Sizing Gas Piping Layout

You may ask why you need to know how to size gas lines. Here are a few reasons to know how to size a new gas line or verify the existing gas line is properly sized.
  • When replacing the heating appliance
  • Replacing a boiler and tank-type water heater with a combi boiler
  • New construction
  • When the existing system cannot maintain proper incoming gas pressure
  • Converting oil to gas
  • Where and if you can add a gas appliance to an existing gas line.
We will show you that sizing gas lines for the home is easy. We will provide the details, accompanied by charts and a few piping layouts. You will also be able to download a couple of worksheets. After completing them, you can check your work by clicking the links with answers.

There are three basic types of calculations for residential gas lines.

  • Longest Pipe Calculation - This is what we will teach here.
  • Branch Calculation
  • Manifold Home Run System -
Let's start with a standard pipe calculation from the meter to the furthest point. We need to gather some information before starting. Once we gather all the info listed below, we must choose the proper 32 charts from the codes for the fuel type, pressures, and piping material. We will only address the use of black pipe. All the charts are applied in the same way.

Sizing Iron Pipe Gas Lines


Here is a list of information we need

  • Total Load - Connected load by appliance and Btu/h
  • Btu's or CFH - Your choice
  • Drawing - Proposed or existing layout
  • Fuel Type - Natural or LP
  • Heating Value - Get this from the fuel supplier if doing CFH calculation
  • Longest Pipe - Longest Pipe from meter to furthest appliance
  • Pressure Drop - Your choice. We suggest .5 for most margin of error

Gathered Information

  • Total Load - 312,000 Btu/h (below)
  • Range 48,000
  • Wall Heater 40,000
  • Gas Grille 32,000
  • Water Heater 42,000
  • Steam Boiler 150,000

Next, we have to know the total length of the gas main in the home. Measure from the meter location to the last appliance off the main gas line. This is shown in red below the list.

  • Btu's or CFH - BTU's
  • Drawing - Proposed layout
  • Fuel Type - Natural
  • Heating Value - ---
  • Longest Pipe - 81 feet (below)
  • 9'
  • 18'
  • 7'
  • 3'
  • 22'
  • 6'
  • 12' (from main to water heater)
  • 4'
  • Pressure Drop - .5

Calculations are usually figured from the farthest point towards the meter. In this case, we will start at the gas grille and size the pipe for the gas grille down to the tee that would go to the steam boiler. We can see from the above drawing that the total line length is 81 ft and the line from the gas grille to the tee to the gas boiler is 22 feet

The pipe from the meter to the gas grille is 81 feet. When we look at the Natural Gas BTU/h chart, it is in 10-foot increments. If you exceed a ten-foot increment, you must jump to the next 10 feet. We will need to read the row for 90 feet and go across the row until we find a number equal to or larger than our load. Then go up and find the pipe size. This means we will size the gas grill pipe for 32,000 btu/h and 90 feet of main.
We see that a 1/2" pipe will supply 33,000 btu/h. So, the pipe diameter from the tee to the boiler to the gas grille is going to be 1/2" black iron. See in blue below. Next, we will size the pipe to the boiler, the main supplying gas to the boiler, and the gas grille. See the yellow pipe below.
This time, we must add together the boiler and gas grille for the total load on this section of pipe. This would be 150,000 for the boiler and 32,000 btu/h for the gas grille, and totals 182,000 Btu/h..
Let's start with the boiler. Looking at the chart, we will always use the entire length of the main pipe for all calculations of the main pipe, and the branches are only calculated. At their length. We will enter the 90-foot row to determine the BTU/h required to cover the load, then proceed to find the corresponding pipe size. The boiler will need to be 3/4", and the main is 1-1/4" black pipe and 1-1/4" to the boiler.

You may wonder why you must run a 1-1/4" pipe to the boiler, which only has a 3/4" gas valve tapping. Refer to the chart to determine the Btu/h capacity of 10 feet of 3/4" pipe. Aim to keep the 3/4" pipe as short as possible, using a minimum number of fittings.

Add on the gas water heater


We will follow the same steps as before. Size the branch using the total length of the main (90 feet) and match it to the appliance load, 42,000. Then size the main including all the loads after the tee that includes the Water heater, gas boiler, and gas grille, which equals 232,200 btu/h. Even though the main pipe length from the tee to the wall heater to the tee for the water heater is only 3 feet, it must be increased to a 1-1/2" pipe due to adding the water heater.

We will do the same calculation for the pipe length from the tee to the wall heater and the meter to the tee for the range, and the two connectors for the wall heater and the range. It would look like this.

Worksheet #1

Here is the first worksheet if you want to practice. You can hand-draw a copy of this, right-click on the drawing, save it as a PNG or JPEG, and then print it.

Click here for the answer (Use your back button to return to this page)

Multiple gas mains from one gas meter

Often, the meter is piped into the gas main at a tee, where the gas main branches off into two different directions. The pipe from the meter to the tee is sized to accommodate the total BTU/h of all appliances and includes the length of run on both upper and lower mains. Calculate which of the loops is the longest. That is the figure you will use to size the common meter pipe. The side of the tee with the longest loop will be used to size the pipes and branches. The smaller main line side of the tee will also use the meter pipe plus the piping on the shorter side of the tee.

How to size the pipes

  • Meter Pipe = 295 btu/h @ 54 feet of pipe. Table, use 60 Feet row
  • Upper Main = 103 btu/h total @ 54 Feet of pipe. Table, use 60 Feet row
  • Lower Main = 192 btu/h @ 39 feet of pipe. Table, use 40 Feet row

This is what the calculation will look like. You can treat this like two separate jobs. Calculate the upper main pipe as we did above, and the lower main as we did above.

What If

What if you are going to remove the water heater tank and install a tankless water heater that has an input of 160,000 btu/h? You probably already realize you will have to change the pipe size to the new tankless water heater. The question is whether the existing piping back to the tee at the meter is correctly sized. What about the pipe to the meter itself, is it the correct size?

Let's do another worksheet. First, show the drawing with the tankless water heater, but with the original pipe sizes. Your job is to see how much piping needs to be changed, if any.

Click here for the answer (Use your back button to return to this page)

Natural Gas BTU/h Pressure Chart

You may hear sometimes that you have to add equivalent feet of pipe for fittings. This is not true for residential gas systems that have the longest loop system. If using the 2 PSI or less chart and .5" water column pressure drop, you do not have to count for fittings. There is enough fudge in the charts to avoid using a fitting equivalent.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property, or bodily injury from applied information found on this website.