Boiler Gas Pipe Size Calculator

Estimate natural-gas or LP vapor boiler piping from appliance input BTU, BTU-to-CFH conversion, longest-length sizing, specific gravity, allowable pressure drop, branch demand, trunk demand, and screened pipe diameter.

BTU/hr to CFH Longest-length method Branch and trunk load Pressure-drop and SG adjustment

🔧Gas Pipe Presets

Pick a boiler gas piping scenario, then adjust the load, gas type, run length, pressure-drop basis, and pipe material for the actual job.

📏Boiler Gas Pipe Inputs

Unit system Metric entries are converted internally to BTU/hr and feet.

Gas specific gravity and heat value CFH demand equals appliance BTU/hr divided by this heat value.

Boiler input load (BTU/hr) Use appliance input rating, not net output or DOE heating capacity.

Other trunk gas load (BTU/hr) Water heaters, ranges, dryers, or other loads carried before the boiler branch.

Boiler branch actual run (ft) Measured pipe path from meter or trunk tee to the boiler connection.

Longest total gas run (ft) Point of delivery to the most remote gas outlet, used for longest-length sizing.

Pressure drop and fittings basis Equivalent length equals measured length multiplied by the fittings allowance.

Pipe material or system CSST, PE, copper, and connectors require listed tables and local approval.

Enter positive boiler load, non-negative other load, positive pipe lengths, and a valid pressure-drop basis.

Longest-length sizing The main branch and trunk diameters use the longest gas run plus the fittings allowance, then compare each pipe section load against adjusted capacity.

Branch versus trunk demand The branch carries only the boiler CFH. The upstream trunk carries boiler CFH plus the other gas loads entered for shared piping.

Pressure and gas correction Capacity is adjusted by the square root of pressure drop and specific gravity, while demand changes with the fuel heat value.

Gas pipe sizing result Enter the boiler load and pipe run to see branch and trunk sizes.

Boiler Branch Size - Longest-length diameter

Shared Trunk Size - Boiler plus other loads

Boiler Gas Flow - CFH from BTU load

Weakest Margin - Adjusted capacity minus demand

Full Boiler Gas Pipe Breakdown

This calculator is a screening aid, not an approval document. Final gas piping must follow the adopted fuel-gas code, appliance instructions, meter/regulator limits, pressure testing rules, and listed manufacturer tables.

⚙Gas Pipe Spec Grid

CFH Fuel Flow

Boiler gas flow is appliance input BTU/hr divided by fuel heating value in BTU per cubic foot.

0.60 SG Natural Gas Basis

Many low-pressure sizing tables are based on natural gas near 0.60 specific gravity.

0.5 in wc Common Drop

Residential low-pressure screens often compare against a half-inch water-column drop basis.

Longest Sizing Method

Each section is checked with the longest run so remote appliances retain enough pressure.

📊Boiler Gas Pipe Reference Tables

BTU Load to CFH Conversion

Gas basis	Heat value	Specific gravity	CFH formula
Typical natural gas	1020 BTU/ft3	0.60	CFH = input BTU/hr / 1020
Code-table natural gas	1000 BTU/ft3	0.60	CFH = input BTU/hr / 1000
Rich natural gas	1050 BTU/ft3	0.62	CFH decreases, SG slightly reduces capacity
LP vapor	2516 BTU/ft3	1.52	Lower CFH demand, higher SG capacity penalty
Low-BTU mixed gas	900 BTU/ft3	0.65	Higher CFH demand for the same boiler input

Representative Low-Pressure Schedule 40 Capacity

Length row	1/2 in	3/4 in	1 in	1-1/4 in	1-1/2 in	2 in
20 ft	120 CFH	250 CFH	465 CFH	950 CFH	1460 CFH	2750 CFH
40 ft	82 CFH	170 CFH	320 CFH	660 CFH	990 CFH	1900 CFH
60 ft	66 CFH	138 CFH	260 CFH	530 CFH	810 CFH	1520 CFH
100 ft	50 CFH	103 CFH	195 CFH	400 CFH	620 CFH	1150 CFH
150 ft	40 CFH	84 CFH	160 CFH	325 CFH	500 CFH	950 CFH

Longest-Length Branch and Trunk Logic

Pipe section	Demand used	Length used	Calculator output
Boiler branch	Boiler CFH only	Longest run x fittings factor	Branch diameter and branch margin
Shared trunk	Boiler CFH plus other CFH	Longest run x fittings factor	Trunk diameter and trunk margin
Branch-only comparison	Boiler CFH only	Actual boiler branch x fittings factor	Shows how conservative longest length is
Pressure-drop correction	Same CFH load	Same sizing length	Capacity x square root of selected drop / 0.5
Specific-gravity correction	Same CFH load	Same sizing length	Capacity x square root of 0.60 / selected SG

Material and System Screening Factors

Material or system	Screen factor	Use in calculator	Field verification
Schedule 40 black steel	1.00	Base low-pressure capacity table	Use adopted fuel-gas table for final size
Schedule 80 steel	0.86	Reduced area screen versus Schedule 40	Confirm internal diameter and table basis
Type L copper tube	0.82	Capacity screen where copper gas piping is allowed	Check gas composition and local code
Underground PE pipe	0.95	Preliminary underground service screen	Use PE manufacturer and utility tables
CSST	0.58	Conservative flexible tubing screen	Use the exact CSST brand sizing chart
Appliance connector	0.38	Short final connector warning only	Use connector rating, length, and appliance listing

Common Boiler Gas Pipe Scenarios

Boiler scenario	Typical input	Natural gas flow	Pipe sizing note
Small wall-hung boiler	60k-90k BTU/hr	59-88 CFH at 1020 BTU/ft3	Often limited by long 1/2 in branches
Standard mod-con boiler	100k-150k BTU/hr	98-147 CFH at 1020 BTU/ft3	Commonly needs 3/4 in or larger on long runs
High-input combi	180k-199k BTU/hr	176-195 CFH at 1020 BTU/ft3	Longest-length method often moves to 1 in
Boiler plus water heater trunk	180k-260k combined	176-255 CFH at 1020 BTU/ft3	Trunk size can exceed the boiler branch size
Light commercial boiler	300k-500k BTU/hr	294-490 CFH at 1020 BTU/ft3	Meter, regulator, pressure, and code table dominate

💡Gas Pipe Sizing Tips

Do the trunk and branch separately

A boiler branch carries only the boiler input, while the upstream trunk must carry every connected gas load downstream of that section.

Use manufacturer tables for flexible systems

CSST, appliance connectors, PE systems, and listed tubing can have very different capacities from nominal steel pipe of similar size.

Sizing a gas pipe for a boiler require that you account for many different variable in your calculations. In order for a boiler to function correctly, the gas pipe that supply the fuel to that boiler must be able to supply enough fuel to the boiler to allow it to perform its function, as well as maintain a proper pressure within the system. If the gas pipe isnt sized correct, the gas pipe may create a pressure drop within the system.

A pressure drop within a gas system that supply fuel to a boiler can lead to many different problem with that boiler, such as the development of a noisy burner flame or the short cycling of the boiler. One of the first variable to consider when sizing a gas pipe for a boiler is the input rating of the boiler. The input rating of the boiler will tell you the maximum amount of fuel that the burner within the boiler want to burn.

How to Size a Gas Pipe for a Boiler

This unit can be converted to unit of cubic feet per hour by understanding the heating value of the gas that will be used to heat the water for the system; natural gas and propane has different heating values. Thus, this factor is important in determining the flow rate of the gas through the gas pipe. The length of the gas pipe is another important factor in determining the size of that gas pipe.

The length of the gas pipe that should be used is the longest run of gas pipe between the meter and the farthest outlet of the system. Using this measurement will ensure that the remote appliance within the system dont experience a drop in pressure within the system when all of the appliances are running at the same time. In addition to the length of the gas pipe, it is also important to add a fittings allowance to the length of the gas pipe.

Elbows and tees within the gas pipe create turbulence within that system; the fittings allowance account for the turbulence that these pipe components create. A twenty percent allowance for the fittings may be used in residential gas pipe systems; however, a fifty percent allowance may be used in systems that contain many direction change for the gas pipe. The branch pipe and the trunk pipe into which the branch pipe is installed must be sized differently.

The branch pipe is only used to supply gas to the boiler; however, the trunk pipe must be capable of supplying gas to the boiler as well as to any appliance that may utilize the trunk pipe. Thus, the trunk pipe must be sized to handle the total load created by the boiler and all other appliance that utilize that trunk pipe. If the trunk pipe is smaller than the boiler require, the trunk pipe will restrict the flow of fuel to the boiler.

The material of the gas pipe is another factor to consider. Black steel pipe is the standard material used in the calculation for gas piping systems. However, corrugated stainless steel tubing, copper and polyethylene tubing has different rates of internal friction.

Because of this, a larger nominal size is often required for flexible tubing compared to rigid gas piping. Additionally, you should always check the manufacturer’s specification for the gas pipe, as the material of the pipe will impact the amount of gas that can flow through the pipe. Two more factor that must be considered are the pressure drop and specific gravity of the fuel.

A half-inch of water column is the standard pressure drop that is allowed in low pressure gas systems. A higher pressure drop can be specified in the system design in order to allow for more capacity of the gas system. The specific gravity of the fuel impact how much resistance the fuel will encounter as it moves through the gas pipe.

Because propane has a higher specific gravity than natural gas, propane will lose more pressure traveling through the gas pipe than natural gas at the same flow rate. Commonly, people make mistake when sizing gas piping systems. Many people make the mistake of only sizing the branch gas pipe and forgetting to size the trunk pipe.

Additionally, many people will use the measured length of the gas pipe instead of the longest run of the gas pipe, or they will forget to allow for the fittings in the system. If these step are skipped, the gas boiler may not be able to reach its full input rating. A calculator can be used to determine both the branch and trunk pipe margin for the system to avoid these mistake.

There are other factor outside the calculation to consider in the sizing of the gas pipe. First consider the capacity of the meter that will be used to measure the gas that is to flow into the system. Additionally, local gas code should also be considered and compared to the size of the gas pipe that was calculated.

Lastly, the goal is for the boiler to receive steady pressure at full fire. If both the branch and trunk pipe have a positive margin when all allowance are made, then the gas pipe will perform as calculated.