Viscosity Converter: Dynamic and Kinematic Viscosity Units

Dynamic viscosity (also called absolute viscosity) measures a fluid's resistance to shear flow: the force needed to move one layer of fluid relative to another. It is measured in pascal-seconds (Pa·s) or the equivalent millipascal-second (mPa·s). Kinematic viscosity is the ratio of dynamic viscosity to the fluid's density, representing how quickly a fluid flows under gravity. Its SI unit is square metres per second (m²/s). Kinematic viscosity is used when gravity drives the flow, such as in capillary-tube viscometers or pipe-flow design.

The centipoise (cP) is a CGS unit of dynamic viscosity equal to one hundredth of a poise (P). It is numerically identical to the millipascal-second (mPa·s): 1 cP = 1 mPa·s exactly. This convenient equivalence arises from the SI definition of the pascal. Water at 20°C has a dynamic viscosity of approximately 1.002 cP (1.002 mPa·s), which made the centipoise a practical everyday unit.

The centistoke (cSt) is a CGS unit of kinematic viscosity equal to one hundredth of a stoke (St). It is numerically identical to the square millimetre per second (mm²/s): 1 cSt = 1 mm²/s exactly. Kinematic viscosity in centistokes is widely used in lubricant specifications (for example, SAE motor oil grades) and in ASTM standards for petroleum products. Water at 20°C has a kinematic viscosity of approximately 1.004 cSt.

For liquids, viscosity decreases as temperature rises: heating disrupts the intermolecular forces that resist flow. Motor oil is a familiar example: it becomes much thinner when hot. For gases, the opposite is true: viscosity increases with temperature because higher-energy molecules collide more frequently and transfer momentum more effectively. This is why viscosity specifications for lubricants always include a temperature, and why multi-grade motor oils (for example, 10W-40) are engineered to maintain acceptable viscosity across a wide temperature range.

SI engineering uses Pa·s for dynamic viscosity and m²/s for kinematic viscosity. The mPa·s and cSt are common in laboratory work because they give convenient numbers for most fluids. US industry and petroleum standards (ASTM, SAE) still commonly use centipoise (cP) and centistoke (cSt), which are numerically identical to mPa·s and mm²/s. Pound-based units such as lb/(ft·s) and ft²/s appear in older American engineering handbooks and some hydraulic calculations.