Fluid Dynamics Calculator

1. Select Reynolds Number, Flow Velocity / Flow Rate, or Pressure Drop.
2. Enter the values required by that mode, then choose the matching unit beside each field.
3. For Flow Velocity / Flow Rate, choose Velocity or Flow rate, then enter Cross-sectional area (A) or select Compute area from diameter (circular pipe: A = π D² / 4).
4. Select Calculate and read the result, Formula & steps, and any warning.
5. Select Copy result, Download result (.txt), or Reset when you are finished.

Calculate Reynolds number and flow regime

Select Reynolds Number and enter Fluid density (ρ), Dynamic viscosity (μ), Pipe diameter (D), and Flow velocity (V). The calculator converts the selected units, applies Re = ρ V D / μ, and labels the result Laminar, Transitional, or Turbulent.

ρ = 998 kg/m³, μ = 0.001 Pa·s, D = 0.05 m, V = 1.5 m/s
Re = 74,850, Flow regime = Turbulent

For a straight circular pipe, the calculator uses Re below 2300 for Laminar, Re from 2300 through 4000 for Transitional, and Re above 4000 for Turbulent flow. The thresholds are approximate because roughness, entrance effects, and disturbances can change the transition.

Calculate flow velocity or flow rate

Choose Flow Velocity / Flow Rate, then select Velocity to solve V = Q / A or Flow rate to solve Q = V × A. Enter Cross-sectional area (A), or select Compute area from diameter (circular pipe: A = π D² / 4) so the calculator derives area from Pipe diameter (D). A flow rate of 0.03 m³/s through 0.02 m² gives 1.5 m/s, and the result also shows the area used.

Calculate pipe pressure drop

Choose Pressure Drop and enter Dynamic viscosity (μ), Pipe length (L), Pipe diameter (D), Flow velocity (V), and Fluid density (ρ). The result uses ΔP = 32 μ L V / D². The generated text includes pressure in Pa, psi, and atm, plus the Reynolds number and flow classification.

Hagen-Poiseuille applies to steady, fully developed laminar flow of an incompressible Newtonian fluid in a straight circular pipe. The calculation leaves out fittings, bends, entrance effects, and pipe roughness. A non-laminar Reynolds number triggers a model warning.

Input units and validation rules

Density accepts kg/m³, g/cm³, or lb/ft³. Dynamic viscosity accepts Pa·s, cP, or lb/(ft·s). Diameter, length, area, velocity, and flow rate each have selectable SI and imperial units.

Every entered value must be greater than zero. Empty, zero, negative, non-numeric, comma-separated, and scientific-notation entries are rejected. Values above 1e15 and non-finite calculated results are rejected too. Changing a mode, solve option, input, or unit select hides the current result until you select Calculate again.

The calculator script processes calculation values in the browser. Copy result and Download result (.txt) act on the generated text.

Choose a mode for the values you have

Use Reynolds Number when you have Fluid density (ρ), Dynamic viscosity (μ), Pipe diameter (D), and Flow velocity (V) and need a flow classification.

Use Flow Velocity / Flow Rate to convert Flow rate (Q) and Cross-sectional area (A) into Flow velocity (V), or calculate Flow rate (Q) from velocity and area.

Choose the right science calculator

Science Calculators groups this fluid-flow calculation with related physics and chemistry tools. Use the Density Calculator when density, mass, or volume is the unknown. Use the Pressure Calculator for force divided by area rather than pipe pressure loss. Use the Acceleration Calculator for change in velocity over elapsed time.

Frequently asked questions