The short answer
A house with an 80,000 Btu furnace, 40,000 Btu water heater, 65,000 Btu range and 22,000 Btu dryer (207,000 Btu in all), 60 ft from the meter, needs a 1 in. Schedule 40 steel main on natural gas, or 3/4 in. on propane. Each branch is sized for its appliance at the same 60 ft.
Natural gas capacity by pipe size and length
Cubic feet an hour (about 1,000 Btu each) of natural gas, Schedule 40 steel, from the code's low-pressure equation (below 1-1/2 psi) with a 0.5 in. water column pressure drop.
| Length | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" |
|---|---|---|---|---|---|---|
| 10 ft | 173 | 362 | 682 | 1,401 | 2,100 | 4,046 |
| 20 ft | 119 | 249 | 469 | 963 | 1,443 | 2,781 |
| 30 ft | 96 | 200 | 377 | 774 | 1,159 | 2,234 |
| 40 ft | 82 | 171 | 322 | 662 | 992 | 1,912 |
| 50 ft | 72 | 152 | 286 | 587 | 880 | 1,695 |
| 60 ft | 66 | 137 | 259 | 532 | 797 | 1,536 |
| 80 ft | 56 | 118 | 222 | 455 | 682 | 1,314 |
| 100 ft | 50 | 104 | 196 | 403 | 605 | 1,165 |
| 150 ft | 40 | 84 | 158 | 324 | 486 | 936 |
| 200 ft | 34 | 72 | 135 | 277 | 416 | 801 |
How it works
The formula
A gas line is big enough when the gas loses no more than a set pressure on its way to the farthest appliance at full load. The fuel gas code's low-pressure equation gives the flow a pipe carries for that loss.
capacity = (diameter × 19.17 × (drop ÷ (Cr × length))^0.206)^(1 ÷ 0.381)
Assumptions
- The equation and gas factors (Cr 0.6094 natural gas, 1.2462 propane) are IFGC Equation 4-1 and Table 402.4, as adopted in the 2021 Seattle Fuel Gas Code. The code allows sizing by the equation or by its tables; the tables round down, so for a load right at a pipe's limit they can call for the next size up (for propane, by about 2%).
- Pressure drop 0.5 in. water column. The equation covers systems below 1-1/2 psi: natural gas at the usual house pressure, and propane after the second-stage regulator at 11 in. water column. Higher-pressure systems use a different equation.
- Natural gas at 1,000 Btu per cubic foot, slightly lean so the pipe errs large; propane at 2,516. Your gas bill or supplier lists the real figure.
- All appliances are assumed on at once, as the code requires unless a smaller diversified load is established. Above 2,000 ft elevation the code adjusts the flow.
- Length is the straight run; elbows and tees add some, so round the length up.
Worked example: 207,000 Btu, 60 ft
- 207,000 ÷ 1,000 = 207 cu ft an hour
- 3/4" at 60 ft carries 137: too small
- 1" main carries 259; the 22,000 Btu dryer branch takes 1/2"
Too big or too small
- Too small: appliances starve. With everything on, the far appliance gets too little pressure: weak flames, sooting and lockouts.
- Adding an appliance? A new range, generator or pool heater adds load to the whole run back to the meter; re-check the main, not only the new branch.
- Check the meter too. The gas company's meter and regulator have a capacity; tell them when your connected load goes up.
Questions people ask
What size gas line do I need?
It depends on the total Btu of everything on the line and the length to the farthest appliance. A 207,000 Btu house (furnace, water heater, range and dryer) 60 ft from the meter needs a 1 in. main of Schedule 40 steel on natural gas. Each branch is sized for its own appliance at the same length.
How many Btu does a 1/2 inch gas line carry?
On natural gas at a 0.5 in. pressure drop: about 173,000 Btu an hour over 10 ft, 96,000 over 30 ft and 66,000 over 60 ft. Longer runs carry less.
Why use the length to the farthest appliance?
That is the longest length method in the fuel gas code: every section of pipe is sized as if it were as long as the run to the most distant outlet, carrying the load downstream of it. It errs on the safe side for the closer appliances.
Is propane piping smaller?
Yes. Propane carries 2,516 Btu per cubic foot against about 1,000 for natural gas, so the same pipe carries more heat. The same house needs a 3/4 in. main on propane at the low-pressure side of the regulator.
Can I size CSST or copper with this?
No. Corrugated stainless tubing and copper have their own tables in the code and the maker's instructions. This is for Schedule 40 steel pipe on low-pressure systems below 1-1/2 psi: a normal natural gas house line, or propane after the second-stage regulator (11 in. water column).