Tiny to Vast
Fluid dynamics

Reynolds Number Calculator

Calculate Re, then test what changes it. Choose your length convention, compare a baseline and plan a model experiment.

10⁻¹ m · ≈ 10 cm (an airfoil chord) · Re = ρUL/μ = UL/ν · dimensionless · Newtonian fluid reference

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Define the flow

01
Viscosity input

Rounded NASA reference at 15 °C and 101.3 kPa. No temperature model.

10 m/s

0.1 m

Use chord c, from the leading edge to the trailing edge, and freestream speed.

1.229 kg/m³

0.0000173 Pa·s

Record temperature, pressure, property source and how U and L were measured. This note travels with your share link and export.

Read the result

02

Reynolds number Re

71,040

Dimensionless. Display rounded to 5 significant digits.

10 × 0.1 / 1.4076e-5 = 71,040

SI check: U in m/s, L in m, ν in m²/s. Both equations use ν = μ/ρ.

Uc
Length convention, schematic

External flow needs more context

A chord-, diameter- or distance-based Re does not give one universal transition threshold. Roughness, pressure gradients, angle and incoming turbulence can change the boundary layer and wake.

Change one thing

03

Each row changes only one input. Fluid properties and all other inputs stay fixed.

ChangeRe factorNew Re
Double speed U×2142,080
Halve length L×0.535,520
Double dynamic viscosity μ×0.535,520
Double density ρ×2142,080

Keep Re, change the scale

04

For the same fluid and geometrically similar models, Umodel = U / scale. A half-size model needs twice the speed to match Re.

Matching speed: 20 m/s

Matching Re is only one similarity condition. Compressibility, roughness, free surfaces and three-dimensional geometry may also matter.

Compare with a baseline

Save one calculation, change an input, then compare. The baseline is included in your share link and JSON.

No baseline saved yet. Try saving the example, then halve its characteristic length.

Reading the number

The same formula, a different length

Re compares inertia with viscosity. Start with the fluid properties at your experiment's temperature and pressure, then choose a speed and length that match the geometry. The calculator converts every active input to SI before computing the result.

Pipe flow

Use the inside diameter, not the length of the pipe, and the mean speed across its cross-section. The approximate tube-flow ranges shown here follow OpenStax. Other conventions often use 2300 and 4000; none is an exact boundary independent of inlet conditions, roughness and disturbances.

Read the OpenStax tube-flow guide

External flow

Use airfoil chord, cylinder or sphere diameter, or distance from a flat plate's leading edge. Record that convention so a comparison is meaningful. Boundary-layer transition and wake behavior need more information than a single Re, so this calculator gives no universal external-flow cutoff.

NASA Glenn on viscosity and similarity

Know where the properties came from

The air preset uses NASA Glenn's rounded 15 °C, 101.3 kPa reference. The water preset combines OpenStax's 20 °C viscosity with a coarse density of 1000 kg/m³. They are teaching references, not property fits. For a measured experiment, enter a cited density and viscosity at the same conditions. This calculator does not model non-Newtonian fluids or changing properties through the flow.

Air reference · Water viscosity reference · Density reference

Take the question to the wind tunnel

Save a baseline and change only speed or length to see the Re ratio. In the wind tunnel, compare shapes at fixed controls and look for differences in their wakes. Its speed, size and smoothing controls do not map to this calculated Re. The visual solver provides qualitative flow patterns, not calibrated engineering drag or lift coefficients.

Open the Wind Tunnel SimulatorExplore the Workshop

Questions about Reynolds number

What is the Reynolds number formula?

Re = ρUL/μ, or Re = UL/ν when kinematic viscosity is known. Here ν = μ/ρ. Use consistent units; the result is dimensionless.

Which characteristic length should I use?

For a circular pipe, use the inside diameter and mean speed. For an airfoil use chord; for a cylinder or sphere use diameter. A flat-plate local Reynolds number uses distance from the leading edge.

Does Reynolds number tell me the drag coefficient?

Re alone does not determine drag. Geometry, angle, surface roughness and flow conditions also matter. The linked wind tunnel is a qualitative demonstration, with no calibrated Reynolds number or engineering drag coefficient.

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