What size gas line for a tankless water heater at 20 feet?
A typical 199,000 BTU tankless water heater on a 20 foot run needs 3/4 inch Schedule 40 pipe for natural gas. That is the smallest size in NFPA 54 Table 402.4(2) whose capacity at 20 feet still meets the demand.
The math, step by step
Whole-house gas tankless units cluster just under 200,000 BTU because that is the largest common non-commercial rating. Always confirm the real rating on the appliance data plate before sizing anything.
- Load: 199,000 BTU per hour.
- Demand: natural gas carries 1,000 BTU per cubic foot, so 199,000 divided by 1,000 gives 199 CFH.
- Length column: a 20 foot run rounds up to the 20 foot column.
- Result: 3/4 inch pipe carries 247 CFH at 20 feet, which covers the 199 CFH demand with 48 CFH to spare.
How the nearby sizes compare at 20 feet
| Pipe size | Capacity at 20 ft | Against 199 CFH |
|---|---|---|
| 1/2 inch | 118 CFH | not enough |
| 3/4 inch | 247 CFH | enough |
| 1 inch | 466 CFH | enough |
| 1-1/4 inch | 957 CFH | enough |
| 1-1/2 inch | 1,430 CFH | enough |
| 2 inch | 2,760 CFH | enough |
If a second appliance shares the run
This is where most undersized lines come from. Put a 100,000 BTU furnace on the same pipe and the connected load becomes 299,000 BTU, or 299 CFH — which needs 1 inch at 20 feet, a size up from the single-appliance answer. Size the pipe for everything that will ever hang off it, not just what you are installing today.
Propane on the same run
On propane the same tankless water heater draws only 79 CFH because LP carries 2,516 BTU per cubic foot, and PlumbCalc returns 1/2 inch for this run. That figure is deliberately conservative — it is sized against the natural gas table, so it will be equal to or larger than a dedicated LP table. Confirm 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
- Full gas pipe sizing chart, every size and length
- Gas line sizing calculator
- How many BTU 3/4 inch pipe carries
- Natural gas pipe sizing explained
- Gas line size for a tankless water heater at 40 feet
- Gas line size for a tankless water heater at 60 feet
Frequently asked questions
What size gas line does a tankless water heater need at 20 feet?
A typical 199,000 BTU tankless water heater on a 20 foot run needs 3/4 inch Schedule 40 pipe for natural gas. The demand is 199 CFH and that size carries 247 CFH at the 20 foot column of NFPA 54 Table 402.4(2).
Can I use a smaller pipe if the run is shorter?
Often yes, because capacity rises as the run shortens. Measure the actual longest run from the meter to this appliance, including vertical rises, then round up to the next table column. Do not measure the straight-line distance across the room.
Do fittings and elbows count toward the length?
NFPA 54 Table 402.4(2) already builds in an allowance for a normal number of fittings, so you generally use the measured pipe length. A run with an unusually high number of elbows, tees or a long vertical drop deserves either the next size up or a proper equivalent-length calculation.
What if the appliance rating is different from 199,000 BTU?
Use the plate rating, not the typical figure. Divide the real BTU input by 1,000 for natural gas to get CFH, then read down the 20 foot column until you find a size that meets it. The PlumbCalc gas line sizing calculator does exactly that.
Does this cover a 2 psi system?
No. This is the ordinary under-2-psi residential case with a 0.5 inch water column drop. A 2 psi system with appliance regulators uses a different table and typically allows smaller pipe over long runs. Size it against the table that matches the system you are installing.