Gas pipe sizing is a safety-critical code and utility coordination task. Model codes such as the IFGC and IRC include fuel-gas sizing methods and tables, but the final answer depends on the adopted edition, local amendments, utility service pressure, meter capacity, material listing, manufacturer instructions, equivalent length, and AHJ interpretation. Use this guide as orientation before working through the current code tables and project-specific review.
Most sizing errors come from using the wrong length basis, not adding downstream loads correctly, treating appliance output ratings as input ratings, ignoring fittings and regulator drops, or applying natural-gas assumptions to LP gas. The ToolGrit calculator works through local formula logic only. It does not reproduce IFGC, IRC, NFPA 54, or NFPA 58 tables and does not authorize installation.
The Longest-Run Method: Why It Matters
The longest-run method is a common code-table workflow for low-pressure fuel-gas piping. The basic concept is to identify the path from the point of delivery to the most distant appliance, then use that developed length as the table length basis while sizing pipe segments for the cumulative downstream input load they carry. The exact method, table, rounding, and exceptions must come from the adopted code and AHJ.
The common mistake is sizing each branch independently. If you have a furnace 30 feet from the meter and a water heater 50 feet from the meter (branching off the main at 20 feet), the critical path is 50 feet to the water heater. The main from the meter to the tee at 20 feet carries the combined BTU load of both appliances. The branch to the furnace carries only the furnace load but is still sized using the 50-foot column, not the 30-foot column, because the total system length governs the pressure drop budget.
The pressure-drop budget is shared across the system. If each branch is treated as if it gets its own full drop allowance, the combined system can be undersized. Current code-table instructions, utility requirements, regulator data, and appliance inlet-pressure limits control how the pressure budget is applied.
A local formula screen can help reveal sensitivity, but it does not replace the adopted table workflow or project review.
Gas Pipe Sizing Calculator
Size natural gas piping per IFGC/IRC using the Spitzglass formula. Select appliances, pipe material, and run length to find minimum pipe diameter.
Reading the Code Tables Correctly
The IRC and IFGC provide separate sizing information for combinations of pipe material, gas type, inlet pressure, and pressure drop. Table numbers, accepted materials, and conditions vary by edition and local adoption. Use the exact adopted table, manufacturer table, or engineered method required for the project.
Code tables commonly organize capacity by pipe diameter and developed length. To use the appropriate table, determine the required length basis, calculate the downstream input load on each segment, and select a row that carries that load under the table conditions. Do not mix table conditions across gas type, pressure, material, or pressure drop.
Developed length includes equivalent lengths for fittings and restrictions. Elbows, tees, valves, meters, regulators, flexible connectors, and manufacturer-specific CSST fittings can materially change the effective length. Use the adopted code, manufacturer, utility, and project data for equivalent lengths rather than relying on a universal shortcut.
If your developed length falls between two columns, always use the longer column. Using the shorter column would overstate the pipe's capacity and could result in undersized pipe with inadequate pressure at the appliance.
L_developed = L_measured + sum of fitting and restriction equivalent lengths
Equivalent lengths are material, size, fitting, manufacturer, and code dependent. Verify before using a shortcut.
Calculating BTU Load Per Segment
Every pipe segment carries the total BTU demand of all appliances downstream of it. The main line from the meter carries the total connected load of the entire building. A branch that serves only a water heater carries only the water heater load. This sounds obvious, but mistakes happen when the piping layout is complex.
Start at the meter and work outward. At the first tee, the main divides into two branches. The main upstream of the tee carries the sum of both branches. Each branch carries its own load. If a branch subdivides further, the same logic applies recursively.
Use input BTU ratings from the appliance nameplate, not output ratings. A furnace with 60,000 BTU output and 96% AFUE has an input rating of about 62,500 BTU. A water heater with 40,000 BTU input is 40,000 BTU for pipe sizing regardless of its efficiency. The gas pipe does not care about efficiency; it cares about fuel volume.
For LP gas, the BTU content per cubic foot is 2,516 (vs 1,000 for natural gas), so LP piping carries a smaller volume of gas for the same BTU load. LP's higher specific gravity (1.52 vs 0.60 for NG) does increase friction loss per cubic foot, but the volume reduction dominates: a given pipe size carries roughly 1.6 times more BTU of LP than of natural gas at the same pressure drop. The net effect is that LP pipe sizes are usually the same as or one size smaller than NG for the same BTU load — which is why LP has its own sizing tables with noticeably higher BTU capacities per pipe size.
CSST: Corrugated Stainless Steel Tubing
CSST (brands like TracPipe, Gastite, and CounterStrike) is flexible corrugated stainless steel tubing that is faster to install than rigid black iron pipe. It uses fewer fittings because it can bend around obstacles. But it has its own sizing rules that differ from rigid pipe.
CSST is sized using manufacturer-specific rows, often using equivalent hydraulic diameter (EHD) or brand-specific designations rather than rigid-pipe nominal size. A local EHD row in a calculator is not a substitute for the selected product manual or listing.
Each CSST manufacturer publishes its own sizing, fitting, routing, bonding, strike-protection, and installation instructions. Use the exact brand and product line instructions plus the adopted code and AHJ requirements.
CSST bonding and protection requirements are commonly enforced, but the exact requirement depends on the adopted code, product instructions, installation configuration, and jurisdiction. Verify before inspection or insurance use.
Common Mistakes That Cause Failed Inspections
The most common gas pipe sizing mistake is not using the longest-run method. Plumbers who size each branch independently often end up with undersized mains that cannot deliver adequate pressure to all appliances running simultaneously. The inspector may not catch it on paper, but the homeowner will notice when the furnace and water heater fire at the same time and the range burners turn yellow.
The second mistake is ignoring fittings. A 30-foot run with six elbows and three tees could have an actual developed length of 50+ feet. If you sized using the 30-foot column, you are significantly undersized. Either count fittings or use the 50% add-on shortcut.
The third mistake is mixing up gas types. LP gas tables are not the same as natural gas tables. If you use an NG table to size LP piping, you will oversize by roughly 60 percent — LP delivers 2.5 times the BTU per cubic foot, which more than offsets its higher specific gravity, so LP tables show higher BTU capacities for every pipe size. Oversizing wastes money rather than creating a hazard, but it is still wrong; and a system later converted from LP back to natural gas with LP-table sizing would be undersized. Always verify you are using the correct table for your gas type.
The fourth mistake is not checking the supply pressure. Standard residential NG is 7 inWC at the outlet of the meter regulator. Standard LP is 11 inWC. If your supply pressure is lower (old regulator, high demand on the distribution main), your pipe capacity is reduced. Tables are published for specific inlet pressures.
The fifth mistake is treating sizing as the only safety step. Fuel-gas work also requires the applicable pressure/leak test, purging/startup process, inspection, and utility or AHJ acceptance required for the jurisdiction and job type.