Natural Gas Pipe Sizing Calculator

The Natural Gas Pipe Sizing Calculator sizes a low-pressure gas supply pipe. You enter the input rating of each appliance served, the developed length to the most remote one, and the pressure drop the system may lose, and it converts the connected load to a volumetric demand, finds the smallest pipe that carries it within that pressure drop, and reports the spare capacity and gas velocity at the size it picked.

It works in United States customary units: BTU per hour for appliance ratings, cubic feet per hour for flow, feet for length, and inches of water column for pressure drop.

Use it for a first-pass sizing of a residential or light-commercial low-pressure gas system. It sizes by the longest-length method, which is the conservative simple approach, and it is not a code table.

Purpose and model boundary

This model sums six appliance heat-input allowances, converts them to volumetric gas demand, and applies the low-pressure Spitzglass relationship to select the smallest embedded Schedule 40 steel pipe that satisfies capacity and velocity criteria over the longest run. It supports a first-pass longest-length sizing check. It does not reproduce NFPA 54, IFGC, utility, or local code tables; design pressure, diversity, regulator behavior, elevation, fittings, branches, and appliance manufacturer requirements still require project-specific review.

Inputs and units

Furnace, water-heater, range/cooktop, clothes-dryer, fireplace, and spare allowances are entered in BTU/hr. Heating value is in BTU/ft³, allowable pressure drop in inches of water column, gas specific gravity is relative to air, longest length is in ft, and velocity limit is in ft/s. The workbook contains six illustrative nominal sizes and inside diameters from 1/2 in through 2 in.

Governing relationships

Connected load and standard volumetric demand are:

Load = sum(appliance loads)

Q_demand = Load / heating value

For each candidate inside diameter d in inches, the workbook uses the Spitzglass low-pressure equation:

Q_capacity = 3550 × sqrt((deltaH × d^5) / (SG × L × (1 + 3.6/d + 0.03d)))

where deltaH is allowable pressure drop in inches water column, SG is gas specific gravity, and L is longest length in ft. Demand velocity is:

velocity = (Q_demand / 3600) / (pi/4 × (d/12)²)

Spare capacity at the selected size is Q_capacity - Q_demand.

Calculation sequence

1. Sum the appliance and reserve loads.
2. Divide by the entered heating value to obtain cfh demand; no diversity factor is applied.
3. Evaluate the six candidate inside diameters in ascending order with the same pressure drop, gas gravity, and longest length.
4. For each candidate, calculate Spitzglass capacity and demand velocity.
5. Select the first candidate whose capacity is at least the demand and whose velocity is no greater than the entered limit.
6. Return the selected nominal size, inside diameter, capacity, velocity, and spare capacity; if no candidate qualifies, return none and a blank numeric ID.
7. Evaluate Model_Status in the order below.

Outputs and interpretation

Recommended_Nominal_Size and Recommended_Pipe_Id identify the first qualifying embedded size. Total_Connected_Load is the heat input basis; Total_Demand is its cfh conversion. Capacity_At_Chosen_Size, Gas_Velocity_At_Chosen_Size, and Spare_Capacity show why the choice qualifies. The chart compares the chosen pipe's capacity over length with the constant demand line.

Validation and status logic

The empty-load check takes precedence over invalid gas/run parameters, which in turn take precedence over the no-size warning.

Assumptions and limitations

• All entered appliance ratings are summed at 100% coincidence; the workbook applies no diversity or load factor.
• Heating value is treated as constant and converts BTU/hr directly to cfh. Pressure/temperature base conditions and gas composition are not normalized separately.
• The same specific gravity, allowable drop, and longest length apply to every candidate. Equivalent length and local fitting losses are not added.
• The public Spitzglass equation is intentionally more conservative than common NFPA 54/IFGC sizing tables in many cases; it is not a substitute for those tables.
• Only six illustrative Schedule 40 steel inside diameters are searched. Other materials, schedules, and larger sizes require external data.
• The model does not size a branching network, account for regulator lock-up or meter capacity, or establish combustion-air and venting requirements.

Restrictions and non-computing states

This calculator rejects negative appliance loads, heating value, pressure drop, gas gravity, and length. The workbook requires at least one positive appliance load and positive heating value, pressure drop, gravity, and length. Velocity limit has no published minimum; an unrealistic nonpositive limit can prevent every candidate from qualifying and produce the controlled CHECK state. A no-size state still reports the connected load and demand but has no valid recommended diameter.

Errors and warnings

A rejected entry means the request did not satisfy the published input rules. Workbook NOT VALID means the load conversion or Spitzglass basis is non-computing. Workbook CHECK means the calculation ran but the embedded catalogue did not contain a qualifying pipe. A connection or calculation-service failure is a service condition, not evidence that a gas run is safe or unsafe.

The workbook derives its relations rather than reproducing any table, chart or figure from a code, standard or agency publication. Capacity is computed from the low-pressure flow relation rather than read from a sizing table.

The sources below are the authoritative methods this tool sits beside and does not implement:

• International Code Council. Sizing and Capacities of Gas Piping, International Fuel Gas Code Appendix A. https://codes.iccsafe.org/content/IFGC2024V1.0/appendix-a-sizing-and-capacities-of-gas-piping-ifgs
• International Code Council. CodeNotes: Fuel Gas Pipe Sizing, a summary of the sizing procedures. https://www.iccsafe.org/building-safety-journal/bsj-dives/codenotes-fuel-gas-pipe-sizing/
• Engineering ToolBox. Natural gas pipe sizing, low-pressure capacity tables for comparison. https://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html

The code adopted where the work is built governs. Its tables also carry the material, joint, support, sleeving, venting and combustion-air requirements that this tool does not address at all, several of which are life-safety provisions. No trademark or organisation name appearing here implies endorsement by its owner.

Additional source notes migrated from Methodology

The workbook uses the low-pressure Spitzglass equation and compares its output with code-table methods without reproducing those tables. The reviewer packet cites Engineering ToolBox's Spitzglass calculator, EngrAssist's low-pressure gas sizing reference, and Reuven Engineering Tools' NFPA 54 comparison. The adopted fuel-gas code and utility requirements govern final sizing.