Gas Piping Sizing Basics
Undersized gas pipe starves appliances, drops manifold pressure, and fails inspections. Here is how to size a system correctly by BTU load, run length, and allowable pressure drop under the fuel gas code.
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In This Guide
Why Correct Sizing Matters
Gas pipe has to deliver enough fuel to every connected appliance running at the same time without letting the pressure at the burner fall below what the manufacturer requires. Undersize it and you get low manifold pressure, lazy yellow-tipping flames, delayed ignition, nuisance lockouts, and appliances that never reach rated input. Oversize it and you have wasted material and, on some LP systems, purge and leak-check headaches.
Pipe sizing is governed by the International Fuel Gas Code (IFGC) in ICC jurisdictions and by NFPA 54 (the National Fuel Gas Code), which the IFGC largely mirrors. Both give you the same tools: capacity tables organized by pipe material, gas type, delivery pressure, and allowable pressure drop. Your job is to pick the right table, then read it correctly.
Scope note: This is a field primer for standard residential and light-commercial low-pressure systems (under 2 psi, delivered around 7 in wc for natural gas or 11 in wc for propane). Two-psi systems, elevated-pressure regulators, and long CSST runs use their own tables and are worth double-checking against the AHJ.
The Four Inputs You Need
Every table lookup comes down to four pieces of information. Nail these down before you touch a chart.
- Total BTU/hr load — the sum of the input ratings on the nameplate of every appliance the section of pipe feeds, assuming they can all run at once.
- Pipe length — the developed length of the longest run from the meter or regulator to the most remote appliance, measured along the pipe (not straight-line).
- Allowable pressure drop — how much pressure you are willing to lose across the piping. The standard low-pressure natural gas table is built on a 0.5 in wc drop; propane tables commonly use 0.5 in wc as well, sometimes 1.0 in wc.
- Gas type & specific gravity — the tables are pegged to a specific gravity: 0.60 for natural gas and 1.50 for undiluted propane. Use the table that matches your fuel.
Heating values to memorize
Natural gas ≈ 1,000–1,030 BTU per cubic foot (use 1,000 for quick math). Propane ≈ 2,500 BTU per cubic foot. These convert your BTU load into the cubic-feet-per-hour (CFH) values the tables actually list.
Converting BTU Load to CFH
Sizing tables are printed in either CFH or BTU/hr. When they use CFH, you have to convert. The relationship is simple:
CFH from BTU Load
Natural gas: divide BTU/hr by 1,000. Propane: divide BTU/hr by 2,500.
A 100,000 BTU/hr furnace on natural gas draws 100,000 ÷ 1,000 = 100 CFH. That same furnace on propane draws 100,000 ÷ 2,500 = 40 CFH. Propane packs far more energy per cubic foot, which is why LP systems move less volume and often use smaller pipe for the same load — but LP's higher specific gravity offsets some of that, so always read the LP table, never the natural gas one.
Use nameplate input ratings, not output. A 96% furnace rated at 100,000 BTU input still draws gas for the full 100,000 — the 4% loss goes up the flue, not into your pipe math.
The Longest-Length Method
The most common approach in the field is the longest-length method (IFGC 402.4.1). It is conservative, simple, and rarely questioned by an inspector. The rule:
- Measure the developed length from the meter/regulator to the most remote appliance. That single number is your length column for the entire system.
- Round up to the next length listed in the table (a 55 ft run uses the 60 ft column).
- For each section of pipe, add up the load of every appliance downstream of it, convert to CFH, and read the smallest pipe in that length column that meets or exceeds the demand.
Because you size every leg off the single longest length, no section can be undersized no matter how the branches actually run. The tradeoff is that near branches get slightly larger pipe than they strictly need. The alternative, the branch-length method (402.4.2), sizes each segment on its own individual length and can save material on sprawling systems, but it takes more bookkeeping.
Developed length means measured along the pipe run, including the rise and fall. For standard systems the code lets you ignore fitting losses in the basic table method — the tables already bake in a fitting allowance — so you generally do not add equivalent lengths for elbows and tees on a typical residential job.
Schedule 40 Sizing Table
Below is a representative capacity table for Schedule 40 metallic pipe, natural gas, inlet pressure less than 2 psi, 0.5 in wc pressure drop, 0.60 specific gravity (values in CFH). This mirrors IFGC Table 402.4(2). Always verify against the current code edition your jurisdiction has adopted — values shift slightly between editions.
| Pipe Size | 20 ft | 40 ft | 60 ft | 80 ft | 100 ft |
|---|---|---|---|---|---|
| 1/2" | 131 | 90 | 72 | 62 | 55 |
| 3/4" | 273 | 188 | 151 | 129 | 114 |
| 1" | 514 | 354 | 284 | 243 | 215 |
| 1-1/4" | 1060 | 726 | 583 | 499 | 442 |
| 1-1/2" | 1580 | 1090 | 873 | 747 | 662 |
| 2" | 3050 | 2090 | 1680 | 1440 | 1280 |
Notice how capacity falls as length grows: a 1" line carries 514 CFH at 20 ft but only 215 CFH at 100 ft. Length is doing most of the work in these numbers.
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Worked Example: A Whole House
The system:
Natural gas, low pressure (7 in wc at the meter). Three appliances: a 100,000 BTU furnace, a 40,000 BTU water heater, and a 65,000 BTU range. The developed length from the meter to the most remote appliance (the furnace in the attic) is 55 feet.
Step 1 — Total the load.
100,000 + 40,000 + 65,000 = 205,000 BTU/hr
Step 2 — Convert to CFH (natural gas, ÷ 1,000).
Total = 205 CFH • Furnace = 100 • Water heater = 40 • Range = 65
Step 3 — Set the length column. Longest run is 55 ft, so round up to the 60 ft column for every section.
Step 4 — Size the main. It carries the full 205 CFH. In the 60 ft column, 3/4" = 151 CFH (too small), 1" = 284 CFH.
Main line = 1" (284 ≥ 205) ✓
Step 5 — Size the branches (still using the 60 ft column).
Furnace branch: 100 CFH → 3/4" (151) ✓
Range branch: 65 CFH → 3/4" (151) ✓
Water heater branch: 40 CFH → 1/2" (72) ✓
Result: 1" main, 3/4" to furnace and range, 1/2" to water heater
Every section clears its demand at 60 ft with margin to spare. If the utility later tells you the load will grow (say a future pool heater), bump the main to 1-1/4" while the trench is open — upsizing the trunk is cheap now and expensive later.
The classic failure
A tech reads the 20 ft column out of habit and sizes the main at 3/4" (273 CFH looks fine for 205 CFH). At the real 55 ft length, 3/4" only carries 151 CFH — the furnace and water heater running together starve, the furnace short-cycles on flame-sense, and the callback lands two weeks later. Always size at the longest length, not the length you wish you had.
Field Tips & Common Mistakes
- Match the table to the fuel and pressure. A natural gas table read for a propane job (or a 0.5 in wc table used on a 2 psi system) gives numbers that are simply wrong. Confirm gas type, specific gravity, and the pressure-drop basis printed in the table header.
- Do not count the appliance connector. The flexible connector at the appliance is not part of the fixed piping and is not included in the developed-length measurement.
- CSST is not steel pipe. Corrugated stainless tubing has its own manufacturer tables keyed to EHD (equivalent hydraulic diameter) sizes, and it carries less than same-nominal steel because of its corrugations. Never substitute the steel-pipe table for CSST.
- Bonding is code, not optional. CSST systems require dedicated bonding per the manufacturer and NFPA 54 to guard against lightning-induced arcing. Verify the bond clamp and conductor size.
- Pressure-test what you build. Most jurisdictions require a pressure test (commonly around 3 psi for 15 minutes on residential low-pressure piping, but follow the AHJ and code) before the gas gets turned on. Leak-check every joint with an approved solution or electronic detector.
- Elevation and long LP runs matter. Undiluted propane is heavier than air and its tables differ; very long or elevated runs may need a two-stage regulator setup. When in doubt, verify against the propane-specific table.
Safety first: Gas work carries real fire and asphyxiation risk. Only perform gas piping and appliance work you are licensed and qualified to do, follow the adopted code edition, and pull the required permit. When a calculation sits near a table boundary, size up — a slightly larger pipe never caused a callback.
FAQ
Do I use appliance input or output ratings?
Always input (the gas the appliance consumes). Output is what it delivers to the space after combustion losses and is irrelevant to pipe sizing.
Why do I size every branch off the longest run?
That is the definition of the longest-length method. It guarantees no section is undersized regardless of branch routing. If you want to trim material on a big system, switch to the branch-length method and size each segment on its own developed length.
Can I ignore fittings?
For standard residential piping using the basic table method, yes — the tables include a fitting allowance. Elevated-pressure and engineered systems may require you to add equivalent lengths for fittings.
What pressure drop should I design for?
Standard low-pressure natural gas systems use the 0.5 in wc pressure-drop table. Keeping the total drop modest ensures the most remote appliance still sees enough manifold pressure at full fire.
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