A Gas Piping Size Calculator is a tool or, more accurately, a process used to determine the correct diameter for the pipes in a natural gas or propane system. Its primary purpose is to ensure that every appliance receives enough fuel at the correct pressure to operate safely and efficiently. Sizing gas pipes correctly is a critical safety measure. Using a pipe that is too small can starve an appliance of fuel, leading to poor performance and potentially dangerous conditions. A properly sized system guarantees that even when all appliances are running, the pressure in the system remains stable and sufficient.
This guide explains the standard, code-approved method for sizing gas pipes. It is intended for educational purposes only. The installation and modification of gas piping systems must be performed by a licensed and qualified professional in accordance with all local codes and regulations. Incorrectly installed gas piping can lead to fires, explosions, and carbon monoxide poisoning.
formula
The process of sizing gas pipes does not use a single complex formula. Instead, it involves a series of calculations to determine gas load and length, followed by a lookup in standardized tables from the International Fuel Gas Code (IFGC).
Step 1: Calculate the Gas Load for Each Appliance and Segment
The first step is to determine how much gas flow in Cubic Feet per Hour, or CFH, each appliance requires. Appliance ratings are usually given in BTU/hour.
Formula 1A: Convert BTU/hr to CFH
CFH = Appliance BTU Rating / Gas Energy Content
Variables:
CFH: The required gas flow in Cubic Feet per Hour.
Appliance BTU Rating: The BTU/hour consumption of the appliance.
Gas Energy Content (BTU per Cubic Foot): A standard value for your gas type.
For Natural Gas (NG): Use 1,100.
For Propane (LP): Use 2,500.
Formula 1B: Calculate Total Load for a Pipe Segment
A pipe segment is any straight run of pipe between two fittings. You must calculate the total gas load that each segment has to carry.
Total Segment Load (CFH) = Sum of CFH for all appliances fed by that segment
Step 2: Determine the “Longest Length” of the System
This is the most critical measurement for the entire system. It is not a calculation but a physical measurement.
Definition:
Longest Length = The measured distance in feet from the gas meter to the inlet of the furthest appliance in the piping system.
You only determine this value once for the entire system. Every pipe segment in the system will be sized using this same “Longest Length” value, regardless of where that segment is located.
Step 3: Determine the Pipe Size Using Sizing Tables
This step uses the results from Steps 1 and 2 to look up the correct pipe size from a standard sizing table. The “formula” here is a lookup process.
Lookup “Formula”:
Pipe Size = Find in Table (Gas Type, Pressure Drop, Longest Length, Segment Load)
How the Lookup Works:
- Select the Correct Table: Choose the sizing table that matches your gas type (Natural Gas or Propane) and your system’s pressure. For most homes, this is a low-pressure system with a standard pressure drop of 0.5 inches of Water Column.
- Find the Length Column: In the table, find the column header that is equal to or the next number higher than your “Longest Length” from Step 2. For example, if your longest length is 47 feet, you would use the “50 feet” column.
- Find the Capacity: Read down this length column until you find a capacity value in CFH that is equal to or greater than the “Total Segment Load (CFH)” you are sizing from Formula 1B.
- Identify the Pipe Size: From that capacity value, read horizontally to the left to the first column of that row. The value in that column is your required pipe size for that specific segment.
Common Appliance Gas Loads (BTU to CFH)
This table shows the approximate gas load in Cubic Feet per Hour (CFH) for common residential appliances. You should always use the specific rating listed on the appliance itself for official calculations.
| Appliance | Typical BTU/hr Rating | CFH for Natural Gas (~1100 BTU/cf) | CFH for Propane (~2500 BTU/cf) |
| Furnace | 100,000 BTU/hr | 91 CFH | 40 CFH |
| Tank Water Heater | 40,000 BTU/hr | 36 CFH | 16 CFH |
| Tankless Water Heater | 199,000 BTU/hr | 181 CFH | 80 CFH |
| Gas Range | 65,000 BTU/hr | 59 CFH | 26 CFH |
| Gas Dryer | 22,000 BTU/hr | 20 CFH | 9 CFH |
| Fireplace Insert | 35,000 BTU/hr | 32 CFH | 14 CFH |
Example
Imagine a simple natural gas piping system.
- The gas meter connects to the house.
- Pipe A runs 20 feet to a tee fitting.
- From the tee, Pipe B runs 30 feet to a furnace (100,000 BTU).
- From the same tee, Pipe C runs 15 feet to a water heater (40,000 BTU).
Step 1: Calculate Loads
First, convert BTU to CFH for each appliance using Natural Gas (1100 BTU/cf).
Furnace Load = 100,000 / 1100 = 91 CFH
Water Heater Load = 40,000 / 1100 = 36 CFH
Next, find the load for each pipe segment.
Load for Pipe B = It only feeds the furnace = 91 CFH.
Load for Pipe C = It only feeds the water heater = 36 CFH.
Load for Pipe A = It feeds both the furnace and water heater = 91 + 36 = 127 CFH.
Step 2: Find the Longest Length
Measure the full distance from the meter to the furthest appliance.
Path to Furnace: Pipe A (20 ft) + Pipe B (30 ft) = 50 feet.
Path to Water Heater: Pipe A (20 ft) + Pipe C (15 ft) = 35 feet.
The furthest appliance is the furnace, so the Longest Length for the entire system is 50 feet.
Step 3: Use a Sizing Table
Now you look up the size for each pipe segment using the Longest Length of 50 feet.
- Sizing Pipe A: Load is 127 CFH. In the sizing table, go to the “50 feet” column. Read down until you find a value of 127 or more. The corresponding pipe size on the left of that row is the correct size for Pipe A.
- Sizing Pipe B: Load is 91 CFH. Use the same “50 feet” column. Read down to find a value of 91 or more. Find the corresponding pipe size.
- Sizing Pipe C: Load is 36 CFH. Use the same “50 feet” column. Read down to find a value of 36 or more. Find the corresponding pipe size.
By following this process for each section, you ensure every part of the system is sized correctly based on its load and the system’s overall length.
Most Common FAQs
This is the most important concept in gas pipe sizing. The pressure drop in a gas system is determined by the total length the gas has to travel and the total load on the system. By using the single longest run to the furthest appliance for all calculations, you are ensuring that even the appliance at the very end of the line will have enough pressure to operate properly when all other appliances are also running. If you used the actual short length of an individual pipe segment, you would select a smaller pipe, which would contribute to a greater overall pressure drop and potentially starve the furthest appliance.
If a gas pipe is too small, it creates excessive friction, which causes the pressure of the gas to drop too much by the time it reaches the appliance. This condition, known as under-firing, can lead to several problems. The appliance may not light at all, or it may operate inefficiently, producing less heat. Most critically, it can lead to incomplete combustion, which can generate dangerous levels of carbon monoxide, a colorless, odorless, and deadly gas.
No, the standard pipe sizing tables discussed here are specifically for rigid iron pipe. Corrugated Stainless Steel Tubing (CSST) is a flexible gas line that has its own unique flow characteristics. You must not use the rigid pipe tables for CSST. Instead, you must use the specific sizing tables provided by the manufacturer of the CSST product you are installing. Using the wrong table will result in an incorrectly sized system.