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Gas Pipe Sizing Calculator - IFGC/IRC Pipe Diameter by BTU Load

Size Natural Gas and LP Piping Using the Spitzglass Formula with Longest-Run Method

Free gas pipe sizing calculator for plumbers, HVAC techs, and mechanical contractors. Enter your appliance BTU loads, select pipe material (black iron, CSST, copper, or PE), set the supply pressure and allowable pressure drop, and the calculator determines the minimum pipe diameter for each segment using the longest-run method per IFGC and IRC code tables. Supports both natural gas (1,000 BTU/ft3) and propane (2,516 BTU/ft3). Automatically identifies the critical path and flags undersized segments.

Pro Tip: Always size gas pipe based on the total connected BTU load of every appliance downstream of that segment, not just the appliance at the end of the run. A common mistake is sizing the branch to the water heater independently from the branch to the furnace, when in reality both share the same main. The main needs to carry the combined load. If your calculation is borderline between two pipe sizes, go up one size. The cost difference is minimal and you avoid a callback when the homeowner adds a gas range or fireplace later.

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Read the complete gas pipe sizing guide

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Start Using This Tool See How It Works

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Gas Pipe Sizing Calculator
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How It Works

  1. Select Gas Type and Supply Pressure

    Choose natural gas or propane. Set the supply pressure (typically 7 inWC for residential NG, 11 inWC for LP). Enter the maximum allowable pressure drop, usually 0.5 inWC for residential systems per IRC G2413.

  2. Add Appliances and BTU Ratings

    Enter each gas appliance with its nameplate BTU/hr input rating. Common values: furnace 60,000-120,000, water heater 30,000-75,000, range/oven 65,000, gas dryer 22,000, fireplace 25,000-40,000. Use the input BTU rating, not the output or efficiency-adjusted number.

  3. Set Pipe Runs and Lengths

    Enter the measured length from the gas meter (or regulator) to each appliance. Use the longest-run method: the calculator identifies the critical path and sizes all segments accordingly. Include fittings as equivalent feet or add 50% to measured length as a rule of thumb.

  4. Choose Pipe Material

    Select black iron (Schedule 40), CSST (corrugated stainless steel tubing), copper Type L or K, or PE (polyethylene for underground). Each material has different internal diameters and friction factors that affect sizing.

  5. Review Results

    The calculator shows the minimum pipe size for each segment, total BTU demand on each segment, and pressure drop per segment. Segments where the selected pipe is undersized are flagged in red. Verify that total pressure drop from meter to farthest appliance stays within the allowable budget.

Built For

  • Plumbers sizing residential gas piping for new construction per IRC fuel gas code requirements
  • HVAC contractors verifying gas pipe adequacy when adding a furnace or water heater to an existing system
  • Mechanical engineers sizing commercial gas distribution per IFGC for restaurants, hotels, and small commercial buildings
  • Home inspectors checking whether existing gas piping can handle added load from a pool heater or fireplace insert
  • LP gas dealers verifying propane line sizing from tank to building for rural installations
  • Fire marshals reviewing gas piping plans for code compliance during permit review
  • DIY homeowners adding a gas appliance who need to verify their existing pipe is large enough

Features & Capabilities

Longest-Run Method

Uses the IFGC/IRC longest-run approach: find the critical path from the meter to the most distant appliance, then size each segment along that path for the cumulative BTU load it carries. Branches are sized from their takeoff point to the appliance.

Spitzglass Formula

Calculates capacity using the Spitzglass formula for low-pressure gas systems (under 1.5 PSI), which is the standard method referenced in IFGC Table 402.4 footnotes. Accounts for specific gravity, pipe diameter, length, and pressure drop.

Multi-Material Support

Size pipe in Schedule 40 black iron, CSST (with manufacturer-specific equivalent lengths), copper Type L and K, and PE for underground sections. Internal diameters and friction factors are specific to each material.

Natural Gas and LP

Supports both natural gas (specific gravity 0.60, 1,000 BTU/ft3) and propane (specific gravity 1.52, 2,516 BTU/ft3). LP gas requires different pipe sizes due to higher heating value and higher specific gravity.

Segment-by-Segment Results

Shows each pipe segment with its total BTU load, selected pipe size, actual capacity at that size, velocity, and pressure drop contribution. Flags undersized segments so you can upsize before ordering material.

Fitting Equivalent Length

Includes an option to add fitting equivalent lengths for elbows, tees, and valves per standard friction loss data, or apply a percentage markup to straight-run measurements as a field shortcut.

Frequently Asked Questions

For standard residential natural gas systems at 7 inWC supply pressure, the IRC limits total pressure drop to 0.5 inWC from the meter to the farthest appliance. Some jurisdictions allow up to 1.0 inWC for systems with higher supply pressure or two-stage regulators. Propane systems typically operate at 11 inWC with a 0.5 inWC drop allowance. Always verify with your local code authority because amendments vary by state and municipality.
There are two approaches. The precise method is to add equivalent feet for each fitting: a 90-degree elbow adds roughly 2-4 equivalent feet depending on pipe size, and a tee adds 4-8 feet. The field shortcut is to measure the straight-run distance and add 50% for fittings. This is conservative but safe. For critical jobs where you are borderline on pipe size, use the precise fitting count method to avoid unnecessary upsizing.
Yes, for low-pressure systems under 2 PSI. The calculator uses the same Spitzglass formula and methodology as IFGC code tables. For medium-pressure commercial systems (2-5 PSI), the Weymouth formula is typically used instead, which is not covered by this calculator. Commercial kitchens with large BTU loads from cooking equipment are a common use case for this tool.
Propane has a higher heating value per cubic foot (2,516 BTU vs 1,000 BTU for NG) but also a higher specific gravity (1.52 vs 0.60). The higher specific gravity increases friction loss, so propane piping carries fewer cubic feet per hour at the same pressure drop. However, because each cubic foot contains more BTUs, the net effect is that LP pipe sizes are often similar to NG for the same BTU load. The calculator handles this automatically when you select the gas type.
In virtually all jurisdictions, yes. Gas piping work requires a plumbing or mechanical permit, an inspection, and a pressure test before the line is put into service. The pressure test is typically 3 PSI for 15 minutes with no drop on a manometer. Some areas require a licensed plumber or gas fitter for any gas work. Never energize a gas line without passing inspection. A leak on a gas system is not a plumbing inconvenience; it is a potential explosion.
Black iron (Schedule 40) is the traditional standard and remains the most common material for exposed interior gas piping. CSST is faster to install and requires fewer fittings but must be bonded per manufacturer instructions and NFPA 54, and some insurers restrict or surcharge for CSST installations. Copper Type L and K are allowed in some jurisdictions but prohibited in others due to reaction with certain gas additives. PE pipe is used for underground exterior runs from the meter to the building. Check your local code for allowed materials before starting work.
Disclaimer: This calculator provides gas pipe sizing estimates based on IFGC/IRC methodology. Gas piping installation requires permits, inspections, and must be performed by qualified individuals in compliance with local codes. Always verify sizing with your local authority having jurisdiction. Improper gas piping can result in gas leaks, explosions, and death. ToolGrit is not responsible for installation decisions or outcomes.

Learn More

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