One length for the whole system
Sizing each segment on its own shorter length reads a higher capacity and produces pipe that is too small. This applies the governing length everywhere, including the short runs at the meter.
Gas pipe by the longest-length method
One length governs the whole system. Sizing each segment on its own shorter length is the mistake, and it is the one that feels right.
Find the longest run from the meter to any single outlet. Then, for every segment below, read the capacity of your candidate pipe size at that one length — not at the segment's own length. Capacities depend on gas type, supply pressure, permitted pressure drop and pipe material all at once, which is why the table is yours and not this page's.
The system
Each segment carries the total demand of everything downstream of it. The segment at the meter carries the whole house.
Segment by segment
How the longest-length method works
Three steps, and the second is the one that gets skipped:
- Find the longest run from the meter to any single outlet.
- Size every segment in the system using that one length — including the short segments near the meter.
- Each segment must carry the total demand of everything downstream of it.
Why one length for everything. The pressure drop the table allows is a budget for the whole path, not for each piece. If each segment were sized at its own length, every segment would read a higher capacity than it is entitled to, the drops would accumulate along the path, and the appliance at the far end would be starved. The length is a property of the system here, not of the segment.
What this does not do. It does not apply the branch-length method, which some codes permit as an alternative and which sizes each branch by its own run from the meter; that is a different method with different rules and it is not interchangeable with this one segment by segment. Nor does it size the meter, regulators, or CSST, which has its own manufacturer tables.
The mistake that looks reasonable
Confronted with a twenty-foot segment near the meter and a sixty-foot run to the furthest appliance, the natural instinct is to size the short one for twenty feet. Shorter run, less friction, higher capacity, smaller pipe. Every step of that reasoning is locally sound and the conclusion is wrong.
What it misses is that the twenty-foot segment is part of the sixty-foot path. The appliance at the end experiences the drop across all the pipe between it and the meter. Give the first segment only the capacity a twenty-foot run would justify and you have spent part of the far appliance's pressure budget on a pipe that appeared, in isolation, to be generously sized.
The error is systematic rather than random — it always undersizes, and it undersizes most on exactly the segments carrying the most gas. That is why this page asks for the governing length once and applies it everywhere, and shows the sum of the segment lengths beside it so you can see that the sum is not what you want either.
Every segment carries what is behind it
The second half of the method is cumulative demand. The pipe leaving the meter feeds everything, so it carries the total of every appliance in the building. A tee splits that, and each leg carries only its own share — but the leg feeding three appliances carries all three.
People size the segment at the meter for the appliance it appears to run toward, which on a simple layout can look like a single furnace. If a water heater and a range hang off a tee further along, that first segment is carrying all three regardless of what it looks like on the drawing.
Demand is input rating, not what the appliance uses
Gas load is the appliance's input rating in BTU per hour, taken from its data plate — not an estimate of consumption and not the output rating. The output is lower by whatever the appliance's efficiency is, and sizing to it undersizes the pipe by that margin. If the plate gives only a nominal figure in kW or an output, get the input rating from the manufacturer's data rather than converting.