Fuel Gas Code tables 402.4(2) and 402.4(28), checked against their own equation
Seattle prints the propane gas pipe table twice, once in its Fuel Gas Code and once in its Residential Code, and the two copies disagree. The residential copy skips the 70 and 90 foot rows without dropping any capacities, so from 80 feet on every row reads a shorter run's figure. At 100 feet a 1/2 inch pipe is listed at 94,000 Btu per hour against the correct 84,000, and a 180,000 Btu/h run sized from that copy comes out at 3/4 inch where the correct table needs 1 inch.
Smallest Schedule 40 pipe for the load and the longest run
thousand Btu/h
ft
Btu/cu ft
Size each section with the load that section carries and the longest length from the meter or regulator to the most remote appliance. Your gas utility and building department may add requirements.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Capacity against load, at the length row used
Worked example
A propane run from the second-stage regulator, 100 feet to the most remote appliance, serving 180,000 Btu per hour of appliance input between them. Then the same idea on natural gas.
Load. The table is in thousands of Btu per hour, so 180.
Length. The longest run is 100 ft, which is a printed row: 100 ft.
Read across. 1/2 in carries 84, 3/4 in carries 175, 1 in carries 330. The first that reaches 180 is 1 in.
The residential copy. Its 100 ft row is really the 80 ft row: 94, 197, 372. There 3/4 in reaches 180, one size too small.
Natural gas, 250,000 Btu/h over 60 ft. 250,000 ÷ 1,037 = 241 cfh. 3/4 in carries 137, 1 in carries 257: 1 in.
The residential copy's answer is wrong by 22,000 Btu per hour: it credits 3/4 inch with 197 where the correct row gives 175, just under the 180 the run needs. It would pass a quick look, because 3/4 inch is a perfectly normal size for that run on a shorter line. That is what makes a shifted column dangerous: nothing looks strange.
The formula
The tables are one equation, for gas under 1 1/2 psi, solved for flow:
actual inside diameter, inches: 0.622 for 1/2 in Schedule 40, 0.824 for 3/4, 1.049 for 1
ΔH
pressure drop allowed, inches of water column: 0.5 for both tables here
Cr
0.6094 natural gas, 1.2462 undiluted propane
L
length, feet: the longest run, by the longest length method
Btu/h ÷ heating value
turns an appliance rating into cfh for the natural gas table
Because capacity depends on length to the power of about 0.54, doubling the run cuts what a pipe carries by roughly a third. That is also the check this page runs on the printed tables: every row has to be consistent with the 10 foot row through that exponent.
Why this page compares two copies of one table
Gas pipe sizing calculators reproduce a capacity table and look a number up in it. Nothing checks the table.
This page read the propane table from two documents the same city publishes and found they disagree from the 80 foot row onwards. The code's own equation settles which one is right, without needing to trust either: every row of a correct table is consistent with its 10 foot row, and the residential copy's rows are not consistent with their labels.
The error overstates capacity, so it undersizes pipe. That is the one direction a sizing table must not err in.
Propane capacity: the Fuel Gas Code table against the residential copy
Length
Fuel Gas Code, 1/2 in
Residential Code, 1/2 in
Printed row really is
Fuel Gas Code, 4 in
Residential Code, 4 in
Capacity overstated
60 ft
110
110
the right row
14,800
14,800
none
80 ft
94
101
70 ft
12,700
13,600
7% high
100 ft
84
94
80 ft
11,200
12,700
12% high
150 ft
67
84
100 ft
9,010
11,200
25% high
200 ft
58
67
150 ft
7,710
9,010
16% high
300 ft
46
58
200 ft
6,190
7,710
26% high
500 ft
35
40
400 ft
4,700
5,300
14% high
1,000 ft
24
25
900 ft
3,230
3,420
4% high
Read 2026-09-13. Both columns are as printed by the City of Seattle in its 2021 Fuel Gas Code and 2021 Residential Code. The row each printed line really belongs to is worked out from the sizing equation.
The same propane table, printed twice by one city, and one copy is wrong
Seattle publishes its Fuel Gas Code and its Residential Code as separate documents, and both carry the Schedule 40 propane table for the low-pressure run from the second-stage regulator to the appliances. The residential copy names its origin in its own heading: Table G2413.4(12) [402.4(28)].
The capacities are the same numbers in the same order. The length column is not. The residential copy goes 50, 60, 80, 100: it has no 70 foot row and no 90 foot row, but it has not lost any capacities. So from the 80 foot label down, every row carries the capacities of a shorter length. Further down, the labels 1,200 to 1,500 are printed twice, and the capacities for 1,900 and 2,000 feet do not appear at all.
Undiluted propane, Schedule 40, 11 in w.c. inlet, 0.5 in w.c. drop, thousands of Btu per hour. Both copies published by the City of Seattle, 2021 editions
Length
Fuel Gas Code, 1/2 in
Residential Code, 1/2 in
Printed row really is
Fuel Gas Code, 4 in
Residential Code, 4 in
Capacity overstated
60 ft
110
110
the right row
14,800
14,800
none
80 ft
94
101
70 ft
12,700
13,600
7% high
100 ft
84
94
80 ft
11,200
12,700
12% high
150 ft
67
84
100 ft
9,010
11,200
25% high
200 ft
58
67
150 ft
7,710
9,010
16% high
300 ft
46
58
200 ft
6,190
7,710
26% high
500 ft
35
40
400 ft
4,700
5,300
14% high
1,000 ft
24
25
900 ft
3,230
3,420
4% high
This is the unsafe direction. Capacity falls as length rises, so a row borrowed from a shorter length overstates what the pipe carries. Size a propane run off the residential copy and the pipe it picks can be one size too small, which shows up as appliances starved of pressure when everything runs at once.
The 2018 edition prints the rows identically, so the fault has been carried forward at least once. Whether it began in Seattle's transcription or in the model residential code it adopts could not be checked: the International Code Council's own text refuses this site's requests.
How to tell which copy is right without trusting either
Both codes print the equation the tables are made from, for gas under 1 1/2 psi:
D = Q0.381 ÷ [ 19.17 × ( ΔH ÷ (Cr × L) )0.206 ]
where D is the inside diameter in inches, Q the flow in cubic feet per hour, ΔH the pressure drop in inches of water column, L the length in feet and Cr 0.6094 for natural gas and 1.2462 for undiluted propane.
Hold the pipe fixed and capacity falls with length to the power 0.206 ÷ 0.381, about 0.54. That means any row of a table can be asked what length it belongs to, using nothing but the 10 foot row above it. Asked of the Fuel Gas Code table, every row names its own printed length. Asked of the residential copy, the row printed as 100 feet names 80.
The natural gas table checks out against the equation directly, within 1.4 per cent on every entry of 100 cubic feet per hour or more, using the actual inside diameters it prints. The code notes that entries are rounded to three significant digits.
The propane table is printed in thousands of Btu per hour, not cubic feet, so a heating value is built into it. Dividing its entries by the equation's flow gives 2,457 to 2,482 Btu per cubic foot across the table. The code does not print the figure it used; that range is this page working it backwards.
Adding up the load, and what it is not
Use the input rating from each appliance's data plate, in Btu per hour. Furnaces and water heaters also print an output or recovery figure, and that is the wrong number for gas piping: it is lower than the gas the burner consumes.
Count every appliance on the section, all at once. The code sizes on total connected load unless a diversity of load can be established, so a range, dryer, water heater and furnace on one line are added together even though they seldom all run.
Plan for what will be connected. A pipe sized exactly for today's appliances has no room for a future generator, fire pit or pool heater, and replacing a buried or concealed line costs far more than one size up at the start.
What this calculator does not size
Only Schedule 40 metallic pipe at a 0.5 in w.c. drop. The code carries separate tables for copper, polyethylene and corrugated stainless steel tubing, and for higher pressure systems.
CSST is sized from the manufacturer. The code permits a listed piping system's own sizing tables, and CSST sizes are given as an equivalent hydraulic diameter that differs between brands.
Propane between the tank and the second-stage regulator is a different table. The one here is for the low-pressure run from the second-stage regulator to the appliances, at 11 in w.c.
Altitude. The code requires flow to be adjusted above 2,000 feet elevation, and that adjustment is not made here.
What the code says to add up, and which length to use
The load is every appliance at once. Section 402.2:
The total connected hourly load shall be used as the basis for pipe sizing, assuming that all appliances could be operating at full capacity simultaneously.
It permits sizing on a lower figure only where a diversity of load can be established. Add the input rating on each appliance's data plate, in Btu per hour, not the output.
The length is the longest one, even for short branches. Section 402.4.1, the longest length method:
The pipe size of each section of gas piping shall be determined using the longest length of piping from the point of delivery to the most remote outlet and the load of the section.
So a section next to the meter is sized with the length out to the most remote appliance and the load that section carries. The code also allows a branch length method, 402.4.2, which sizes branches off the main run with their own length to their own most remote outlet.
Natural gas tables are in cubic feet per hour, so an appliance rating in Btu per hour has to be divided by the heating value of the gas. The US average delivered to consumers was 1,037 Btu per cubic foot in 2024 according to the EIA, and the calculator uses that unless you enter your utility's figure.
The sources, and why Seattle
Read 2026-09-13.
2021 Seattle Fuel Gas Code, Chapter 4 Gas Piping Installations, City of Seattle, Department of Construction and Inspections, the 2021 International Fuel Gas Code as adopted and amended by Seattle. Sections 402.2, 402.3, 402.4 and 402.4.1, Equation 4-1, Table 402.4 (Cr values), Table 402.4(2) natural gas and Table 402.4(28) undiluted propane, both Schedule 40 metallic pipe at 0.5 in w.c. pressure drop.
2021 Seattle Residential Code, Chapter 24 Fuel Gas, City of Seattle, Department of Construction and Inspections, the 2021 International Residential Code as adopted and amended by Seattle. Table G2413.4(12) [402.4(28)], the residential copy of the propane table, read to compare against the Fuel Gas Code. The 2018 edition (2018SRCChapter24.pdf) prints the same rows.
Heat Content of Natural Gas Delivered to Consumers, U.S. Energy Information Administration, annual, Btu per cubic foot. US figure 1,037 Btu per cubic foot for 2024. Used only as the default for converting appliance ratings in Btu/h into cubic feet per hour; your gas utility publishes the figure for your supply.
The International Fuel Gas Code is paywalled and codes.iccsafe.org returns HTTP 403 to this site. Seattle publishes both its fuel gas and residential codes as free PDFs from the city, with the edition printed on every page, which is why a city's adoption is the text read here and why the two copies could be compared at all.
Your jurisdiction may have adopted a different edition or amended it. Gas piping is also commonly subject to the gas utility's own requirements for pressure and meter set. Ask the building department which code applies before sizing.
Frequently asked questions
How do you size a gas pipe?
Add the input ratings of every appliance the section serves, find the longest length of piping from the meter or regulator to the most remote appliance, and read the code table at that length for the smallest pipe whose capacity reaches the load. For natural gas, convert Btu per hour to cubic feet per hour first by dividing by the heating value.
What size gas line do I need for 100 feet?
It depends entirely on the load. On the propane table at 100 feet, 1/2 inch Schedule 40 carries 84,000 Btu/h, 3/4 inch 175,000 and 1 inch 330,000. On the natural gas table at 100 feet, 1/2 inch carries 50 cubic feet per hour, 3/4 inch 104 and 1 inch 195.
How many Btu can a 1/2 inch gas line carry?
On the Fuel Gas Code propane table, 291,000 Btu/h at 10 feet, 110,000 at 60 feet and 84,000 at 100 feet. On the natural gas table, 172 cubic feet per hour at 10 feet and 50 at 100 feet, which at 1,037 Btu per cubic foot is about 178,000 and 52,000 Btu/h.
Why does gas pipe capacity drop with length?
Friction. The tables are built on a fixed pressure drop of half an inch of water column, and a longer pipe uses that drop up at a lower flow. Capacity falls with length to the power of about 0.54.
What is the longest length method?
The code's default for picking the length to read the table at. Every section of piping is sized with the length from the point of delivery to the most remote outlet, together with the load of that section. The code also permits a branch length method and a hybrid method for two-pressure systems.
How many Btu are in a cubic foot of natural gas?
The EIA reports the US average heat content of natural gas delivered to consumers as 1,037 Btu per cubic foot for 2024. It varies by supply, and your gas utility publishes the figure for yours.
Which Seattle table is wrong?
The propane table in the Seattle Residential Code, Table G2413.4(12) [402.4(28)], in both the 2018 and 2021 editions as published by the city. The same table in the 2021 Seattle Fuel Gas Code, Table 402.4(28), is correct. Whether the model residential code carries the same fault could not be checked, because its publisher refuses this site's requests.
Can I use this for CSST?
No. CSST is sized from the manufacturer's tables, which use an equivalent hydraulic diameter that varies by brand. This page covers Schedule 40 metallic pipe only.
Check these numbers yourself
2021 Seattle Fuel Gas Code, Chapter 4 Gas Piping InstallationsCity of Seattle, Department of Construction and Inspections, read 2026-09-13. Sections 402.2, 402.3, 402.4 and 402.4.1, Equation 4-1, Table 402.4 Cr values, Table 402.4(2) natural gas and Table 402.4(28) undiluted propane, Schedule 40 metallic pipe.
2021 Seattle Residential Code, Chapter 24 Fuel GasCity of Seattle, Department of Construction and Inspections, read 2026-09-13. Table G2413.4(12) [402.4(28)], the residential copy of the propane table, compared row by row. The 2018 edition prints the same rows.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.
The tables are quoted from the City of Seattle's 2021 Fuel Gas Code, read on 13 September 2026. Your jurisdiction may have adopted a different edition or amended it, and gas utilities set their own requirements. The comparison with the residential copy is this page's reading of two published documents, checked against the code's own equation. Gas piping is commonly permit work that must be installed and pressure tested by a qualified person; this page sizes pipe and does nothing else.