Skip to content
WaterHeaterCalc

Install calculator

Gas Line Size Calculator for Water Heaters

Quick answer

A 199,000 BTU tankless unit needs about 193 cubic feet per hour of natural gas. Over a 40 foot run at a half inch water column drop, the NFPA 54 table gives 3/4 inch pipe a capacity of 170 cubic feet per hour, so that run needs 1 inch pipe.

Your numbers

Results update as you type. Nothing you enter leaves your browser.

Result

Minimum pipe size 1 in Published standard
Required flow 193.2 CFH Published standard
Table capacity at that size 320 CFH Published standard
Table length used 40 ft Published standard

Gas piping, flue and vent work is for a licensed plumber or gas fitter. An undersized gas line starves a burner, and a starved burner produces carbon monoxide. This figure is for planning a conversation, not for doing the work.

The formula

Required CFH = BTU/h / 1,030 BTU per cubic foot. Then read the smallest pipe size whose NFPA 54 capacity at that run length meets or exceeds it.

The table is Schedule 40 metallic pipe, natural gas at 0.60 specific gravity, less than 2 psi inlet, half inch water column pressure drop. Code sizing uses the longest run from the meter to any outlet, and every elbow and tee adds equivalent length that this does not account for. This is a planning figure. The person who signs the permit does the real sizing.

The figures above are computed with these exact inputs: Appliance input 199000 BTU/h, Longest run from the meter 40 ft, Fuel naturalGas.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

AssumptionKindWhere it comes from
NFPA 54 and IFGC pipe capacity tablePublished standardNFPA 54 and IFGC pipe capacity table, Schedule 40 metallic pipe, natural gas at 0.60 specific gravity, less than 2 psi inlet, 0.5 in water column drop
Btu Per Cuft Ng: 1030Published standardTypical published higher heating value of pipeline natural gas. Your utility publishes its actual figure on the bill

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Frequently asked questions

What size gas line does a tankless water heater need?
Usually 3/4 inch at short runs and 1 inch beyond about 30 feet, because a 199,000 BTU unit draws roughly 193 cubic feet per hour. The exact answer depends on the longest run from the meter and on what else is on the same line. Upgrading the gas line is often the largest single cost in a tankless retrofit, and it is the item most likely to be missed in a quote.
Can I use this calculator for propane?
No, and it returns nothing rather than a number if you select propane. NFPA 54 publishes a separate capacity table for undiluted propane at 11 inches water column, this calculator does not reproduce it, and sizing propane pipe from the natural gas table understates what the run needs. Use the propane table or have the gas fitter size it.
Does the existing gas line to my tank work for a tankless unit?
Often not. A 40,000 BTU tank and a 199,000 BTU tankless unit differ by a factor of five in gas demand, so a line that was generously sized for the tank can be badly undersized for the tankless. Check the input on both data plates and run the length before assuming the pipe is reusable, because this is where tankless budgets go wrong.
Do fittings and elbows count toward the length?
Yes. Every elbow, tee and valve adds equivalent length to the run, and code sizing accounts for it. This calculator uses only the straight run length, so treat its answer as the minimum. A run with many turns can size up a pipe diameter on the fittings alone, which is one of several reasons this is a gas fitter decision.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.