How to Size a Gas Line
Sizing a gas line comes down to two numbers: how much fuel your appliances demand (their total BTU/hr) and how far the pipe has to carry it (the longest run). Get those, and the pipe diameter falls out of a capacity table. But sizing is planning only — all gas work must be done by a licensed professional with a permit and inspection.
Safety first — this is planning, not a permit
Everything below is for understanding and budgeting. Gas work is dangerous and licensed. Undersized or leaking gas piping risks fire, explosion and carbon-monoxide poisoning, and gas piping is inspected work in every jurisdiction. Use these tools to plan and to sanity-check a contractor’s proposal — then hire a licensed gas fitter, pull the permit, and have the work inspected. Nothing here substitutes for that.
Step 1: total the appliance load
Every gas appliance has a BTU/hr input rating on its label. Add up everything the line will feed. The appliance gas load calculator totals them.
Worked example. A furnace at 80,000 BTU/hr, a water heater at 40,000, a range at 65,000 and a dryer at 22,000 total 207,000 BTU/hr. That connected load — not any single appliance — is what the trunk of the line must carry.
Step 2: convert BTU to CFH
Gas capacity tables are in cubic feet per hour (CFH), so convert your BTU load using the fuel’s heating value. The BTU-to-CFH calculator does it:
CFH = BTU/hr ÷ heating value (natural gas ≈ 1000, propane ≈ 2516)
Worked example. 100,000 BTU/hr of natural gas is 100,000 ÷ 1000 = 100 CFH. The same 100,000 BTU/hr of propane is only 100,000 ÷ 2516 ≈ 39.7 CFH, because propane packs far more energy per cubic foot — which is why propane lines can be smaller than natural-gas lines for the same appliances.
Step 3: size the pipe by longest run
Capacity falls as the pipe gets longer, so gas sizing uses the longest-length method: find the distance from the meter to the farthest appliance, and size every section of the trunk for that length. The gas pipe size calculator reads the diameter, and the gas pipe capacity table lists the bands.
Worked example. A 200,000 BTU/hr natural-gas load (about 200 CFH) over a 40-foot longest run lands on a 1-inch schedule-40 line. Stretch that run to 80 feet and the same load needs a larger pipe, because capacity drops roughly with the square root of length — double the distance and a pipe carries only about 70% as much.
Why longest length, not each branch?
Using the single longest run to size the whole system is a deliberately conservative simplification: it guarantees every appliance gets enough gas even at the worst-case distance, without solving the network branch by branch. More detailed methods (branch-length, or the pressure-drop approach) can shave a pipe size, but the longest-length method is the standard planning shortcut and errs safe.
Pressure drop is the real limit
Under the hood, what a table encodes is allowable pressure drop — typically about 0.5 inch of water column on a standard low-pressure system. A pipe is “big enough” when it delivers the required CFH without dropping more pressure than that. The capacity vs pressure-drop calculator shows how much a given size and length can carry within that limit, which is useful when you are checking whether an existing line can take a new appliance.
Adding an appliance to an existing line
The common real-world question is “can my current line feed a new tankless heater (or range, or generator)?” Answer it in order: add the new appliance’s BTU to the existing load, convert to CFH, and check whether the existing pipe size and run still fall within capacity on the pressure-drop tool. If not, the line needs upsizing — a job for the licensed installer who will pull the permit.
Pipe material changes the numbers
Gas piping is not one material, and the choice affects both capacity and cost. Black iron (steel) pipe is the traditional workhorse: rigid, threaded, and the basis for most schedule-40 capacity tables like the one behind the gas pipe size calculator. CSST (corrugated stainless-steel tubing) is flexible and fast to route, but its corrugated bore has more friction than smooth steel, so a given CSST size carries less than the same nominal black-iron size — it uses its own capacity tables and its own bonding requirements for lightning safety. Copper is used for gas in some regions and not others. The practical upshot: a capacity band is tied to a material, and you cannot read a black-iron table and then run CSST of the “same size.” Because these rules and the allowed materials vary sharply by jurisdiction, the material decision belongs to the licensed installer who knows local code — another reason to treat this as planning, not a build spec.
Pressure, regulators and two-stage systems
The ordinary capacity tables assume a standard low-pressure system — roughly 0.5 inch of water column allowable drop downstream of the meter’s regulator. Some larger homes and longer runs use a two-pounds (elevated-pressure) system, delivering gas at higher pressure to a smaller trunk and then dropping it back down with an appliance regulator near each unit; this lets a smaller pipe carry more gas over a long distance. It is a legitimate design, but it is a different set of tables and a different install, and it is firmly licensed-professional territory. If a contractor proposes it for a far appliance, that can be the right call — the point here is only that the simple longest-length band assumes standard pressure, so a proposal that departs from it is not wrong, just outside what a planning tool models. Confirm any elevated-pressure design with the installer and the inspector.
Labeled bands — and a licensed pro
The capacity figures here are NFPA-54-style planning bands, not a code citation; pipe material, fuel pressure, fittings and local code all change the real answer. Confirm every gas design with a licensed professional, pull the permit, and have the work inspected. Gas leaks are dangerous — treat this as planning only.