Gas Line Sizing Calculator

Size gas lines for your appliances so you get proper flow and pressure. Works in Imperial and metric units.

System Parameters

Gas Type

Pipe Length (feet) *

Inlet Pressure (in WC)

Pipe Material

Code Standard

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Quantity

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Pipe Capacity Reference

Natural Gas capacity in CFH for different pipe lengths

Pipe Size	10 ft	20 ft	50 ft	100 ft
1/2"	132	92	56	38
3/4"	278	190	115	80
1"	520	350	215	150
1-1/4"	890	610	375	260
1-1/2"	1400	950	580	405
2"	2100	1420	880	620

* Capacities shown for 0.5" WC pressure drop, black iron pipe

Standard Pipe Sizes & Applications

Residential Applications

1/2" (15mm) Black Iron

Small appliances: single dryer, fireplace, range

Capacity: 132 CFH @ 10 ft

3/4" (20mm) Black Iron

Multiple appliances: range + water heater

Capacity: 278 CFH @ 10 ft

1" (25mm) Black Iron

Main house lines: furnace + water heater + range

Capacity: 520 CFH @ 10 ft

Commercial Applications

1-1/4" (32mm) Black Iron

Small commercial: restaurant equipment

Capacity: 890 CFH @ 10 ft

2" (50mm) Black Iron

Large commercial: multiple units, boilers

Capacity: 2100 CFH @ 10 ft

3" (80mm) Black Iron

Industrial: large boilers, process equipment

Capacity: 4020 CFH @ 10 ft

Gas Line Sizing Formulas

Basic Calculations

BTU to Gas Flow Conversion

CFH = Total BTU ÷ BTU per cubic foot

Natural Gas: 1,000 BTU/cf (37.3 MJ/m³)
Propane: 2,500 BTU/cf (93.2 MJ/m³)

Example: 120,000 BTU furnace ÷ 1,000 BTU/cf = 120 CFH

Safety Factor

Safety Factor = Pipe Capacity ÷ Required Flow

Minimum: 1.0 (meets demand)
Recommended: 1.2 - 1.5 (20-50% margin)

Example: 150 CFH capacity ÷ 120 CFH demand = 1.25 safety factor

Unit Conversions

Flow: CFH × 0.0283 = m³/h

Pressure: inches WC × 0.249 = kPa

Length: feet × 0.3048 = meters

Diameter: inches × 25.4 = millimeters

Advanced Calculations

Gas Velocity (Approximate)

V = (Q × 0.0283) ÷ (π × (D/24)²)

V = velocity (ft/s)
Q = flow rate (CFH)
D = pipe diameter (inches)
Maximum recommended: 60 ft/s (18.3 m/s)

Pressure Drop (Simplified)

ΔP ≈ (Q ÷ Capacity) × 0.5" WC

ΔP = pressure drop (inches WC)
Q = actual flow (CFH)
Capacity = pipe capacity at length (CFH)
Note: Simplified approximation for black iron pipe

Code Compliance Check

NFPA 54/IPC: ΔP ≤ 0.5" WC (standard) or 1.0" WC (high pressure)

UPC: ΔP ≤ 0.3" WC (standard) or 0.8" WC (high pressure)

Safety Factor: ≥ 1.0 minimum

Velocity: ≤ 60 ft/s maximum

Professional Engineering Formulas

For precise calculations, engineers use more complex equations that account for gas properties, temperature, and specific pipe characteristics:

Weymouth Equation: Accounts for gas specific gravity, temperature, and compressibility

AGA Method: American Gas Association standard for distribution systems

Panhandle Equation: Used for high-pressure transmission lines

NFPA 54 Appendix B: Simplified tables based on tested pipe capacities

This calculator uses NFPA 54 table lookup method for ±15% accuracy on straight black iron runs.

Frequently Asked Questions

What's the real difference between sizing for natural gas vs propane?

Natural gas has about 1,000 BTU per cubic foot (37.3 MJ/m³) while propane packs 2,500 BTU per cubic foot (93.2 MJ/m³). So if you've got a 100,000 BTU furnace, you need 100 cubic feet per hour (2.83 m³/h) of natural gas but only 40 cubic feet per hour (1.13 m³/h) of propane. That's why you can often get away with smaller pipes for propane, but you need different regulators and the safety rules are stricter.

How much pressure drop can I get away with?

Most places follow NFPA 54 or IPC, which lets you lose up to 0.5 inches water column (0.124 kPa) on standard 7" WC (1.74 kPa) systems. If you're running high pressure at 14" WC (3.48 kPa), you can lose up to 1.0" WC (0.249 kPa). But if you're in UPC territory, they're tighter - only 0.3" WC (0.075 kPa) for standard and 0.8" WC (0.199 kPa) for high pressure. Check what your local inspector wants before you start.

How much gas can different pipe sizes actually carry?

For a typical 50-foot (15.2m) run, 1/2" (15mm) black iron gives you about 56 CFH (1.6 m³/h), 3/4" (20mm) gets you 115 CFH (3.3 m³/h), 1" (25mm) handles 215 CFH (6.1 m³/h), and 1-1/4" (32mm) can do 375 CFH (10.6 m³/h). Double that length to 100 feet (30.5m) and those numbers drop to 38, 80, 150, and 260 CFH respectively. Longer runs kill your capacity fast.

Why can't I use this for CSST flexible gas line?

CSST is a whole different animal. Each brand - TracPipe, HomeRun, Gastite - has their own capacity charts because the corrugated tubing and fittings create different pressure losses. A 1/2" CSST typically carries 30-40% less gas than regular 1/2" black iron. You really need to use the manufacturer's own sizing charts or software. Don't guess on this stuff.

How do I figure out what my appliances actually need?

Most residential furnaces run 80,000 to 200,000 BTU, so that's 80-200 CFH (2.3-5.7 m³/h) of natural gas. Water heaters are usually 32,000-76,000 BTU (32-76 CFH or 0.9-2.2 m³/h). A typical gas range is around 65,000 BTU total. Add them all up - don't try to get fancy with diversity factors unless you really know what you're doing. Better to oversize slightly than have problems later.

What safety margin should I build into my pipe sizing?

I always aim for at least 20-50% extra capacity if possible. So if I need 100 CFH (2.83 m³/h), I want pipe that can handle 120-150 CFH. This covers future appliance additions, gives you some wiggle room for measurement errors, and keeps the inspector happy. Never size exactly to the load - that's asking for trouble down the road.

Why does pipe length matter so much?

Friction is a killer. Take a 1" (25mm) pipe - at 10 feet (3m) it'll give you 520 CFH (14.7 m³/h), but stretch that to 100 feet (30.5m) and you're down to just 150 CFH (4.2 m³/h). That's a 71% drop! Long runs need bigger pipes, period. Sometimes it's worth planning a shorter route even if it costs more in fittings.

How fast can gas move through the pipe?

Keep it under 60 feet per second (18.3 m/s) or you'll get noise, erosion, and unstable pressures. I've seen pipes whistle like tea kettles when the velocity gets too high. In commercial kitchens, I try to stay under 30 ft/s (9.1 m/s) because nobody wants to hear that racket during service. If your calculations show high velocity, go up a pipe size.

How accurate is this calculator compared to engineering software?

For straight runs of black iron, this calculator is pretty solid - within about 15% of what you'd get from expensive engineering software. The pros use more complex equations that account for gas temperature, elevation changes, and all sorts of other factors. For most residential jobs, this simplified approach works fine. Complex commercial systems? Get an engineer.

When do I need permits and professional design?

All gas work needs permits and a licensed tech to do the installation - no exceptions. For design, most residential single-run jobs under 100 feet (30.5m) can use simplified methods like this calculator. But anything commercial, over 200 feet (61m), or more than 1,000,000 BTU/hr (293 kW) needs an engineer to stamp the drawings. When in doubt, ask your local inspector what they want to see.

Should I oversize my gas line "just to be safe"?

Within reason, sure - going up one pipe size isn't going to hurt anything and gives you headroom for future additions. But don't go crazy - oversized lines can actually cause problems with proper gas valve operation and regulator performance on some equipment. Stick with the calculated size or maybe one size larger. If you think you might add more gas appliances later, calculate for that total load instead of oversizing arbitrarily.

Complete Your Gas System Design

Gas line sizing requires understanding total appliance loads. Use these calculators to plan your complete gas system:

Expansion Tank Sizing•Pump Head & Power•Hot Water Recirculation•Pressure Drop