Viscosity is a liquid's resistance to flow. Water has low viscosity; honey has high viscosity. It appears in almost every liquid product specification because it affects how a product is perceived, how it performs and whether it can be handled by the equipment that fills and applies it.
Viscosity matters at several points. In manufacturing it determines whether a product can be pumped, mixed and filtered. In filling it determines whether the material runs cleanly into containers at line speed. In use it decides whether a coating runs down a vertical surface, whether an adhesive stays where it is placed, and whether a liquid pours acceptably.
Two things need stating whenever viscosity is specified: temperature and measurement method. Viscosity changes substantially with temperature - often by a factor of several between a cold store and a warm production area - so a figure without a temperature is incomplete. Different instruments and conditions also give different values for the same material, so specification and testing must use the same method.
Many products are not simply viscous but change viscosity under shear, thinning when stirred or brushed and thickening again at rest. That behaviour is not captured by a single figure and belongs to rheology. See thickeners and rheology modifiers.
Always specify viscosity with a temperature and a measurement method. A specification stating a figure alone is not testable in any consistent way, and disputes over whether a batch conformed usually trace back to that omission.
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Last reviewed: August 2026. Regulation and guidance change — verify anything you rely on against the primary source. Tell us if something here is out of date.
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