Gas Pipe Capacity vs Pressure Drop (CFH)
Find the CFH capacity of a gas pipe size and run length at a typical pressure drop, using a labeled NFPA-54-style band.
Calculator
A 1 in Schedule-40 line about 40 ft long carries roughly 205 CFH at a typical 0.5 in WC drop (labeled NFPA-54-style band) — about 205,000 BTU/hr on natural gas. ⚠️ Gas work is for licensed professionals only.
This is the inverse of pipe sizing: instead of asking "what size do I need," it asks "how much can this size carry?" Pick a nominal pipe size and a run length, and the tool returns the capacity in CFH (and the equivalent natural-gas load in BTU/hr) at a typical 0.5 in WC pressure drop. It is the quick check you run when you already have pipe in the wall and want to know whether it can feed a new appliance.
Capacity is set by three things: the pipe diameter, the length of the run, and the allowable pressure drop. Bigger and shorter carries more; longer carries less. The band here uses the standard low-pressure natural-gas assumption so it lines up with the gas pipe size tool.
Formula
Capacity scales from the 40 ft reference run:
capacity(length) ≈ capacity(40 ft) × √(40 ÷ length)
The reference capacities (Schedule-40 metallic pipe, natural gas, ~0.5 in WC drop) are in the table below. The equivalent load in BTU/hr is simply CFH × 1000 on natural gas.
Worked example
A 1 in Schedule-40 line over a 40 ft run sits right at its reference capacity: about 205 CFH, or roughly 205,000 BTU/hr on natural gas. Double the run to 80 ft and capacity drops to about 205 × √(40 ÷ 80) ≈ 145 CFH — a vivid reminder that length costs you capacity. Step up to 1-1/4 in and the same 40 ft run carries about 435 CFH.
Pressure drop, 2-psi systems and CSST
The "0.5 in WC drop" is the allowable pressure loss the table is built around — a common design target for a low-pressure (about 7 in WC) residential natural-gas system. If your system is designed for a different drop, or runs at 2 psi with a downstream regulator (common for longer propane runs), the capacities change and you need the matching table. Likewise, CSST and copper have their own charts.
Use this as a feasibility check, not a final design. If the existing pipe's capacity comfortably exceeds the new appliance's CFH, you likely have room; if it is marginal, a licensed gas fitter should do the full longest-length calculation for the whole system, since adding a load can affect every upstream segment. Gas work requires a licensed professional, a permit and an inspection.
Reference table
Schedule-40 metallic pipe, natural gas (~0.60 sp. gr.) at a typical 0.5 in WC drop, tabled at the 40 ft reference run. Capacity falls on longer runs (≈ 1 ÷ √(length ÷ 40)). Labeled NFPA-54-style planning band — your gas fitter and local code set the real sizing.
| Nominal size | Capacity (CFH) | ≈ Natural-gas load (BTU/hr) |
|---|---|---|
| 1/2 in | 95 | 95,000 |
| 3/4 in | 190 | 190,000 |
| 1 in | 205 | 205,000 |
| 1-1/4 in | 435 | 435,000 |
| 1-1/2 in | 650 | 650,000 |
| 2 in | 1,250 | 1,250,000 |
Frequently asked questions
How much gas can a 1 inch pipe carry?
On a low-pressure natural-gas system, a 1 in Schedule-40 line over a 40 ft run carries about 205 CFH (~205,000 BTU/hr) at a typical 0.5 in WC drop. Longer runs carry less — an 80 ft run drops to roughly 145 CFH.
Why does capacity fall on longer runs?
Friction. Over a longer pipe, more of the limited pressure is spent overcoming friction, so less gas reaches the end. Capacity falls roughly with the square root of the length ratio, which is why the tool scales from the 40 ft reference.
What is a 0.5 inch WC pressure drop?
It is the amount of pressure the pipe is allowed to lose along the run, measured in inches of water column. Residential natural gas is delivered at about 7 in WC, and tables commonly design for a 0.5 in WC drop so appliances still get enough pressure.
Can I add an appliance to an existing line?
Maybe — check whether the existing pipe's capacity comfortably exceeds the combined CFH of the old and new appliances. If it is close, a licensed gas fitter should recheck the whole system, because a new load can overstress upstream segments.
Does this apply to CSST or 2-psi systems?
No. The band is for Schedule-40 metallic pipe on a low-pressure natural-gas system. CSST, copper and 2-psi (regulated) designs use different capacity tables — size those with the manufacturer's chart and a licensed installer.