Gas Pipe Sizing Calculator

The load alone does not answer it: the length does half the work, and so does everything else on the same system. Convert first, because the sizing equation works in cubic feet per hour rather than Btu per hour. At a heating value of 1,000 Btu per cubic foot, which you should confirm with your gas supplier rather than assume, 100,000 Btu/h is 100 cubic feet per hour. One straight Schedule 40 run carrying 100 cubic feet per hour of natural gas at a half inch water column drop needs 1/2 inch at 20 feet, 3/4 inch at 60 feet and 1 inch at 150 feet. And if that furnace shares the system with a water heater and a range, the pipe upstream of the branch carries all three, which is the part a single-pipe chart cannot tell you.

Section by section, not pipe by pipe. Draw the system as a tree from the meter outward, put each appliance's nameplate input on the section that ends at it, and then every section carries its own appliance plus everything downstream of it. The length each section is sized at depends on the method: under the longest length method every section is sized at the single longest run from the point of delivery to the most remote outlet, and under the branch length method the sections on that longest run keep it while every other section uses the longest run within its own branch. Both are permitted. The example on this page is a 235,000 Btu/h house with a 57 foot longest run: the main off the meter comes out 1 inch while the furnace drop is only 3/4 inch, because the main carries everything and the drop carries one appliance.

Which length each section gets sized at. The longest length method (IFGC 402.4.1) uses one length for the whole system, the longest run from the point of delivery to the most remote outlet, so every section is sized as though it were feeding that farthest appliance. The branch length method (IFGC 402.4.2) keeps that length only for the sections that actually sit on the longest run and lets every other section use the longest run inside its own branch, which is shorter and therefore permits smaller pipe on those branches. The informative appendix that describes them calls the branch length method less conservative than the longest length method (IFGC A103.2, which is guidance rather than code). The calculator on this page shows both, side by side, so you can see exactly which sections the choice moves and defend the one you install.

Yes. IFGC 402.3 authorises the sizing tables and the equations as alternatives, so a size computed from the equation is code-compliant on its own. This tool computes, which is why it is not locked to one drop, one length row or one edition. The honest catch is that the two routes do not agree perfectly. Measured across the 1,110 cells of the two published natural gas capacity tables, where no assumed heating value can enter the comparison, the equation as printed runs about 0.9 percent less conservative than the tables themselves. Across all 1,470 published cells of the three tables we transcribed there is no cell where the equation picks a smaller pipe than the table does at a capacity that table publishes, but in a narrow band just above a published capacity the table has already stepped up a size and the equation has not. So a plans examiner checking your job against the table can legitimately land one size larger. Both answers are code-compliant, and the larger one is never wrong to install.

No, and that is deliberate rather than a gap we have not got to. Corrugated stainless steel tubing is not smooth-wall pipe, so the code's own equations do not reach it: the section that prints them applies them where the pipe or tubing has smooth inside walls. CSST is sized from its listed system's own tables, keyed to that manufacturer's equivalent hydraulic diameter, and the code gives those manufacturer tables the authority. Sizing CSST from this equation would publish a number the code does not authorise this tool to compute, so the tool refuses by name and sends you to the manufacturer's listed instructions for the system actually being installed.

Because the sizing equation works in cubic feet per hour and your appliances are rated in Btu per hour, and the number that converts between them is the heating value of the gas actually being delivered. The code does not publish one. The appendix says it can be obtained from the serving gas supplier, and that is the honest route. Assuming a value undersizes the pipe wherever the real gas is leaner than the assumption, so this tool refuses to assume one: it asks, and it tells you it is asking. Undiluted propane is nowhere near natural gas on this figure, which is why changing the gas clears the field rather than carrying a natural gas number across.

No. It is what most published capacity tables ASSUME, printed in their headings, and it is a design decision rather than a code requirement. What the code actually requires (IFGC 402.6) is that the pressure at every appliance inlet meets that appliance's own minimum inlet pressure. A borrowed table assumption is not a design drop: on a long run with a low supply pressure, half an inch of water column may be more than you have to give away. State the drop you are designing to, and if you also give the tool the appliance's minimum inlet pressure and the pressure at the point of delivery it will tell you whether your drop actually fits inside what is available. Without those two it says the check is incomplete rather than implying it passed.

The one your jurisdiction adopted, which is why this tool asks. The arithmetic is the same in every book and the address is not. International Fuel Gas Code, and the International Residential Code for one- and two-family dwellings: the sizing provisions are at IFGC 402.4, published in the IRC as G2413.4 (402.4). A homeowner pulling a residential permit is inspected against the residential code's own numbering, and printing only the commercial fuel gas address to them is a right value under the wrong authority. Two further families are offered and both refuse rather than answer, because their own coefficients were never read from their own text: putting one of those labels on this equation's answer would be a guess about most of a state's permit process.