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WaterHeaterCalc

Gas Piping

Gas Pipe Size Chart for Water Heaters

Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated .

Quick answer

A 3/4 inch Schedule 40 pipe carries 199 cubic feet of natural gas per hour up to 30 feet at the standard 0.5 inch water column pressure drop, which is just enough for a 199,000 BTU tankless water heater at that distance. Longer runs or smaller pipe carry less gas at the same pressure drop, and every number on this page is for natural gas only, never for propane.

This chart reproduces the NFPA 54 (National Fuel Gas Code) and International Fuel Gas Code low pressure capacity table for Schedule 40 steel pipe carrying natural gas, at the standard design conditions of a 0.5 inch water column pressure drop and a 0.60 specific gravity gas. Each column is a straight run length in feet, and each cell is the maximum cubic feet of gas per hour that pipe size delivers to the appliance over that distance without exceeding the allowed pressure drop.

A single tankless water heater can call for 150,000 to 199,000 BTU per hour, several times what a smaller tank-type heater or a furnace branch needed, so a gas line sized correctly for an older appliance is frequently the wrong size once a tankless unit replaces it. This page is a reference for checking that arithmetic, not a substitute for a licensed gas fitter working out the load on the whole house piping system.

The distinction between a whole-house main and an individual branch line matters here. The main, running from the meter to the point where it splits toward each appliance, has to be sized for every BTU downstream of it at once. A branch, running from that split point to a single appliance such as the water heater, only has to be sized for that one appliance's own input, measured over its own shorter run. Confusing the two, by checking a branch's length and capacity against the combined load of the whole house, produces a pipe size far larger than the branch actually needs.

What does the NFPA 54 gas pipe capacity table show?

Every value below is cubic feet of natural gas per hour, the maximum a given pipe size delivers over a given run length before the pressure drop across that run exceeds 0.5 inch water column, at a 0.60 specific gravity gas. Capacity falls as the run gets longer because friction inside the pipe consumes more of the allowed pressure drop over distance.

NFPA 54 / IFGC low pressure natural gas capacity, Schedule 40 pipe, 0.5 in w.c. drop, 0.60 specific gravity
Pipe size10 ft20 ft30 ft40 ft50 ft60 ft70 ft80 ft90 ft100 ft
1/2 in1721189581726560565250
3/4 in360247199170151137126117110104
1 in678466374320284257237220207195
1-1/4 in1390957768657583528486452424400
1-1/2 in209014301150985873791728677635600
2 in4020276022201900168015201400130012201160

Published standard Source: NFPA 54 / International Fuel Gas Code low pressure natural gas pipe sizing table, Schedule 40 steel pipe, 0.5 inch water column pressure drop, 0.60 specific gravity.. Figures are for straight Schedule 40 steel pipe. Each elbow, tee or valve in the run adds equivalent length that a gas fitter accounts for separately, on top of the measured straight distance.

How many cubic feet per hour does a water heater BTU rating need?

The capacity table above is written in cubic feet per hour, while a water heater nameplate is written in BTU per hour. Converting between the two uses the standard heat content of pipeline quality natural gas, about 1,030 BTU per cubic foot.

Water heater BTU input converted to cubic feet per hour of natural gas
Appliance input (BTU per hour)Cubic feet per hour (at 1,030 BTU per cubic foot)
4000038.8
5000048.5
7500072.8
10000097.1
120000116.5
150000145.6
180000174.8
199000193.2

Published standard Source: Appliance BTU input divided by 1,030 BTU per cubic foot, the standard heat content figure NFPA 54 uses for natural gas.. A specific appliance rating plate is the number to use; this table exists to translate that BTU figure into the units the pipe capacity table above uses.

What minimum pipe size covers common water heater BTU ratings?

Cross referencing the two tables above gives the smallest Schedule 40 pipe size that covers a given appliance input at a few typical residential run lengths. A pipe size that works at 30 feet does not necessarily work at 80 feet for the same appliance, which is why the minimum size grows with distance in the table below.

Minimum Schedule 40 pipe size by BTU input and run length, natural gas
BTU inputMinimum size at 30 ftMinimum size at 50 ftMinimum size at 80 ft
400001/2 in1/2 in1/2 in
500001/2 in1/2 in1/2 in
750001/2 in3/4 in3/4 in
1000003/4 in3/4 in3/4 in
1200003/4 in3/4 in3/4 in
1500003/4 in3/4 in1 in
1800003/4 in1 in1 in
1990003/4 in1 in1 in

Published standard Source: Each appliance BTU input converted to cubic feet per hour at 1,030 BTU per cubic foot, matched against the smallest pipe size in the NFPA 54 table above whose capacity meets or exceeds that figure at the given distance.. Several rows sit close to the limit of the smaller size. A run that is a few feet longer than assumed, or that carries extra equivalent length for fittings, can push a borderline case into the next pipe size up.

Does this chart work for sizing a propane line?

No. Every figure on this page is for natural gas at 0.5 inch water column pressure drop and 0.60 specific gravity. NFPA 54 publishes a separate low pressure capacity table for undiluted propane at 11 inches water column, and this page does not reproduce it. Propane is metered at a different pressure and carries a different heat content per cubic foot than natural gas, so reading a propane run off the natural gas numbers above understates what that run actually needs. A propane installation should be sized from the NFPA 54 propane table or by a licensed gas fitter, not from this page.

Propane installations also bring their own regulator staging, tank or cylinder placement rules, and vapor withdrawal limits in cold weather, none of which show up anywhere in a natural gas capacity table regardless of which pressure column is used. A household converting from natural gas to propane, or building new on propane, needs a fitter familiar with that fuel specifically rather than an adaptation of the chart above.

An undersized gas line starves the burner instead of simply stopping it. The appliance may still light and run underfired, producing incomplete combustion and carbon monoxide rather than failing outright. Gas piping and any flue or venting work belongs to a licensed plumber or gas fitter, not a table lookup.

What if the water heater shares a gas line with a furnace or range?

NFPA 54 sizes a shared gas line by the longest length method: the segment of pipe closest to the meter has to carry the combined BTU load of every appliance downstream of it, not just the water heater's own input, and the run length used for that segment is measured out to the farthest appliance on the line. A branch that only feeds the water heater is sized to that appliance alone, using the run length from where it splits off the main line. Mixing these up, by sizing the shared trunk for one appliance while three others draw from it, is a common source of an underperforming furnace or water heater on an older house that has had equipment added over time.

Adding a tankless water heater to an existing gas system is exactly the scenario where this trips people up: the new unit's 150,000 to 199,000 BTU input joins whatever the furnace, range and dryer already draw from the same meter, and the segment nearest the meter has to be re-checked against that new combined total, not just against the tankless unit's own branch. A load calculation across the whole system, not a single branch lookup, is what a permit review actually checks.

Fittings for the last few feet

The chart above sizes the rigid branch line. Most installs still finish with a flexible connector between the shutoff valve and the appliance, sized short enough that it never becomes the bottleneck in the run.

Frequently asked questions

What size gas pipe do I need for a 199,000 BTU tankless water heater?
At 30 feet, a 3/4 inch Schedule 40 pipe carries 199 cubic feet per hour, just enough for a 199,000 BTU appliance, which needs about 193 cubic feet per hour. Past 30 feet, 3/4 inch pipe capacity falls below what a 199,000 BTU unit needs, and the run has to step up to 1 inch pipe, per the NFPA 54 low pressure natural gas capacity table.
How far can 1/2 inch gas pipe run to a water heater?
It depends on the load. NFPA 54 shows 1/2 inch Schedule 40 pipe carrying 172 cubic feet per hour at 10 feet, dropping to 50 cubic feet per hour at 100 feet. That covers a 40,000 or 50,000 BTU tank-type water heater at almost any residential distance, but it falls short of most tankless units past roughly 30 feet.
Does gas pipe capacity really change with distance?
Yes. Friction inside the pipe consumes pressure over distance, so a longer run delivers fewer cubic feet per hour at the same 0.5 inch water column pressure drop. A 3/4 inch pipe carries 360 cubic feet per hour at 10 feet but only 104 cubic feet per hour at 100 feet, roughly a 71 percent drop over that added distance.
What does 0.5 inch water column pressure drop mean?
It is the maximum pressure loss NFPA 54 allows across a pipe run at the design flow rate, measured in inches of water column, a small unit used for low pressure gas systems. The capacity table on this page is built around that specific drop and a 0.60 specific gravity gas; a different allowed drop or a different gas would change every number in the table.
Can this chart size a propane gas line?
No. This table covers natural gas only, at 0.5 inch water column and 0.60 specific gravity. NFPA 54 publishes a separate capacity table for undiluted propane at 11 inches water column, which this page does not reproduce. Reading a propane run off the natural gas numbers above understates what that line actually needs.
What happens if a gas line is undersized for a tankless water heater?
An undersized line cannot deliver the appliance's full rated BTU input, so the burner runs underfired instead of simply failing to turn on. That can mean poor combustion and carbon monoxide production rather than an obvious failure. Repiping to the correct size is a job for a licensed gas fitter, not a homeowner repair.

Treat this chart as a starting point, not a permit. A local code official can require additional derating for fittings, regulators, or several appliances sharing one line, and gas piping plus any flue or venting work belongs to a licensed plumber or gas fitter. An undersized line is a combustion and carbon monoxide problem, not just a performance one.