Drag coefficient
（重定向自Coefficient of Drag）
![Drag coefficients in fluids with Reynolds number approximately 104](/Images/godic/202501/05/14ilf1l.svg0728.png")
![Flow around a plate, showing stagnation.](/Images/godic/202501/05/Cd_flat_plate.svg0728.png")
![Drag coefficient Cd for a sphere as a function of Reynolds number Re, as obtained from laboratory experiments. The solid line is for a sphere with a smooth surface, while the dashed line is for the case of a rough surface. The numbers along the line indicate several flow regimes and associated changes in the drag coefficient:
•2: attached flow (Stokes flow) and steady separated flow,
•3: separated unsteady flow, having a laminar flow boundary layer upstream of the separation, and producing a vortex street,
•4: separated unsteady flow with a laminar boundary layer at the upstream side, before flow separation, with downstream of the sphere a chaotic turbulent wake,
•5: post-critical separated flow, with a turbulent boundary layer.](/Images/godic/202501/05/Drag_sphere_nasa.svg0728.png")
![Trade-off relationship between pressure drag and friction drag](/Images/godic/202501/05/Pressure_Drag_and_Friction_Drag0728.png")
In fluid dynamics, the drag coefficient (commonly denoted as: cd, cx or cw) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equation, where a lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag. The drag coefficient is always associated with a particular surface area.