The Ford viscosity cup is a simple gravity device that permits the timed flow of a known volume

Volume

Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

 of liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 passing through an orifice located at the bottom. Under ideal conditions, this rate of flow

Volumetric flow rate

The volumetric flow rate in fluid dynamics and hydrometry, is the volume of fluid which passes through a given surface per unit time...

 would be proportional to the kinematic

Kinematics

Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces that cause the motion....

 viscosity

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 (expressed in stokes and centistokes) that is dependent upon the specific gravity

Specific gravity

Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

 of the draining liquid. However, the conditions in a simple flow cup are seldom ideal for making true measurements of viscosity. It is important when using a Ford Cup and when retesting liquids that the temperature of the cup and the liquid is maintained, as ambient temperature makes a significant difference to viscosity and thus flow rate.

The original Ford Cup was based on Imperial (US) measurement of the aperture, the world standard is DIN 4 which is a true 4mm (metric).