CHW pipe size calculator

Size a chilled or heating water pipe from a target velocity, or check an existing pipe's velocity, Reynolds number and pressure drop — Darcy-Weisbach friction physics with real water density and viscosity at your system's mean temperature.

Darcy-Weisbach + Colebrook-White Hydronic velocity guideline Free, no sign-up

Pipe & flow data

Inputs only — every result is on the right.

Water

Mean water temperature °C

Flow rate

Flow unit

Solve for

Mode

Target velocity typically 0.9–3.0 m/s

Unit

Inner diameter

Unit

Pipe

Pipe length

Unit

Pipe material surface roughness

Display units

Pressure display unit

Velocity display unit

Diameter: —
Velocity: —
Reynolds number: —
Darcy friction factor: —
Pressure drop: —
Total pressure drop: —

Link copied — it reopens with these exact inputs.

How to use this calculator

Enter the mean water temperature.
Enter the flow rate.
Choose the mode — size a new pipe to a target velocity, or check an existing diameter.
Enter the pipe length and material, for roughness.
Read the velocity, Reynolds number and pressure drop.

What this calculates

This reuses two engines already verified elsewhere on this site rather than reimplementing anything: the same Darcy-Weisbach + Colebrook-White friction physics used by the Duct Pressure Drop and Refrigerant Pipe Pressure Drop calculators, and the same water density/viscosity lookup table (steam-table-style values, interpolated at your mean water temperature) as the Water Flow Power Calculator.

v   = V̇ / A                                          velocity from flow
Re  = ρ·v·D / μ                                    Reynolds number
1/√f = −2·log₁₀( (ε/D)/3.7 + 2.51/(Re·√f) )     Colebrook-White
Δp/L = f·(1/D)·(ρ·v²/2)                          Darcy-Weisbach

"Required diameter" mode rearranges the velocity equation directly (D = √(4·V̇/(π·v_target))) rather than iterating — pick the nearest standard pipe size above the result, then optionally switch to "check" mode with that exact size to see its actual velocity and pressure drop. "Check" mode runs the same physics the other direction, from a diameter you already have.

Why density and viscosity matter here

Both water properties are temperature-dependent enough to matter for pipe sizing: viscosity in particular roughly triples between 60°C heating water and 7°C chilled water, which shifts the Reynolds number and friction factor meaningfully. Rather than using one fixed "room-temperature water" constant, this tool looks up density and viscosity from the same standard water-property table as the Water Flow Power Calculator, interpolated at your entered mean temperature.

Velocity guideline

The 0.9–3.0 m/s range checked here is a commonly cited hydronic design range across HVAC references — low enough to control noise and erosion, high enough to avoid oversizing pipe and wasting insulation/material cost. It's a general range, not a precise per-diameter table (larger pipes can often tolerate somewhat higher velocity before noise becomes an issue) — treat a result outside the range as worth a second look, not an automatic fail, and check it against your own project standard for critical designs.

Friction loss uses Darcy-Weisbach with Colebrook-White friction factors, verified against published Moody-chart reference points (same engine as the Duct Pressure Drop and Refrigerant Pipe Pressure Drop calculators). Water density and viscosity are standard reference values (steam-table style, atmospheric pressure), linearly interpolated between 10°C table points — pure water only, not valid for glycol/brine mixtures. The velocity guideline is a commonly cited hydronic design range, not a single citable standard's exact figures. Provided for engineering guidance — verify against project-specific design standards for critical designs.