How the longest-length method works
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.
The longest-length method is three steps, and the second is the one that gets skipped.
The method
- Find the longest run from the meter to any single outlet.
- Size every segment using that one length — including the short segments near the meter.
- Give each segment the total demand of everything downstream of it.
The mistake that looks reasonable
Faced with a twenty-foot segment near the meter and a sixty-foot run to the furthest appliance, the 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 looked, in isolation, generously sized.
The error is systematic rather than random: it always undersizes, and it undersizes most on exactly the segments carrying the most gas.
Not the sum of the lengths either
The governing figure is the longest single run, not the total of all the pipe in the building. Adding the segments together gives a larger number and a more expensive system that is wrong in the other direction. The tool shows both side by side so the distinction is visible.
Every segment carries what is behind it
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 a 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 the drawing suggests.
Demand is the input rating
Gas load is the appliance's input rating in BTU per hour from its data plate — not an estimate of consumption and not the output rating. Output is lower by whatever the efficiency is, and sizing to it undersizes the pipe by that margin.
A note on methods
Some codes permit a branch-length method as an alternative, sizing each branch by its own run from the meter. That is a different method with different rules; the two are not interchangeable segment by segment, and this tool implements the longest-length method only.
What this tool will not do
It contains no code tables. It does not know which edition your jurisdiction has adopted, what local amendments apply, or anything about your job, and it makes no determination of compliance. A licensed professional and the authority having jurisdiction decide that, and where they or the published code differ from the page, they are right.