How big a gas line for a tankless water heater?

Many 199,000 BTU units need ¾ in; check the IFGC/NFPA 54 tables for your length.

What pressure drop is allowed?

US tables typically use 0.5 in w.c. (≈ 1.2 mbar) for low-pressure natural gas.

Yes, if sized from the maker's tables and bonded per code.

What size gas line for a tankless heater?

Often ¾ in; check your local code tables for length and BTU.

Natural gas or propane?

Propane has higher heating value, so it needs less flow and smaller pipe.

What is equivalent length?

Actual length plus fittings converted to feet of pipe.

Gas pipe sizing calculator

Appliance input, pipe length, gas type and allowed pressure drop: you get gas flow, minimum inside diameter and the copper tube size.

Minimum inside diameter

15.94 mm Ø

Gas flow1.318 CFM
Minimum inside diameter15.94 mm Ø
Copper tube size (outside diameter)18 mm Ø

Use my prices

Your prices per language/currency. Used when you enable “Use my prices” in calculators. My prices →

Calculate labour, margin & VAT

How it's calculated (visible formula)

Q = input ÷ heating value; Renouard: D = (23,200 × SG × L × Q^1.82 ÷ ΔP)^(1/4.82)

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Take the appliance input rating (nameplate or spec sheet).
Convert to gas flow with the gas's heating value.
Measure pipe length and add about 20% for fittings.
Set the allowable pressure drop.
Work out diameter and pick the next larger stock size.

Worked example: 82,000 BTU/h (24 kW) tankless on natural gas, 33 ft, 1 mbar drop

Q = 24 ÷ 10.7 ≈ 2.24 m³/h (79 CFH)
D ≈ 15.9 mm

→ 18 mm copper (16 mm ID) — about a ⅝–¾ in line.

Common mistakes

Ignoring equivalent length of fittings.
Using propane pressure drops for natural gas.
Skipping permit and inspection.

Pro tips

With several appliances, size each run for everything downstream.
Pressure-test before putting the system in service.
Keep code clearances from electrical systems.
Gas work needs a licensed installer, permit and inspection.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

Create quote Calculate margin & VAT

FAQs

How big a gas line for a tankless water heater?: Many 199,000 BTU units need ¾ in; check the IFGC/NFPA 54 tables for your length.
What pressure drop is allowed?: US tables typically use 0.5 in w.c. (≈ 1.2 mbar) for low-pressure natural gas.
Can I use CSST?: Yes, if sized from the maker's tables and bonded per code.
What size gas line for a tankless heater?: Often ¾ in; check your local code tables for length and BTU.
Natural gas or propane?: Propane has higher heating value, so it needs less flow and smaller pipe.
What is equivalent length?: Actual length plus fittings converted to feet of pipe.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.