Gas Pipe Sizing Calculator

If the size of gas lines is too small, appliances may not operate properly due to insufficient gas supply. If they are too large, unnecessary material and installation costs will be incurred. This tool is based on the pipe sizing calculation formulas in NFPA 54's "National Fuel Gas Code" and "International Fuel Gas Code", and supports both low-pressure systems with a pressure of less than 1.5 psi and high-pressure systems with higher pressures.

When you enter the gas flow and length of pipe, it calculates the minimum inside diameter required to deliver the amount of gas needed for the appliance without exceeding the allowable pressure rating of the appliance. This value can be used regardless of whether steel (Schedule 40), copper or CSST is being used.

Problems that can be solved by calculating the gas pipeline size:

This task can be reduced to a single value: the minimum pipe diameter required to ensure sufficient gas supply throughout the entire pipeline segment while keeping the pressure within the range tolerated by the device. Each device has an output in BTUs which can be directly converted into a volumetric flow rate of cubic feet per hour.

To calculate the volumetric flow rate (CFH) for natural gas, divide the BTU per hour value by 1,000; for propane, divide it by 2,516. Add up the individual flows of all appliances served by the same pipe segment and use the total volume flow to determine the size of that pipe segment.

Is it a low pressure or high pressure system?

The formula used depends on the pressure of the system. Low-pressure systems have a pressure less than 1.5 psi, which is typical for most residential homes. For this type of system, allowable pressure loss is expressed in inches of water column.

High pressure systems have a pressure of 1.5 psi or more and are often used in commercial and industrial settings. In this case, the compressibility of the gas is taken into account when calculating, so the formula uses not just the simple pressure loss but the difference between absolute pressures squared. Selecting a pressure range at the top of the tool will display the corresponding input fields.

Here's how to use this calculator:

First select the pressure range and type of gas. As natural gas and propane have different specific gravity coefficients, the flow available for each pipe diameter will change.

To determine the pipe size, enter the flow in CFH (cubic feet per hour) and equivalent length in feet. For low-pressure systems also enter allowable pressure loss; for high-pressure systems both inlet and outlet pressures must be entered. If the field for pipe diameter is left blank, the calculation tool will automatically populate the results. Conversely, you can enter a pipe diameter and clear the flow to get the maximum flow that this pipe can provide.

Understanding of calculation formula.

Both formulas are derived from the Mueller equation, an empirical potential model that describes how the friction coefficient changes with respect to a change in diameter. They are more accurate than simpler models based on square roots because they use more general exponents.

The formula for low pressure pipe diameter is:

where Q is the flow rate in CFH, L is the equivalent length expressed in feet, Cr is the relative density coefficient of the gas and delta H is the pressure drop expressed in inches water column. If we have natural gas with a flow rate of 200 CFH, a relative density coefficient of 0.6094, an equivalent length of 50 feet and an allowable pressure loss of 0.5 inch water column, then the calculated minimum diameter is approximately 0.89 inches. So we select a pipe with a nominal size of 1 inch type 40 which has an inside diameter of 1.049 inches.

The high-pressure form of the formula uses a difference in squares of pressure to account for compressibility.

The constants change from 19.71 to 18.93 as this version is calibrated for absolute pressure in psia. The compressibility factor Y corrects the behavior of real gases. For natural gas it's about 0.9992, and for propane it's about 0.9910. In these cases, the molecules do not fully follow the ideal gas law.

If this same relationship is reversed to calculate flow instead of pipe diameter, a formula for calculating volume results.

The table below shows the meaning of each symbol and an example value.

Areas of application:

Residential installers use low-pressure formulas to determine the size of piping from gas meters to furnaces, water heaters and cooktops. The equivalent length calculations must take into account not only straight lengths of pipe but also additional lengths due to fittings such as elbows and tees. These values are available in Appendix C of NFPA 54.

In commercial and industrial applications, formulas for high pressure are used to determine the size of main lines, branch lines to rooftop equipment, and process gas lines. CSST manufacturers also create capacity tables based on these same formulas. Therefore, if you want to verify a table or determine the size of pipe sections outside of those listed, then these formulas are your starting point. The formulas are directly stated in Annex B of NFPA 54.

Tips for accurate sizing:

Use not only the actual measured length of pipe sections but also equivalent length and add in additional lengths for fittings as given in appropriate handbooks. A 90-degree elbow on a 3/4-inch pipe adds about 2.5 feet.

After you have determined the pipe size, round it up to the next standard size. Never round down even if your calculated value is just slightly below the next larger size. Also pay attention to gas velocity. If it exceeds about 100 feet per second, noise in the pipes and corrosion at connection points can occur. This is another reason for choosing a larger pipe size.