The one question gas sizing answers
Gas line sizing comes down to a single test: can the pipe deliver enough cubic feet of gas per hour to satisfy every appliance at full fire, with pressure to spare? Everything else is detail. A line carries a flow in cubic feet per hour (CFH); multiply that by the heating value of the gas and you have the BTU per hour it can supply. This gas pipe sizing calculator works that out in capacity mode, and turns it around in sizing mode to give the smallest bore that meets a load.
Use the real bore, not the nominal label
Capacity depends sharply on the inside diameter, and the nominal pipe size is not the bore — an NPS ½ line is about 0.622 in inside, not 0.5 in. Feed the tool the actual inside diameter; if you only know the nominal size and wall, the pipe size conversion calculator will give you the bore first.
The low-pressure gas formula
For low-pressure fuel gas (below about 1.5 psi) the public Spitzglass equation relates flow, bore, pressure drop, length and gas gravity:
Sizing mode solves the same equation the other way, for the diameter d, given the flow the load needs. Because d is inside a fifth-power term the bore dominates: a small increase in diameter buys a large increase in capacity, which is why rounding up to the next real pipe almost always works in your favour.
Pressure drop is the factor people forget
As gas travels it loses pressure to friction, and the farther it goes the less it can deliver: the very same pipe that carries a healthy flow over a few feet may carry a fraction of that over a long run. The allowable pressure drop is therefore central, not incidental. For a typical low-pressure house line near 7 inches of water column after the regulator, a commonly used allowance is about 0.5 inch water column, with some rules holding to 0.3 inch. The right figure depends on your system pressure and your code, so it is an input here, not a built-in assumption — and a tighter allowable drop calls for a larger pipe.
Count the fittings
Every elbow, tee and valve adds resistance equivalent to extra straight pipe, so sizing on the measured straight length alone flatters the result. A practical habit is to add a margin to the run — often something like 10 to 20 percent, and as much as 50 percent on a fitting-heavy run — or to add a set equivalent length per fitting. The fitting-allowance box adds a percentage to the length you enter so the fittings aren't simply ignored; for a careful design, count the actual fittings against your code's equivalent-length figures.
Check every branch, not just the longest run
It's tempting to size the whole system from the longest run, but a short branch feeding a large appliance can demand a bigger pipe than a long branch feeding a small one, because capacity turns on both the load a section carries and its own length. Size each section for the load it actually carries over its length, then confirm the most remote and the largest outlets still get enough. This tool sizes one run at a time, so step through your branches one by one.
Natural gas and propane size differently
Two gas properties pull in opposite directions. Propane packs far more energy per cubic foot — about 2,516 BTU against roughly 1,030 for natural gas — so the same appliance needs fewer cubic feet per hour of propane, which tends to allow a smaller pipe. But propane is also much denser, with a specific gravity near 1.52 against about 0.60 for natural gas, and a heavier gas moves less freely through the same bore, which pushes the other way. Both effects are built in: pick the gas, or type your own gravity and heating value, and the formula and the BTU conversion use them.
| Gas | Specific gravity (air = 1) | Heating value (BTU/ft³) |
|---|---|---|
| Natural gas | ≈ 0.60 | ≈ 1,030 (editable 900–1,100) |
| Propane / LPG | ≈ 1.52 | ≈ 2,516 |
Worked examples
Example 1 — capacity of a known pipe
A 0.622 in inside-diameter natural gas line, 100 ft long, at 0.5 inch water column drop (SG 0.6) carries roughly 38 CFH, which at 1,030 BTU/ft³ is about 39,000 BTU/hr. Open it with ?mode=capacity&id=0.622&len=100&drop=0.5&gas=ng.
Example 2 — sizing for a load
For a 100,000 BTU/hr natural gas load over 100 ft at 0.5 inch drop, that's about 97 CFH, and the formula returns a minimum bore of roughly 0.86 in — so you'd round up to a real pipe whose published inside diameter meets or exceeds that and verify it against your code's tables.
Gas pipe sizing FAQ
How do I size a gas pipe by BTU load?
First convert the connected load to a flow in cubic feet per hour by dividing the BTU per hour by the heating value of the gas — about 1,030 BTU per cubic foot for natural gas, about 2,516 for propane. Then find a pipe whose carrying capacity, at your run length and allowable pressure drop, is at least that flow. The sizing mode does the whole chain: enter BTU per hour, length, allowable drop and gas type and it returns the minimum inside diameter, which you round up to a real pipe and verify. Size on the connected load with diversity, not the simple sum of every appliance nameplate.
What pressure drop should I use for low-pressure gas sizing?
For a typical low-pressure house line near 7 inches of water column after the regulator, a commonly used allowable drop is about 0.5 inch water column, with 0.3 inch used by some stricter rules. The drop is not fixed — it depends on the system pressure and which code applies — so it's an input here, not an assumption, and the figure you choose must match your local code. Capacity falls as the allowable drop falls, so a tighter drop calls for a larger pipe.
Why does propane size differently from natural gas?
Two properties differ. Propane carries far more energy per cubic foot — about 2,516 BTU against roughly 1,030 for natural gas — so the same appliance needs fewer cubic feet per hour of propane, which tends to allow a smaller pipe for the same load. But propane is also much denser, with a specific gravity near 1.52 against about 0.60, and a heavier gas flows less freely, which works the other way. The calculator carries both effects because you pick the gas (or enter your own gravity and heating value) and it uses them in the flow formula and the BTU conversion.
Do I need to account for fittings and elbows?
Yes. Every elbow, tee and valve adds resistance equivalent to extra straight pipe, so sizing on the measured straight length alone overstates the capacity. A common allowance is to add a percentage to the straight run — often around 10 to 20 percent, sometimes 50 percent for fitting-heavy runs — or to add an equivalent length per fitting. This tool lets you add a percentage to the length so the fittings aren't ignored; for a careful design, count the actual fittings against your code's equivalent-length figures.
Should I size on the longest run or check every branch?
Check every branch, not just the longest run. A short branch feeding a large appliance can need a bigger pipe than a long branch feeding a small one, because capacity depends on both the load that section carries and its length. Size each section for the load it actually carries over its own length, then confirm the whole system still delivers enough pressure at the most remote and the largest outlets. This tool sizes one run at a time, so run it for each section.
Is this gas pipe sizing calculator accurate enough to install from?
No — treat it as an estimate and a sanity check, not an installation document. It uses a public low-pressure gas-flow formula and standard published gas properties, which is close to what design software gives for straight low-pressure runs, but it does not reproduce or replace the code sizing tables, and it can't see your meter capacity, regulator, elevation changes or local amendments. Fuel-gas work must be designed, installed and verified by a licensed professional and comply with your local code. Pull a permit and have it inspected.
Is this tool free and private?
Yes. It's free, needs no signup or download, and runs entirely in your browser, so nothing you enter leaves your device. You can copy a shareable link that reopens it with your mode, diameter or load, length, drop and gas type already filled in, and it keeps working offline once loaded.