Gas Line Sizing Calculator — Free Online Calculator
The IFGC Table 402.4 method selects the minimum Schedule 40 iron pipe size (IPS) that delivers enough gas capacity to your appliance without exceeding the 0.5 in.w.c. allowable pressure drop. Enter the total appliance BTU/h input rating and the total equivalent pipe run length to get the correct pipe size for natural gas or LP at low or medium supply pressure.
Enter gas line parameters
Use the appliance nameplate input BTU/h rating. For multiple appliances on one run, add all loads.
Use total equivalent length (TEL) — straight pipe plus equivalent length of fittings. Add ~25% to straight-pipe measurement for typical residential piping.
Minimum pipe size (IPS)
Capacity at selected size (BTU/h)
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Approx. gas velocity (ft/min)
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Engineering review required
The required load exceeds the capacity of 2" Schedule 40 pipe at this run length. A pipe larger than 2" or a pressure regulator upgrade is needed — consult a licensed gas engineer.
Results based on IFGC Table 402.4 (Schedule 40 iron pipe, natural gas, 0.5 in.w.c. drop, 0.60 SG) with linear interpolation between table lengths. LP capacity adjusted by factor 0.634. Always verify with the AHJ and a licensed gas fitter.
IFGC Table 402.4 — Natural gas capacity (1,000 BTU/h)
Schedule 40 metallic pipe, 0.5 in.w.c. pressure drop, natural gas SG = 0.60. Source: IFGC Table 402.4 / NFPA 54 Appendix C.
| Length | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" |
|---|---|---|---|---|---|---|
| 10 ft | 103 | 199 | 372 | 673 | 1,000 | 1,902 |
| 20 ft | 72 | 138 | 258 | 468 | 693 | 1,319 |
| 30 ft | 58 | 112 | 209 | 379 | 562 | 1,070 |
| 50 ft | 44 | 85 | 159 | 289 | 428 | 815 |
| 75 ft | 36 | 69 | 129 | 235 | 348 | 662 |
| 100 ft | 31 | 59 | 111 | 202 | 300 | 571 |
| 150 ft | 25 | 48 | 90 | 163 | 242 | 461 |
| 200 ft | 21 | 41 | 77 | 140 | 208 | 396 |
Capacities are in thousands of BTU per hour. LP gas capacity = natural gas capacity × 0.634. Medium pressure (3 in.w.c.) capacity ≈ natural gas capacity × 2.0.
How the Gas Line Sizing Calculator Works
The IFGC table method selects the smallest pipe size whose listed capacity at the given run length equals or exceeds the required appliance load. The table is based on the Spitzglass formula for gas flow through pipes with a 0.5 in.w.c. allowable pressure drop. Capacities are interpolated linearly between the standard table lengths (10, 20, 30, 50, 75, 100, 150, 200 ft). LP and medium-pressure adjustments are applied as multipliers to the base natural gas table values.
Total equivalent length (TEL)
IFGC requires using total equivalent length, not just straight pipe footage. Each fitting adds resistance equivalent to several feet of straight pipe. A 90° elbow in 3/4" pipe adds roughly 1.5 ft equivalent. Add 20–30% to your measured straight length for a typical residential installation to account for fittings without performing a full fitting count.
Critical path sizing
When multiple appliances share a trunk, the critical path is the run to the appliance farthest from the meter with the highest simultaneous demand. Size the trunk for total connected load at the longest run. Each branch is sized for the load it serves at its own TEL from the last tee.
LP gas sizing
LP (propane) has a specific gravity of approximately 1.53 vs. 0.60 for natural gas. The heavier gas flows more slowly through the same pipe at the same pressure drop, resulting in lower BTU/h capacity per pipe size. The IFGC adjustment factor is 0.634 × the natural gas table value for the same pipe size and length.
Medium vs. low pressure
Low-pressure systems (under 0.5 psi) use the 0.5 in.w.c. drop tables. Medium-pressure systems (0.5–2 psi) can use a larger allowable pressure drop, significantly increasing pipe capacity and allowing smaller pipe sizes. A dedicated pressure regulator at the appliance is required when dropping from medium to low pressure at the appliance connection.
Worked example
A single-family home has a 100,000 BTU/h furnace and an 80,000 BTU/h water heater on the same gas trunk. The meter is 40 ft from the furnace (the farthest appliance). The plumber estimates 10 ft of equivalent fitting length for elbows and tees, making TEL = 50 ft.
Total load and TEL
Straight run = 40 ft; fittings equivalent = 10 ft
TEL = 40 + 10 = 50 ft
Look up pipe size at 50 ft
1" pipe → 159,000 BTU/h — insufficient (need 180,000)
1-1/4" pipe → 289,000 BTU/h — sufficient
Result: Use 1-1/4" Schedule 40 IPS for the main trunk. The 1-1/4" pipe has 289,000 BTU/h capacity at 50 ft, giving a 61% safety margin above the 180,000 BTU/h demand.
Branch sizing — water heater
Branch TEL from last tee = 15 ft
IFGC at 10–20 ft interpolation for 3/4": 170,000 BTU/h at 15 ft — sufficient
Result: Water heater branch uses 3/4" Schedule 40 IPS. The trunk is 1-1/4" from the meter to the first tee, then branches reduce as loads decrease toward the appliances.
Frequently asked questions
Common questions about gas line sizing, IFGC tables, and pipe selection.
What IFGC table does this use?
This calculator uses IFGC Table 402.4 (Schedule 40 metallic pipe) for natural gas at 0.5 in.w.c. pressure drop and 0.60 specific gravity. The same table appears in NFPA 54 Appendix C. Always verify with the authority having jurisdiction (AHJ) — local codes may require a different table or pressure drop value.
Does pipe length include fittings?
IFGC uses total equivalent length (TEL) — straight pipe plus the equivalent length of all fittings. A standard elbow adds 1–2 ft equivalent, a tee adds 3–5 ft. Add 20–30% to measured straight length to account for fittings, or use the fitting equivalent table in IFGC Appendix C.
Should I size for just the furnace or all appliances?
Size each branch for the appliances it serves. Size the main trunk for the total connected load of all simultaneously operating appliances (typically all appliances). The longest most-demanding run is the critical path — size the full pipe tree from the meter to each appliance.
Can I use copper or CSST for gas lines?
IFGC Tables 402.5 (copper) and 402.9 (CSST) give capacities for those materials — capacities differ from iron pipe. This calculator uses Schedule 40 iron pipe (IPS) values only. CSST generally has higher capacity than iron pipe of the same nominal size due to lower resistance per foot.
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