About the Reynolds Number Calculator
The Reynolds number compares the push of a moving fluid with the drag of its own stickiness, and it tells you whether the flow is smooth or chaotic. Choose a fluid such as water at 20 degrees, air or oil, or type your own density and viscosity. Enter the flow speed and the pipe diameter, and the calculator returns the Reynolds number, the flow regime (laminar, transitional or turbulent), the Darcy friction factor, the kinematic viscosity and the flow rate. You can also add the roughness of the pipe wall. It is meant for full circular pipes and is calculated in your browser.
Updated October 2026
How to use the Reynolds Number Calculator
- 1Pick the fluid, or choose your own and type its density and viscosity.
- 2Enter the flow speed in metres per second.
- 3Enter the inside diameter of the pipe in millimetres.
- 4Optionally enter the wall roughness.
- 5Read the Reynolds number, the regime and the friction factor.
Why flow changes character
Keep in mind that the speed in the formula is the average speed across the pipe, found by dividing the flow rate by the cross-sectional area. Near the wall the fluid moves more slowly, and this is where the sticky layer forms.
At low speed or high viscosity, fluid moves in smooth layers and the flow is laminar. As the speed rises, small disturbances grow, layers mix and the flow becomes turbulent. Honey in a thin tube stays laminar at speeds where water in the same tube is already turbulent, because honey is far more viscous.
The regime changes how much energy is lost to friction. Laminar friction falls as the Reynolds number rises, while turbulent friction stays high and depends on the roughness of the wall.
Typical values
- Water at 1.5 m/s in a 50 mm pipe: Re around 75,000, turbulent.
- Blood in a small artery: Re around a few hundred, laminar.
- Air at 10 m/s in a 100 mm duct: Re around 66,000.
- Heavy oil at 0.5 m/s in a 25 mm pipe: often below 2,300.
- A swimming fish or boat: Re in the millions.
Related tools
To size a pipe for a target flow, the Pipe Flow Calculator is a natural next step, while the Density Calculator helps you find a density from mass and volume. Pressure answers can be converted with the Pressure Converter, and the Ideal Gas Law Calculator provides gas density from pressure and temperature. Real installations need professional design checks.
Features
- Presets for water, air, oil and glycerin
- Custom density and viscosity
- Laminar, transitional and turbulent labels
- Darcy friction factor with roughness
- Kinematic viscosity and flow rate
- Metric units
- Free and private
Frequently asked questions
What is the Reynolds number?
It is a dimensionless number equal to density times speed times diameter divided by dynamic viscosity. It predicts whether flow is laminar or turbulent.
At what value does flow become turbulent?
In a pipe, flow is usually laminar below about 2,300 and turbulent above about 4,000. In between it is transitional and hard to predict.
What is the friction factor?
The Darcy friction factor is used to find the pressure loss in a pipe. For laminar flow it is 64 divided by Re, and for turbulent flow this page uses the Swamee-Jain equation.
What viscosity should I enter?
Use the dynamic viscosity in pascal seconds. One centipoise is 0.001 pascal seconds. Water at room temperature is about 0.001.
Does temperature matter?
Yes. Viscosity and density change with temperature, especially for oils, so pick the preset or enter values for your temperature.
Can I use it for a rectangular duct?
Use the hydraulic diameter, which is four times the area divided by the wetted perimeter, in place of the diameter.
Is this accurate enough for design?
It is fine for estimates and teaching. Real systems have bends, valves and changing temperatures, so use a full pipe-flow analysis for design work.