Pipe Flow Calculator

Pipe flow analysis connects a pipe's geometry, a fluid's properties, and its speed to quantities engineers use when sizing pumps, checking plumbing capacity, and estimating energy losses. This calculator assumes steady flow of an incompressible Newtonian fluid through a full, straight, circular pipe. It first converts the entered diameter and roughness from millimeters to meters so every equation uses a consistent SI basis. Cross-sectional area is pi times diameter squared divided by four, and volumetric flow rate is that area multiplied by average velocity. The displayed flow rate is converted from cubic meters per second to liters per second for convenient reading. The Reynolds number compares inertial and viscous effects. It is calculated as density times velocity times diameter divided by dynamic viscosity. Values below roughly 2,300 normally indicate laminar flow, while values above roughly 4,000 normally indicate turbulent flow. The interval between them is transitional, where disturbances, fittings, entrance conditions, and surface imperfections can strongly affect behavior. Reynolds number is dimensionless, so it is useful for comparing geometrically similar systems even when their scale, fluid, or speed differs. Pressure loss is estimated with the Darcy-Weisbach equation. The equation multiplies the Darcy friction factor by the length-to-diameter ratio and the velocity pressure. For laminar flow, the calculator uses the exact circular-pipe relation of 64 divided by Reynolds number. For turbulent flow, it uses the explicit Swamee-Jain approximation, incorporating both Reynolds number and relative roughness. This approximation closely represents the Colebrook equation across ordinary engineering ranges without requiring an iterative solver. The resulting pressure drop is shown in kilopascals. Pipe roughness represents the characteristic height of internal surface irregularities, not the total variation in pipe diameter. New plastic or drawn tubing is very smooth, while commercial steel, concrete, and aged pipes can be much rougher. Published roughness values are estimates and may not capture corrosion, deposits, joints, bends, valves, or manufacturing variation. Actual systems also include minor losses from fittings and changes in area; those losses are not included here and should be added separately using loss coefficients when designing a complete network. Use the results as a transparent first-pass engineering estimate. Confirm fluid properties at the operating temperature because density and especially viscosity can change significantly. For gases, large pressure changes, noncircular ducts, open-channel flow, non-Newtonian fluids, or safety-critical designs, use a model suited to those conditions and verify it against applicable standards and measured performance.