave it blank for a tee. Add a typical appliance adds a section with a usual input, for a first sketch only. The results give every section’s load (Btu/h and cubic feet an hour), the length it is sized at, the size and what that size carries, and a drawing of the tree with the longest run marked. The last column is the pressure left at the end of each section with the sizes chosen, over each section’s own length — the worst appliance is shown at the top.
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Longest length (IFGC 402.4.1): every section is sized at the length from the meter to the most remote outlet.
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Branch length (IFGC 402.4.2): sections on that longest run likewise; each other section uses the length from the meter to the farthest outlet it feeds. Branches come out smaller.
A fitting allowance adds a percentage to every length if you measured the pipe only.
Capacity table — capacities for every size at lengths from 10 to 500 ft, laid out like the code’s tables (cubic feet an hour; thousands of Btu/h for propane), with the code table for those conditions named so you can compare. Give a load and a length to highlight the size it needs.
Btu ↔ cfh — appliance input in Btu/h, MBH or kW to cubic feet an hour, cubic metres an hour and therms.
Method — the steps and the equations.
How it is worked out
Instead of transcribing the tables, the tool solves the equations they come from — IFGC Section 402.4, the same as NFPA 54 — for the flow each size carries:
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under 1½ psi (Equation 4-1): D = Q0.381 ÷ [19.17 (ΔH ÷ (Cr L))0.206];
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1½ psi and above (Equation 4-2): D = Q0.381 ÷ [18.93 P12 − P22) Y ÷ (Cr L0.206],
with D the inside diameter (in), Q in cubic feet an hour, ΔH the drop in inches of water column, P1 and P2 absolute pressures (psia), L in feet, and Cr and Y from IFGC Table 402.4: 0.6094 and 0.9992 for natural gas, 1.2462 and 0.9910 for propane. Appliance inputs are divided by the heating value (1,000 Btu/ft³ for natural gas and 2,516 for propane unless you set your own).
Checked against the 2018 IFGC tables, the bare equations agree within about 2 % for schedule 40 steel, copper and polyethylene at low pressure and for the propane tables; ½″ steel carries 173 cfh over 10 ft against 172 in Table 402.4(2). But the printed tables run a little under the equations — 1–2 % from rounding, and up to 12 % for the 2 psi and 5 psi natural-gas steel tables, 402.4(5) and 402.4(7), at short lengths — and an equation alone could then pick a smaller pipe than the table. So the tool never uses more than the table allows: like someone reading a table, it rounds each length up to the next row (55 ft is read at 60 ft), and it scales the equation by that row’s smallest table ÷ equation ratio across the sizes, worked out from the 2018 tables for every pressure and material the code tabulates. Every capacity is then at or under the printed figure — the tests check over 700 table entries from 10 to 500 ft — and at most a few per cent under for most sizes (up to 12 % on 2 psi and 5 psi steel). A pipe with no code table at that pressure (copper or polyethylene at 3″ w.c. drop or 5 psi, polyethylene at 10 psi) borrows the steel factors for the same pressure. One printed cell is treated as a misprint and left out: Table 402.4(36), 3″ polyethylene at 400 ft, shows 12,000 between 21,500 at 350 ft and 18,700 at 450 ft.
Corrugated stainless steel tubing has no such equation, and each maker publishes its own tables. Here it uses Q = K (ΔH ÷ (Cr L))b with K and b fitted for each flow designation (EHD) to the generic CSST tables, IFGC 402.4(15)–(19) and (32)–(34), then lowered so the result sits at or a little under them (typically 5–10 % under). For the tubing you install, the maker’s table governs; the generic tables include four 90° bends and two end fittings per run, and 1.3 ft is added for each one more.
Copper tubing uses the inside diameters the code tables list (type K, the smaller of K and L); polyethylene uses the SDRs in the tables (SDR 9 for ½″, SDR 10 for 1¼″, SDR 11 otherwise).
Limits
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Hybrid-pressure systems (2 psi to a line regulator, then low pressure) are sized in two parts: size the high-pressure part to the regulator as one tree, then each low-pressure part from its regulator. Regulator capacity, lock-up and venting are not covered.
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No diversity: every appliance is assumed to run at full input at once, as the code does unless a diversity can be shown.
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No altitude correction (the code asks for one above 2,000 ft), no appliance connectors, no combustion air, no venting.
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Smooth-pipe capacities are deliberately at or under the 2018 IFGC tables, so near a size boundary the tool can choose one size larger than the table would (a load exactly at a table figure, for example). Past the longest row (950 ft) it uses the equation at your length with the case’s smallest factor. Other editions of the code, or NFPA 54, may print slightly different figures.
Related: plumbing sizing, heat loss and HVAC sizing, pipe offsets and takeoffs, pumps, tanks and hot water (tankless heater inputs).
Linking to an example
The tree itself travels in Share link's compressed ?s=; the settings can be written by hand.
| Parameter | Meaning | Example |
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gas and pressure: |
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heating value, Btu/ft³ |
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fitting allowance, per cent |
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appliance inputs in |
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one of the presets below; other parameters override it |
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Presets:
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house-steel— a house on natural gas in schedule 40 steel, longest-length method -
house-branch— the same house by the branch-length method -
house-csst— the same house in CSST -
propane-cabin— a propane cabin in copper tubing from the second-stage regulator -
two-psi— a 2 psi CSST trunk to a line regulator feeding a big load
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