# Reynolds Number Calculator (Re=ρvD/μ, Laminar/Transitional/Turbulent)

Compute the Reynolds number Re = ρ·v·D/μ (dynamic viscosity) or Re = v·D/ν (kinematic viscosity, ν = μ/ρ) and classify the flow regime: Re < 2300 laminar, 2300 ≤ Re < 4000 transitional, Re ≥ 4000 turbulent. Diameter accepted in m/cm/mm. In dynamic mode the kinematic viscosity ν = μ/ρ is also returned. For internal pipe flow and general fluid-mechanics regime analysis.

> Canonical page: https://elysiatools.com/en/tools/reynolds-number-calculator

- **Category:** Math & Numbers

- **Keywords:** Reynolds number, Re, laminar, turbulent, transitional, flow regime, dynamic viscosity, kinematic viscosity, fluid mechanics, pipe flow, hydraulic diameter, ρvD/μ, viscosity, fluid dynamics

## Overview

Calculate the Reynolds number for pipe flow and fluid-mechanics analysis using dynamic or kinematic viscosity. Enter velocity, diameter, and the required viscosity values to get Re, the flow regime, and, in dynamic mode, kinematic viscosity.

## Inputs

- **Solve For** (select): Dynamic mode takes ρ, v, D, μ and also computes ν = μ/ρ. Kinematic mode takes v, D, ν directly.
- **Density ρ (kg/m³)** (number): Fluid density ρ (kg/m³). Required when Solve For = Dynamic.
- **Velocity v (m/s)** (number): Mean flow velocity v (m/s).
- **Diameter D** (number): Characteristic length / hydraulic diameter D, in the selected Diameter Unit.
- **Diameter Unit** (select)
- **Dynamic Viscosity μ (Pa·s)** (number): Dynamic viscosity μ (Pa·s). Required when Solve For = Dynamic.
- **Kinematic Viscosity ν (m²/s)** (number): Kinematic viscosity ν (m²/s). Required when Solve For = Kinematic.
- **Decimal Places** (number)

## When to use

- Classify an internal pipe flow as laminar, transitional, or turbulent.
- Compare flow regimes for different fluid velocities, diameters, or viscosities.
- Use a hydraulic diameter with fluid properties for general flow-regime analysis.

## How it works

- Choose Dynamic mode and enter density, velocity, diameter, diameter unit, and dynamic viscosity, or choose Kinematic mode and enter velocity, diameter, and kinematic viscosity.
- The calculator converts the diameter from meters, centimeters, or millimeters to meters.
- It calculates Re using Re = ρvD/μ in Dynamic mode or Re = vD/ν in Kinematic mode.
- The result classifies the flow as laminar below 2300, transitional from 2300 to below 4000, or turbulent at 4000 and above.

## Use cases

- Pipe and duct flow checks using a diameter or hydraulic diameter.
- Fluid-mechanics coursework and engineering calculations.
- Evaluating how viscosity and flow speed affect laminar or turbulent behavior.

## Frequently asked questions

### What is the Reynolds number formula?

Dynamic mode uses Re = ρvD/μ. Kinematic mode uses Re = vD/ν.

### What inputs are required in Dynamic mode?

Enter density in kg/m³, velocity in m/s, diameter, diameter unit, and dynamic viscosity in Pa·s.

### What inputs are required in Kinematic mode?

Enter velocity in m/s, diameter, diameter unit, and kinematic viscosity in m²/s.

### How is the flow regime classified?

Re < 2300 is laminar, 2300 ≤ Re < 4000 is transitional, and Re ≥ 4000 is turbulent.

### What additional value does Dynamic mode return?

It also returns kinematic viscosity calculated as ν = μ/ρ in m²/s.

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