Hydronic Pipe Sizer

Darcy-Weisbach pipe sizing for hydronic systems — steel, copper, and plastic, water or glycol.

Inputs
Sizing Method
Results
Recommended Size
— in. nominal
Velocity
—
Head / 100 ft
—
Total Head
—
Pressure Drop
—
Adjust Size
in.
⚠ Velocity exceeds 8 ft/s — noise and erosion risk. Consider upsizing.
⚠ Re below 4,000 — laminar or transitional flow. Friction between Re 2,300 and 4,000 is uncertain; treat the head loss as approximate.
Fluid Properties at Temperature
Density
—
Viscosity
—
Reynolds No.
—
All Sizes —
Nom. (in.) ID (in.) Vel. (ft/s) HL / 100ft OK?
Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This hydronic pipe sizer selects a nominal pipe size for chilled water, hot water, and glycol systems by computing velocity and friction head loss for every size in the chosen material schedule. Enter the flow in GPM, fluid temperature, and equivalent length; the tool returns the smallest size that meets your sizing target, along with velocity, head loss per 100 ft, total head, and pressure drop.

Two sizing methods are supported. Equal Friction picks the smallest size at or below a target friction rate (ft per 100 ft) that also stays at or below 8 ft/s; Velocity picks the smallest size at or below a target velocity. Fluid density and viscosity are interpolated by temperature from published property data for water and for 30, 40, and 50% (by volume) ethylene or propylene glycol, so antifreeze systems size correctly.

Formula & Method
VelocityV (ft/s) = Q (GPM) × 0.002228 ÷ A   A = π · (d ÷ 24)²
ReynoldsRe = V · D · (ρ ÷ gc) ÷ μ   (ρ in lb/ft³, μ in lbf·s/ft²)
Head losshL = f · (L ÷ D) · V² ÷ (2 · g)
Friction fColebrook–White (turbulent); f = 64 ÷ Re (laminar)

Sizing uses the Darcy–Weisbach equation with the Colebrook–White friction factor, solved iteratively, the standard basis for hydronic pipe sizing in ASHRAE Fundamentals. The constant 0.002228 converts GPM to cubic feet per second (1 GPM ≈ 0.002228 ft³/s); g = gc = 32.174. For Reynolds numbers below 2,300 the flow is laminar and f = 64 ÷ Re. Pipe roughness ε is set per material (steel ≈ 0.00015 ft, copper and plastic much smoother). Inside diameters: ASME B36.10M (steel), ASTM B88 (copper), ASTM F441 (CPVC), ASTM F876 (PEX). Head is in feet of the flowing fluid; pressure drop = head × ρ ÷ 144. Water properties follow IAPWS (NIST); glycol viscosity is Dow inhibited-glycol data as tabulated in ASHRAE Fundamentals (secondary coolants), and glycol density is from MEGlobal (ethylene) and Melinder (propylene) correlations. The 8 ft/s ceiling is a common noise and erosion rule of thumb, not a code limit.

Frequently Asked Questions
What head loss should I target when sizing hydronic pipe?
For most chilled and hot water mains, a friction rate of about 1–4 feet of head per 100 feet of pipe is common practice, with 2.5 ft/100 ft a frequent default. Lower rates save pump energy but increase pipe cost; higher rates reduce pipe size but raise pumping power and noise. This tool defaults to 2.5 ft/100 ft and lets you adjust the target.
Why does the calculator cap velocity at 8 ft/s?
Above roughly 8 ft/s, water velocity causes noise and accelerates erosion-corrosion at fittings and elbows; it is a common rule of thumb rather than a code limit. A practical minimum near 2 ft/s helps carry entrained air and dirt toward separators. In Equal Friction mode the tool will not recommend a size above 8 ft/s even when the friction target allows it, and it flags any size above 8 ft/s in either mode.
How does glycol change the pipe size?
Glycol antifreeze raises both density and viscosity relative to water. The higher viscosity increases friction loss for the same flow, so a glycol line often needs to be one size larger than a water line. This tool uses temperature-dependent density and viscosity for 30, 40, and 50 percent (by volume) ethylene and propylene glycol from published manufacturer data, so the Reynolds number and friction reflect the actual mix at your temperature. Cold, concentrated propylene glycol can be several times as viscous as water.
What is Reynolds number used for here?
The Reynolds number determines whether flow is laminar or turbulent and which friction equation applies. Below about 2,300 the flow is laminar and friction factor equals 64 divided by Reynolds number; above that the tool solves the Colebrook-White equation iteratively. A warning appears when Reynolds number drops below 4,000, because friction in the 2,300 to 4,000 transition band is uncertain and the head loss should be treated as approximate.
Related Tools
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AIM Works · launches Oct 15, 2026

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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.