I'm interested in understanding the natural gas capacity at my house using the standard longest length method. I'm slightly confused which column in the IFGC Table 402.4 to use though because a 1" diameter pipe comes out of my gas meter and then goes into a 1 1/4" at the house.

I read in an article on sizing gas pipes:

Note: Any nipple or other piece that is smaller in the system and then goes larger again is to be rated from the reduced piece onward. For instance, if a 1 inch pipe runs 30' then drops to 1/2" for 6" and then someone installed 3/4" pipe and ran an additional 40 feet, the 40 feet of 3/4" pipe would all be considered 1/2". You will never get more gas through the 3/4" pipe than will pass thorough the 1/2" nipple.

Does this apply in this situation? Should I be looking at the 1" column or the 1 1/4" column (the red column or the blue column I highlighted below) for making calculations of the capacity from the trunk line?

Annotated Table 402.4 Table

Attached is a picture of my gas meter. There is a 7" WC regulator before the meter so I know I'm looking at least at the right table. The gas meter is connected to a tee which goes from 1" to 1 1/4" going into the house.

1" Pipe coming out from my Gas Meter

  • 1
    The thinnest pipe determines the maximum flow rate, just as it would for water.
    – keshlam
    Commented Mar 5, 2023 at 16:02
  • 1
    I'd take a more nuanced view, but presumably you have to follow whatever established code practice is. I'd say you have about 10 feet of 1", (probably less, but that's what's in the table at the short end) and then you have 1-1/4" that is not the same as more 1" in terms of pipe friction, so the stuff going off the 1-1/4" is pretty much like it was going off 10 feet of 1", friction-wise, if there's less than 40 feet of 1-1/4" - presumably the builder was allowing for an upgrade to 1-1/4" through the meter if you wanted more capacity.
    – Ecnerwal
    Commented Mar 5, 2023 at 17:38

1 Answer 1


The answer is likely to be somewhere in between those values. I looked up the tables you cited, but didn't see the paragraph you quoted. However, the final sentence of that paragraph is somewhat misleading.

You will never get more gas through the 3/4" pipe than will pass thorough the 1/2" nipple.

That is true, but you can get more gas through 40 feet of 3/4" pipe with 6 inches of 1/2" pipe, than you can get through 40 1/2 feet of 1/2" pipe.

As gas flows through a length of pipe, the pressure continually drops due to losses caused by friction along the walls, and losses caused by flow disturbances such as elbows, tees, valves, the flowmeter, and size changes. All the pressure losses increase as the flow rate increases. The maximum flow is the flow at which the total losses are equal to the allowable pressure drop (the inlet pressure minus the required minimum pressure for the gas appliance).

As Ecnerwal noted in a comment, you may have to simply pick the smaller pipe size based on codes if you're wanting to install a new appliance (or have an engineer sign off on a capacity calculation).

But if you're asking more out of curiosity, you can probably estimate an answer using the IFGC tables. The table you included specifies that the values are calculated to give less than 0.5 inWC pressure drop. There are other tables based on different pressure drops, including 0.3, 3.0, and 6.0 in WC. So as an example, suppose you have 50 feet of 1 1/4 inch pipe, and 10 feet of 1 inch pipe, and an allowable pressure drop of 1 inch WC.

Using your table, you can get 583 scfh through 50 ft of 1 1/4" pipe, with a pressure drop of 1/2 inch WC. And you can get 678 scfh through 10 ft of 1" pipe with a pressure drop of 1/2 in WC. Therefore, you can get 583 scfh through the combined pipes with a pressure drop of less than 1 in WC.

  • I'm sorry, i wasn't clear that the quoted statement was from an article I was reading about how to properly size a system. The article may not be correct either. Thanks for explaining and running those calculations. Commented Mar 6, 2023 at 21:58

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