Reynolds number for an object moving at a speed V through a fluid with density ρ and viscosity μ is defined as

where L is a representative scale of the object (the object length, for example). According to Reynolds Number Scaling in an incompressible (constant ρ) fluid, C_D is the same for two identically shaped objects if the Reynolds number is the same for the two objects. This is true even if the two objects are in different fluids. For example, a large object in a very viscous fluid (high μ) can have the same Reynolds number as a small object in a thin fluid (low μ). Likewise, a small object in a thin fluid can have the same C_D as a large object in a viscous fluid if they both have the same Reynolds number and the same shape.