What size gas line for a furnace?
The gas line size for a furnace is 1/2 inch Schedule 40 pipe on runs up to 20 feet, then 3/4 inch from 30 to 100 feet, then 1 inch from 125 to 200 feet. A typical residential furnace is rated 100,000 BTU per hour, which is 100 CFH of natural gas, and the required pipe grows with the length of the run because capacity falls as gas travels.
Required gas line size for a furnace at every run length
A mid-size residential forced-air furnace. High-efficiency units often run lower and older units higher, so read the plate. Measure the longest run from the gas meter to the appliance, round that measurement up to the next row, and read the size. Rounding down is the single most common way a line ends up undersized. Demand here is fixed at 100 CFH; only the length changes.
| Longest run | Smallest pipe that works | Its capacity | Margin over 100 CFH |
|---|---|---|---|
| 10 ft | 1/2 inch | 172 CFH | 72 CFH spare |
| 20 ft | 1/2 inch | 118 CFH | 18 CFH spare |
| 30 ft | 3/4 inch | 199 CFH | 99 CFH spare |
| 40 ft | 3/4 inch | 170 CFH | 70 CFH spare |
| 50 ft | 3/4 inch | 151 CFH | 51 CFH spare |
| 60 ft | 3/4 inch | 137 CFH | 37 CFH spare |
| 70 ft | 3/4 inch | 126 CFH | 26 CFH spare |
| 80 ft | 3/4 inch | 117 CFH | 17 CFH spare |
| 90 ft | 3/4 inch | 110 CFH | 10 CFH spare |
| 100 ft | 3/4 inch | 104 CFH | 4 CFH spare |
| 125 ft | 1 inch | 173 CFH | 73 CFH spare |
| 150 ft | 1 inch | 157 CFH | 57 CFH spare |
| 175 ft | 1 inch | 144 CFH | 44 CFH spare |
| 200 ft | 1 inch | 134 CFH | 34 CFH spare |
Where the size steps up
The answer is not one size. It is 1/2 inch through 20 feet, then 3/4 inch through 100 feet, then 1 inch through 200 feet. Each step happens at the first column where the smaller pipe can no longer carry 100 CFH within the code allowance of 0.5 inches water column. Anyone quoting a single size for a furnace without asking how far the run is has skipped the half of the calculation that actually varies.
Worked examples at the four most common runs
Each of these shows the full NFPA 54 lookup for that distance, the nearby sizes and the margin:
- Gas line size for a furnace at 20 feet — 1/2 inch
- Gas line size for a furnace at 40 feet — 3/4 inch
- Gas line size for a furnace at 60 feet — 3/4 inch
- Gas line size for a furnace at 100 feet — 3/4 inch
If a second appliance shares the run
The code sizes the total connected load, so this is where undersized lines come from. At 60 feet a furnace alone needs 3/4 inch. Add a 100,000 BTU furnace to the same pipe and the connected load becomes 200,000 BTU, or 200 CFH, which needs 1 inch at that same 60 feet — a size up from the single-appliance answer. Size the pipe for everything that will ever hang off it.
Propane instead of natural gas
Propane carries 2,516 BTU per cubic foot against natural gas at 1,000, so the same furnace draws only 40 CFH on LP. PlumbCalc returns 1/2 inch at 20 feet and 1/2 inch at 100 feet. Those figures are deliberately conservative — they are read off the natural gas table, so they come back equal to or larger than a dedicated LP table would give. Confirm any LP work against an LP table and local code.
Before you cut any pipe
These figures come straight from NFPA 54 Table 402.4(2) for Schedule 40 metallic pipe, natural gas at 0.60 specific gravity, inlet pressure under 2 psi and an allowable pressure drop of 0.5 inches water column. That is the ordinary residential case, and it is the same table the PlumbCalc gas line sizing calculator uses.
Two things change the answer on a real job. First, the code sizes the total connected load on a run, not one appliance in isolation — if a second appliance shares the pipe, add its BTU rating before you size. Second, a 2 psi system, a CSST or copper material, or a longer allowable pressure drop all use different tables entirely. Size to the longest run, size to the full load, and confirm against your local code and the appliance data plate before installing.
The governing standard is the National Fuel Gas Code, published as NFPA 54 and adopted in most of the United States — see the National Fuel Gas Code for background on the code itself.
Related sizing references
Frequently asked questions
What size gas line does a furnace need?
A typical 100,000 BTU furnace draws 100 CFH of natural gas and needs 1/2 inch up to 20 feet, 3/4 inch from 30 to 100 feet, 1 inch from 125 to 200 feet, in Schedule 40 metallic pipe. The size depends on the longest run from the meter, so measure the run before you buy pipe.
Why does the run length change the gas line size for a furnace?
Gas loses pressure to friction as it travels, and NFPA 54 Table 402.4(2) is built around a fixed allowable drop of 0.5 inches water column. A longer run spends that allowance sooner, so the same diameter delivers fewer CFH. That is why the table has a column for every length from 10 to 200 feet.
What if my furnace is rated higher than 100,000 BTU?
Use the rating on the appliance data plate, never the typical figure. Divide the real BTU input by 1,000 for natural gas to get CFH demand, then read down the column for your run length until you reach a capacity that meets it. The PlumbCalc gas line sizing calculator does exactly that in one step.
Can a furnace share a gas line with another appliance?
Yes, but you must size for the combined load. At 60 feet the furnace alone needs 3/4 inch; add a 100,000 BTU furnace and the 200 CFH total needs 1 inch. Add every BTU rating on the run together before you look at the table.
Does this cover CSST, copper or a 2 psi system?
No. These sizes are Schedule 40 metallic pipe on an ordinary under-2-psi residential system with a 0.5 inch water column drop. CSST, copper tubing and polyethylene each have their own capacity tables, CSST varies by manufacturer, and a 2 psi system with appliance regulators uses a different table entirely and usually allows smaller pipe over long runs.