Chilled Water Pipe Sizing Calculator

Schedule 40 steel, chilled water at 49 °F mean, sized with a 4 ft/s velocity limit at 2″ and below and a 4 ft per 100 ft friction limit with an 8 ft/s cap above 2″. Tonnage is converted at a 10 °F ΔT, which gives 2.4 GPM per TR. Values come from the same engine as the calculator above.

For condenser water at 3 GPM per TR, read the chart at 1.25 times your tonnage. For copper Type L the internal diameter is smaller than steel at the same nominal size, so a copper line often lands one size larger — use the calculator with copper selected rather than this chart.

1. Pick the fluid. Chilled water, condenser water or heating hot water. This sets the design temperature, the default ΔT and the velocity limits — condenser water is normally allowed a little more velocity in plant rooms.
2. Pick the material. Schedule 40 black steel is standard for mains; Type L copper is common on small branches and has a smaller bore at the same nominal size.
3. Enter tonnage or flow. If you work from equipment schedules in TR or kW, use the load option and the tool converts with GPM = TR × 24 ÷ ΔT. If you already have a flow rate from a coil schedule, switch to flow.
4. Enter the pipe run length including the equivalent length of elbows, valves, strainers and balancing devices. The tool returns total head loss, which is the piping contribution to pump head.
5. Check the two candidate sizes. The tool shows the selected size and the size below it with the reason that smaller size was rejected, so you can see how close the call was.
6. Read the warnings before committing. Very low velocity traps air; very high velocity causes noise and erosion.

This is preliminary sizing. Final design must account for coil and equipment pressure drop, control valve authority, balancing, and any project specification that overrides these limits.

The three-band sizing rule

Chilled water pipe sizing is not a single rule — it is three bands, and getting this wrong is the most common mistake in quick calculators:

• 2″ and below — velocity governs. Limit to about 4 ft/s (1.2 m/s). Friction loss on small pipes runs far above 4 ft per 100 ft at this velocity, and that is accepted; branch runs are short.
• Above 2″ — friction governs. Limit to about 4 ft of water per 100 ft (roughly 120 Pa/m) to keep pump energy sensible over long runs.
• Large pipes — velocity governs again. From roughly 6″ upward the friction rule alone allows very high velocity. At 14″ a 4 ft per 100 ft target works out at about 14 ft/s, which is well into erosion-corrosion territory. A cap of 8 ft/s (2.4 m/s) takes over.

Without that third band a calculator will undersize mains badly. At 6,000 GPM the friction rule alone points at 14″, while the correct answer with the velocity cap applied is 20″ — two sizes larger.

Converting cooling load to flow rate

A higher ΔT means less flow for the same tonnage, which means smaller pipes and lower pump energy. Moving a chilled water system from a 10 °F to a 16 °F ΔT cuts design flow by 37% and can drop the pipe two sizes — this is the core idea behind low-flow, high-ΔT plant design.

Recommended chilled water velocities

Why velocity and friction both matter

Velocity drives noise, vibration and erosion. In steel pipe, sustained velocity above about 10 ft/s strips the protective oxide layer at bends and tees and accelerates wall loss. In copper the practical limit is lower still, around 8 ft/s for cold water and 5 ft/s where the water is warm.

Friction drives pump head, and pump energy runs 24/7 for the life of the building. Halving the design friction rate roughly halves the piping share of pump head, at the cost of a larger pipe, more insulation and more support steel. The 4 ft per 100 ft target is the long-standing compromise between the two.

Typical applications

• CHW mains, risers and headers in plant rooms
• AHU and FCU chilled water coil connections
• Primary / secondary and variable primary chilled water systems
• Condenser water between chillers and cooling towers
• District cooling distribution and ETS connections

Frequently asked questions

Related tools: TR to CFM Calculator · Ductulator — Duct Size Calculator · Pump Head Calculator

References

• ASHRAE Handbook - Fundamentals, Ch.22 Pipe Sizing
• ASHRAE Handbook - HVAC Systems and Equipment
• Carrier System Design Manual, Part 3 - Piping Design
• ASME B36.10M - Welded and Seamless Wrought Steel Pipe
• ASTM B88 - Seamless Copper Water Tube

Last updated: 2026-07-26