Use this Reynolds number calculator to find the Reynolds Number (Re) for a given scenario. The result helps predict if a fluid’s flow is laminar (smooth), transitional, or turbulent (chaotic).
How to Use
Reynolds Number Calculator
How to Calculate Reynolds Number
Our calculator is designed to be flexible and user-friendly, accommodating various scenarios you might encounter. Here’s a breakdown of its features and the calculations we carry out in order to determine the results.
1. Flow Type (Internal vs. External):
- Internal Flow (e.g. through a pipe or duct): Select this if your fluid is confined within a boundary. For these cases, the characteristic length (L) in the Reynolds number formula is typically the diameter for circular pipes or the hydraulic diameter for non-circular ducts.
- External Flow (e.g. over a flat plate, around a sphere): Choose this when the fluid flows around an object. Here, the characteristic length (L) is a dimension of the object, such as the length of a plate or the diameter of a sphere. The Reynolds number for external flow often dictates where boundary layers transition from laminar to turbulent.
2. Fluid Properties (Kinematic vs. Dynamic Viscosity & Density):
The Reynolds Number can be calculated using either kinematic or dynamic viscosity. Our calculator allows you to choose based on the data you have:
- Kinematic Viscosity (ν): This option uses the formula Re = (v x L) / ν. Kinematic viscosity already accounts for the fluid’s density and is often provided for common fluids. Units are typically m²/s or cSt (centistokes).
- Dynamic Viscosity (μ) & Density (ρ): If you have dynamic viscosity and density separately, select this. The calculator will use the formula Re = (ρ x v z L) / μ. Dynamic viscosity (also known as absolute viscosity) represents a fluid’s resistance to shear flow. Units are typically Pa·s (Pascal-seconds) or cP (centipoise).
- Need to convert? Remember, kinematic viscosity (ν) can be calculated from dynamic viscosity (μ) and density (ρ) using the relationship: ν = μ / ρ.
3. Input Parameters:
- Fluid Velocity (v): The average speed of the fluid.
- Pipe Diameter (D) / Characteristic Length (L):
- For Internal, Circular Flow: Enter the pipe’s diameter.
- For Internal, Rectangular Flow: You’ll input the duct’s width and height. The calculator will automatically calculate the Hydraulic Diameter (D_h) using the formula D_h = (4 x Area) / Perimeter. This equivalent diameter is used as the characteristic length for non-circular ducts.
- For External Flow: Enter the characteristic length relevant to the geometry of the object (e.g., length of a plate, diameter of a cylinder).
- Kinematic Viscosity (ν) or Dynamic Viscosity (μ) and Density (ρ): Input these values based on your “Fluid Properties” selection.
4. Units:
We’ve included common units for all inputs, and the calculator will handle the conversions to ensure accurate results in SI units internally. Just select the unit you’re working with for each parameter.
Frequently Asked Questions
The Reynolds number is a dimensionless quantity used in fluid mechanics to predict flow patterns. It represents the ratio of inertial forces (a fluid’s tendency to keep moving) to viscous forces (a fluid’s internal friction or “stickiness”).
The flow regime has major real-world consequences:
Pressure & Energy: Turbulent flow dissipates energy faster and causes a significantly higher pressure drop in pipes, requiring more powerful pumps.
Drag: Flow over a car or airplane wing creates drag. The type of flow in the boundary layer determines the amount of drag.
Heat Transfer: Turbulent flow transfers heat much more effectively than laminar flow, which is critical in designing heat exchangers or cooling systems.
Mixing: If you need to mix chemicals, turbulent flow is far more effective
This is a common point of confusion.
Dynamic Viscosity (μ): This is the fluid’s fundamental resistance to flow. Think of it as the fluid’s absolute “thickness” or internal friction. Its common units are Pa·s or cP.
Kinematic Viscosity (ν): This is the dynamic viscosity divided by the fluid’s density (ν = μ/ρ). It describes how easily a fluid flows under the force of gravity. Its common units are m²/s or cSt.
For non-circular pipes or ducts (like triangles or ovals), you need to use the Hydraulic Diameter (D_h) as the characteristic length. The general formula is:
D_h = (4 x Cross-Sectional Area) / Wetted Perimeter
Our calculator automatically computes this for rectangular ducts, but you can use this formula to find the hydraulic diameter for any shape and use it in calculations.
No, they are rules of thumb, not strict physical laws. The transition from laminar to turbulent flow can be influenced by a multitude of factors.
Yes. Gases like air, nitrogen, and steam are also fluids. The Reynolds number is used in exactly the same way to predict whether the flow of a gas is laminar or turbulent, which is essential for aerodynamics and HVAC design.
