Calculator Inputs
Example Data Table
These examples show how different loads and lengths can change the suggested planning size.
| Scenario | Connected Load | Developed Length | Supply / Minimum | Typical Result | Planning Note |
|---|---|---|---|---|---|
| Range and water heater | 105,000 BTU/h | 45 ft | 11 / 10 in WC | Often 3/4 in | Short run with moderate demand. |
| Furnace, dryer, range | 190,000 BTU/h | 80 ft | 11 / 10 in WC | Often 1 in | Longer main needs more diameter. |
| House plus standby generator | 360,000 BTU/h | 120 ft | 11 / 10 in WC | Often 1 1/4 in or larger | Generator demand can dominate sizing. |
Formula Used
This calculator uses a simplified Darcy-Weisbach gas flow method for planning. It does not replace code tables or manufacturer instructions.
Standard Flow, SCFH = Design Load ÷ Heating Value
Total Length = Actual Length + Equivalent Length + Fitting Adders
Available Drop = Supply Pressure - Minimum Pressure - Reserve Margin
Pressure Drop = f × (L ÷ D) × (ρ × v² ÷ 2)
Reynolds Number = ρ × v × D ÷ μ
The script estimates gas density from pressure, temperature, and specific gravity. It calculates velocity for each pipe inside diameter. It then estimates friction factor with laminar flow logic or the Swamee-Jain equation. The smallest size that passes pressure drop and velocity checks is selected.
How to Use This Calculator
- Enter each appliance nameplate input rating in BTU per hour.
- Leave manual total at zero unless you already know the segment load.
- Set the supply pressure, minimum inlet pressure, and reserve margin.
- Add actual developed length plus fittings, valves, and special allowances.
- Choose the closest pipe material roughness for planning.
- Press the calculate button and review the suggested size table.
- Export CSV or PDF for notes, then verify the design locally.
LP Gas Pipe Sizing Guide
Why pipe size matters
LP gas systems need careful pipe sizing. A small pipe can starve appliances. A large pipe may cost more than needed. This calculator gives a practical planning estimate before final design.
Start with demand
Good sizing starts with total demand. Each appliance has a rating in BTU per hour. Add the ratings, then apply any diversity factor. Add future allowance when a grill, dryer, heater, or generator may be installed later.
Length and fittings
Length matters as much as load. Gas loses pressure while moving through pipe walls, bends, valves, tees, and risers. Longer runs need larger inside diameters. Fittings also act like extra pipe. This tool lets you add both actual length and equivalent fitting length.
Pressure checks
Pressure limits control the result. The supply pressure must stay above the minimum inlet pressure required by the appliance. The difference becomes usable pressure drop. The calculator compares that available drop with the calculated drop for each common pipe size.
Calculation method
The method uses a simplified fluid flow model. It estimates vapor flow, velocity, Reynolds number, friction factor, and pressure loss. It then chooses the smallest size that passes the drop and velocity checks. This makes the output useful for early planning.
Safe planning
Always treat the answer as guidance. LP gas work is regulated. Local codes, regulator settings, altitude, material rules, burial rules, and manufacturer tables can change the final requirement. A licensed gas professional should approve the installed system.
Better inputs
Use clean inputs. Enter realistic appliance loads, developed length, fitting allowance, gas gravity, heating value, and pressure settings. Review the warning messages. Compare the example table with your project. Then export the result for your job notes.
Layout review
Planning should also consider layout. A straight main with short branches often performs better than a maze of small lines. Keep regulators accessible. Avoid guessing hidden lengths. Note every appliance served by the same segment. When several fixtures fire at once, the shared pipe must carry the combined load. For branch sizing, run this tool once for the main, then again for each branch. Record assumptions beside every export. Clear notes help inspectors, installers, and future owners understand why a size was selected. Recheck inputs whenever a new appliance or longer route is added.
FAQs
1. Is this calculator suitable for final installation approval?
No. It is a planning tool. LP gas piping must be verified against local code, approved tables, appliance instructions, regulator details, and inspection requirements.
2. What does SCFH mean?
SCFH means standard cubic feet per hour. The calculator converts BTU load into standard gas flow using the heating value entered by the user.
3. Why does longer pipe need a larger size?
Longer pipe creates more friction loss. More friction lowers pressure at the appliance. A larger inside diameter reduces velocity and pressure drop.
4. Should fittings be included?
Yes. Elbows, tees, valves, unions, and regulators create added resistance. Include them through the count fields or extra equivalent length field.
5. What heating value should I enter?
Propane vapor is often estimated near 2516 BTU per cubic foot. Use a supplier value when available for better planning accuracy.
6. Can I use this for CSST?
Only as a rough comparison. CSST products must be sized with the exact manufacturer tables, connector rules, and installation guide.
7. Why did no pipe size pass?
The load, run length, pressure drop, or velocity limit may be too strict. Consider a larger design pressure, shorter run, split branches, or professional redesign.
8. Does elevation change matter?
Elevation can slightly affect gas pressure. The calculator adds lift loss for positive rise. Long vertical runs should still be reviewed professionally.