Natural Gas Pipe Sizing — BTU Demand, Longest Run & the Tables
Gas pipe sizing is a lookup, not a guess: add up the appliance BTU inputs, measure the longest run, and read the pipe size from the table for your material and pressure. The mistakes happen in the adding and the measuring.
Worth knowing
- Adding one big appliance — a tankless water heater at 150–199k BTU is the classic — can undersize the whole existing system, not just the new branch. Re-run the table for every segment.
- A 3/4 in. steel line that fed a 40k BTU furnace fine will starve a 199k tankless at any real length: this is the most common “no hot water in winter” service call.
- Two-psi systems with pounds-to-inches regulators use different table rows and can move far more gas in the same pipe — common in newer tract housing.
- State amendments matter here (several states modify CSST and bonding rules) — and utilities have their own meter-sizing requirements on top of the code.
Common questions
What does the code say about demand for Gas Pipe Sizing?
Piping is sized for the total input BTU/h of all connected appliances, taken from nameplates — not an assumed diversity. (IRC G2413.2)
What does the code say about longest-length method for Gas Pipe Sizing?
The default method sizes every segment using the table row for the distance from the meter to the farthest appliance. (IRC G2413.4.1)
What does the code say about sizing tables for Gas Pipe Sizing?
Capacity tables by material (Schedule 40 steel, CSST, copper, PE) at the system pressure and a standard 0.5 in. w.c. pressure drop. (IRC Table G2413.4(1) et seq.)
Adopted edition by state
Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.