m2/s 1 3.6 2.24 196.9 3.28 1
m2/h 0.278 1 0.62 54.68 0.91 1
cm2/s 0.45 1.609 1 88 1.47 1
ft2/s 5.08 10-3 1.83 10-2 1.14 10-2 1 1.67 10-2 1
ft2/hr 0.305 1.097 0.682 60 1 1
in2/s 0.305 1.097 0.682 60 1 1


Kinematic viscosity is a measure of a fluid's internal resistance to flow under gravitational forces. It is determined by measuring the time in seconds, required for a fixed volume of fluid to flow a known distance by gravity through a capillary within a calibrated viscometer at a closely controlled temperature.

This value is converted to standard units such as centistokes (cSt) or square millimeters per second. Viscosity reporting is only valid when the temperature at which the test was conducted also is reported - for example 23 cSt at 40 degrees C.

Of all the tests employed for used oil analysis, none provides better test repeatability or consistency than viscosity. Likewise, there is no property more critical to effective component lubrication than base oil viscosity. However, there is more to viscosity than meets the eye. Viscosity can be measured and reported as dynamic (absolute) viscosity or as kinematic viscosity. The two are easily confused, but are significantly different.

Most used oil analysis laboratories measure and report kinematic viscosity. By contrast, most onsite viscometers measure dynamic viscosity, but are programmed to estimate and report kinematic viscosity, so that the viscosity measurements reported reflect kinematic numbers reported by most labs and lube oil suppliers.

Given the importance of viscosity analysis coupled with the increasing popularity of onsite oil analysis instruments used to screen and supplement offsite laboratory oil analysis, it is essential that oil analysts understand the difference between dynamic and kinematic viscosity measurements.

Generally speaking, viscosity is a fluid’s resistance to flow (shear stress) at a given temperature. Sometimes, viscosity is erroneously referred to as thickness (or weight). Viscosity is not a dimensional measurement, so calling highly viscous oil thick and less viscous oil thin is misleading.

Likewise, reporting viscosity for trending purposes without a reference to temperature is nonsensical. The temperature must be defined to interpret the viscosity reading.