Fluid Flow Calculator
Find Reynolds number, flow regime, and volumetric flow rate from fluid velocity, characteristic diameter, and kinematic viscosity.
Enter your values
Velocity, geometry, and fluid kinematic viscosity.
How this calculator works
Reynolds number is a dimensionless ratio of inertial to viscous forces, and determines whether flow is laminar or turbulent:
V is fluid velocity, D is the characteristic length (internal diameter for pipe flow), and ν is kinematic viscosity. Volumetric flow rate for a circular cross-section follows from continuity:
Typical kinematic viscosity (at ~20°C)
| Fluid | ν (mm²/s, cSt) |
|---|---|
| Water | 1.0 |
| Air | 15.0 |
| Light hydraulic oil (ISO VG32) | 32 (at 40°C) |
| SAE 30 motor oil | ~100 – 120 |
Frequently asked questions
How is this different from the pipe flow calculator?
This tool focuses on the flow regime itself — Reynolds number and its classification — plus resulting flow rate. The pipe flow calculator goes a step further and also computes friction-based pressure drop along a specified pipe length.
What if my characteristic length isn't a circular pipe diameter?
For non-circular ducts, use the hydraulic diameter (D_h = 4 × cross-sectional area / wetted perimeter) in place of D.