# Reynolds Number Calculator

URL: https://speedycalculator.com/calculators/physics/reynolds-number/
Last updated: 2026-10-25

> The Reynolds number calculator predicts whether a fluid flow is laminar or turbulent, from velocity, a characteristic length (such as pipe diameter), density and viscosity.

## Inputs

- Fluid density (kg/m³, g/cm³, g/mL, lb/ft³)
- Flow velocity (m/s, km/h, mph, ft/s, knots)
- Pipe diameter (m, cm, mm, km, in, ft, yd, mi)
- Dynamic viscosity (Pa·s)

## Results

- Reynolds number

## Formula

```
Re = ρ v D ÷ μ
```

## Explanation

Low Reynolds numbers mean smooth, layered flow; high values mean chaotic, turbulent flow. In pipes, transition is around 2,300–4,000.

### How to use the Reynolds Number Calculator

- **Fluid density**
- **Flow velocity**
- **Pipe diameter**
- **Dynamic viscosity**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Multiply density, velocity and diameter and divide by dynamic viscosity.

### Inputs explained

FieldWhat to enterFluid densityUnits: kg/m³, g/cm³, g/mL, lb/ft³Flow velocityUnits: m/s, km/h, mph, ft/s, knotsPipe diameterUnits: m, cm, mm, km, in, ft …Dynamic viscosityIn Pa·s

### Example values

How the main result changes when **Fluid density** changes and everything else stays at the example values above:

Fluid density (kg/m³)Reynolds number49937,425748.556,13899874,8501,247.593,5631,497112,2751,996149,700

## Worked example

Inputs — Fluid density: 998 kg/m³; Flow velocity: 1.5 m/s; Pipe diameter: 0.05 m; Dynamic viscosity: 0.001 Pa·s.

Results — **Reynolds number = 74,850**.

## FAQ

**What is water’s viscosity?**
About 0.001 Pa·s at 20 °C.

**Do the thresholds apply to all shapes?**
No, they are for flow in round pipes.

**What formula does the Reynolds Number Calculator use?**
Re = ρ v D ÷ μ.