Gas Pipe Sizing Using Code Tables

Why Pipe Sizing Matters

Under-sized gas pipe starves appliances of fuel, causing incomplete combustion, pilot outages, and dangerous carbon monoxide production. Over-sized pipe wastes money and can create low-velocity conditions that allow condensate and debris to accumulate. Every segment of a residential gas distribution system must be sized to deliver the required flow at adequate pressure. The International Fuel Gas Code (IFGC) provides prescriptive sizing tables that make this process straightforward when you follow a systematic method.

Understanding Gas Demand

Natural gas appliances are rated in British Thermal Units per hour (BTU/h) of input — not output. The input rating is the fuel consumed, and it appears on the appliance nameplate and in the manufacturer's data sheet. To use IFGC pipe-sizing tables, you must convert BTU/h to cubic feet per hour (CFH) of natural gas.

Natural gas has a heating value of approximately 1,000 BTU per cubic foot at standard conditions. Use this factor for all residential natural-gas sizing unless your jurisdiction or the gas utility specifies a different heating value.

Conversion formula:

> CFH = BTU/h ÷ 1,000

For example, a furnace rated at 80,000 BTU/h requires 80 CFH. A water heater rated at 40,000 BTU/h requires 40 CFH.

The Longest-Length Method — Step by Step

The IFGC longest-length method (sometimes called the single longest-length method) uses one conservative pipe length — the longest run from the meter or service regulator to any appliance — to size all segments in the system. This approach is intentionally conservative and slightly over-sizes upstream segments, but it is fast, permitted by code for typical residential systems, and eliminates the need for pressure-drop calculations on each segment.

Step 1 — Determine the longest length.

Trace every possible path from the meter outlet to each appliance. Measure or scale the total pipe length along each path. The longest of those measured distances is your longest length. Use this single value for every segment in the system.

Step 2 — Convert all appliance ratings to CFH.

List every appliance and convert its BTU/h input rating to CFH using the formula above.

Step 3 — Calculate the demand for each segment.

Work from the meter toward each appliance. Each segment carries the combined demand of all appliances it feeds. The segment nearest the meter carries the total system demand. At each tee, the flow splits: each branch carries only the appliances downstream of that branch point.

Step 4 — Enter the IFGC sizing table.

Select the table that matches your pipe material (Schedule 40 steel, CSST, etc.) and the system pressure (typically 0.5 psi or less for residential low-pressure systems). Find the row for your longest length, then read across to find the column whose CFH capacity equals or exceeds your segment demand. The column header is your minimum pipe size. Never round down.

Step 5 — Record and check.

Record the pipe size for every segment and confirm that the table capacity at that size meets or exceeds the demand for that segment. Show your work — inspectors and instructors both require it.

Reading the IFGC Tables Correctly

The tables are organized by pipe material and pressure. Rows represent pipe lengths in feet; columns represent nominal pipe sizes. The table value is the maximum allowable CFH for that length and size. If your demand falls between two row values, always use the longer length row (conservative). If your CFH demand falls between two column capacities, always choose the larger pipe size (conservative).

For systems with CSST (corrugated stainless-steel tubing), use the CSST-specific table because CSST has different flow characteristics than steel pipe. Never mix the two tables for the same segment.

Meter and Total Demand

The gas utility and the meter must also be capable of supplying the total connected load. Sum the CFH for every appliance in the building. This total is the demand at the meter. In practice, not all appliances run simultaneously, but the IFGC longest-length method assumes simultaneous operation — a conservative and code-compliant assumption.

Safety Points

• Leak testing is mandatory before any gas system is put into service. Sizing a pipe correctly does not eliminate the need for a pressure test. Follow IFGC and local requirements for test medium, pressure, and duration.
• Never use an open flame to test for leaks. Use an approved gas-leak detector, listed leak-detection solution, or a calibrated pressure gauge.
• Work on energized gas lines requires shutoff. Always close the upstream shutoff valve before breaking a joint. Confirm gas is off with a manometer or detector before cutting pipe.
• Ventilation: Even during sizing and layout work in an occupied space, be aware that residual gas may be present from previous work. Maintain ventilation.
• Yellow CSST bonding: CSST must be bonded per the manufacturer's listing and code requirements to protect against lightning-induced arc damage. Pipe sizing does not substitute for proper bonding.

Practical Tip

Draw a simple isometric or single-line sketch of every system you size. Label each segment with its length and the appliances it feeds. This makes it easy to identify the longest path, calculate segment demands, and catch errors before you enter the table.