Gas pipe size calculator
Gas sizing has one rule that catches people: the longest-length method sizes the entire system off the single longest run, not off each branch separately. Add up every appliance that can fire at the same time, measure to the outlet furthest from the meter, and read one size for the whole system.
Pre-filled with a furnace, water heater, range and dryer on a 60 ft longest run.
Minimum size for the whole run
- Demand
- 197CFH
- Length bucket used
- 60ft
- Table capacity at that length
- 400CFH
- Heating value assumed
- 1,050BTU / ft³
| Size | 20 ft | 40 ft | 60 ft | 100 ft | 150 ft |
|---|---|---|---|---|---|
| 1/2" | 90 | 62 | 50 | 38 | 31 |
| 3/4" | 188 | 129 | 103 | 78 | 63 |
| 1" | 353 | 243 | 195 | 148 | 119 |
| 1-1/4" | 726 | 499 | 400 | 304 | 244 |
| 1-1/2" | 1090 | 747 | 600 | 455 | 366 |
| 2" | 2090 | 1440 | 1160 | 877 | 704 |
How it sizes
The table is indexed by the length to the furthest outlet and read against demand in cubic feet per hour. Natural gas is taken at 1,050 BTU per cubic foot, so a 207,000 BTU/h connected load is about 197 CFH. The length column rounds up to the next published bucket — 60 ft of run reads the 60 ft column, 61 ft reads 80.
Every size in the system is then that one size, or larger. You may reduce at a branch only if you re-size that branch on its own longest length, which on a house is rarely worth the fittings.
Simultaneous, not total
Worked example
An 80,000 BTU furnace, a 40,000 BTU water heater, a 65,000 BTU range and a 22,000 BTU dryer come to 207,000 BTU/h, or 197 CFH. The dryer sits 58 ft of developed pipe from the meter, so the 60 ft column applies. Schedule 40 at 60 ft carries 195 CFH in 1" — one CFH short. The system goes to 1-1/4".
That is not pedantry. Two CFH of margin on paper is what absorbs a future tankless swap, a regulator that drifts, or a run that measures longer than the drawing.
Code notes
NFPA 54 IFGC The capacity tables are identical in substance between the National Fuel Gas Code and the International Fuel Gas Code. CSST is sized by equivalent hydraulic diameter (EHD), not by nominal size, and every manufacturer publishes its own EHD-to-part mapping — check the one in the box. Bonding of CSST is a separate requirement and is enforced.
Code violation