In order for two flows to be similar, they must have the same geometry and equal Reynolds and Euler numbers. When comparing fluid behavior at corresponding points in a model and a full-scale flow, the following holds:
where  is the Reynolds number for the model, and  is full-scale Reynolds number, and similarly for the Euler numbers.
The model numbers and design numbers should be in the same proportion, hence
This allows engineers to perform experiments with reduced scale models in water channels or wind tunnels and correlate the data to the actual flows, saving on costs during experimentation and on lab time. Note that true dynamic similitude may require matching other dimensionless numbers as well, such as the Mach number used in compressible flows, or the Froude number that governs open-channel flows. Some flows involve more dimensionless parameters than can be practically satisfied with the available apparatus and fluids, so one is forced to decide which parameters are most important. For experimental flow modeling to be useful, it requires a fair amount of experience and judgment of the engineer.
An example where the mere Reynolds number is not sufficient for the similarity of flows (or even the flow regime – laminar or turbulent) are bounded flows, i.e. flows that are restricted by walls or other boundaries. A classical example of this is the Taylor–Couette flow, where the dimensionless ratio of radii of bounding cylinders is also important, and many technical applications where these distinctions play an important role. Principles of these restrictions were developed by Maurice Marie Alfred Couette and Geoffrey Ingram Taylor and developed further by Floris Takens and David Ruelle.
;Typical values of Reynolds number
* Dictyostelium amoebae: ~ 1 × 10
* Bacterium ~ 1 × 10
* Ciliate ~ 1 × 10
* Smallest fish ~ 1
* Blood flow in brain ~ 1 × 10
* Blood flow in aorta ~ 1 × 10
* Onset of turbulent flow ~ 2.3 × 10 to 5.0 × 10 for pipe flow to 10 for boundary layers
* Typical pitch in Major League Baseball ~ 2 × 10
* Person swimming ~ 4 × 10
* Fastest fish ~ 1 × 10
* Blue whale ~ 4 × 10
* A large ship (Queen Elizabeth 2) ~ 5 × 10
* Atmospheric tropical cyclone ~ 1 x 10