Calculation Breakdown
Full Calculation Table
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How to Use the Darcy-Weisbach Calculator
Choose water or a glycol concentration. Glycol's higher viscosity and density change the result meaningfully compared to water at the same flow rate, which is exactly what this calculator is built to capture. Check the glycol concentration calculator first if you have not settled on a percentage.
Enter flow rate, inside diameter, equivalent length, and pipe roughness. Use the fitting equivalent length calculator to get an accurate length if the run includes several fittings.
Click Calculate Pressure Drop to see Reynolds number, the Colebrook-White friction factor, and total pressure drop, with the full intermediate calculation shown step by step.
Why Darcy-Weisbach Handles Any Fluid Correctly
The Darcy-Weisbach equation calculates friction loss from first principles: fluid density, velocity, pipe geometry, and a friction factor that itself comes from the fluid's actual viscosity through the Reynolds number. This is what makes it accurate across water and glycol mixtures at different concentrations, unlike the Hazen-Williams equation, which uses a single roughness coefficient calibrated specifically to water and does not have a direct way to account for a different fluid's viscosity. Glycol loops show up well beyond hydronic heating too; the solar thermal calculator covers another common glycol application where the same viscosity-driven pressure drop applies. This calculator is built for liquid hydronic fluids specifically; refrigerant piping involves phase change and compressible flow effects that need the dedicated refrigerant line sizing calculator instead.
The Friction Factor Comes From Colebrook-White
The Darcy friction factor in turbulent flow is found using the Colebrook-White equation, which relates friction factor to Reynolds number and relative pipe roughness. Colebrook-White cannot be solved algebraically and needs an iterative solution, which this calculator performs automatically. The U.S. Bureau of Reclamation's engineering monograph on friction factors covers the history of the Moody diagram and Colebrook-White in more depth, along with measured roughness values for various pipe materials. Use the Reynolds number calculator to check the flow regime on its own, or the Moody chart calculator to look up a friction factor directly from Reynolds number and roughness without running a full pressure drop calculation.
Why Glycol Changes the Result So Much
Glycol is more viscous and slightly denser than water, and both properties feed directly into this calculation through the Reynolds number and the velocity head term. Higher viscosity lowers Reynolds number for the same flow rate, which generally raises the friction factor, while higher density increases the velocity head term directly. NIST's reference correlation for propylene glycol viscosity is the kind of underlying data this calculator's fluid property table is built from. The combined effect is why a 50% glycol mixture can show a meaningfully higher pressure drop than plain water at the identical flow rate and pipe size, an effect Hazen-Williams cannot capture since it has no fluid property input beyond an empirical roughness number.
Picking a Roughness Value
Absolute roughness, not the C coefficient from Hazen-Williams, is the roughness input Darcy-Weisbach needs. Drawn copper and plastic pipe have a very smooth roughness around 0.0015mm, while commercial steel pipe runs rougher at roughly 0.045mm. Compare materials on the pipe material comparison tool if you are choosing between options for a new design.
A Note on Accuracy
This calculator solves the Colebrook-White equation iteratively to a tight tolerance and uses standard published fluid property values for water and propylene glycol mixtures at typical hydronic temperatures. For critical commercial piping or permit-stamped mechanical drawings, confirm fluid properties against the specific manufacturer's data sheet for the actual product being used, since formulations vary slightly between glycol brands, and watch for a data sheet reporting viscosity in a different unit than this calculator uses; the viscosity converter handles that conversion directly.
Frequently Asked Questions
Darcy-Weisbach calculates friction loss from a friction factor that itself depends on Reynolds number and relative pipe roughness, and the relationship between them, the Colebrook-White equation, cannot be solved directly and needs an iterative or approximate solution. Hazen-Williams sidesteps this by using a single empirical roughness coefficient calibrated specifically to water, which is faster to use but only accurate for water at typical temperatures.
Darcy-Weisbach works directly from the fluid's actual viscosity and density through the Reynolds number, so it automatically accounts for glycol being more viscous than water. Hazen-Williams uses a roughness coefficient calibrated to water's properties and does not have a direct way to account for a different fluid's viscosity, which makes it increasingly inaccurate as glycol concentration rises.
PEX and ACR copper both have smooth interior surfaces similar to drawn copper, so the 0.0015mm default used for water is a reasonable starting point for either. If a manufacturer publishes a specific roughness value for their product, use that instead, since slight variations in manufacturing process can shift the actual surface finish.
Yes. Below a Reynolds number of about 2,300, flow is laminar and the friction factor is calculated directly as 64 divided by Reynolds number, without needing Colebrook-White at all. Most hydronic systems operate in turbulent flow at design conditions, but laminar flow can occur at very low part-load flow rates, and this calculator switches to the correct laminar formula automatically when that happens.
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