Viscosity Unit Converter: Centipoise, Pascal Seconds, Centistokes, and SSU

Two kinds of viscosity, one confused conversation

Viscosity conversions cause more genuine confusion than almost any other unit problem in engineering, and the reason is that there are really two different physical quantities hiding behind one word. Dynamic viscosity, measured in centipoise or pascal seconds, describes how hard a fluid resists being sheared between two surfaces. Kinematic viscosity, measured in centistokes or Saybolt seconds, describes how readily the fluid flows under its own weight. They are related through density, and the converter above handles that bridge for you, but you cannot convert between the families without knowing what the fluid weighs.

The relationship is simple once stated plainly: dynamic viscosity in centipoise equals kinematic viscosity in centistokes multiplied by density in grams per cubic centimeter. Water at room temperature has a density of almost exactly 1.0 g/cm3, which makes its cP and cSt values numerically identical, about 1.0 of each. That happy coincidence is also the source of the confusion, because people who learned viscosity on water assume the two units are interchangeable for everything. For a heavy drilling mud at 1.6 g/cm3 or a light condensate at 0.7, the difference is very real.

The units in this converter

• Centipoise (cP): the workhorse dynamic unit. Water is about 1 cP, typical drilling mud plastic viscosity runs 10 to 40 cP, honey is around 10,000 cP.
• Pascal second (Pa-s): the SI dynamic unit. 1 Pa-s = 1,000 cP, so most everyday fluids are small decimal values.
• Poise (P): the older CGS dynamic unit. 1 P = 100 cP.
• Centistokes (cSt): the kinematic unit used across the lubricant and fuel industries. ISO viscosity grades for oils are defined in cSt at 40 deg C.
• Saybolt Universal Seconds (SSU): literally the time in seconds for 60 mL of fluid to drain through a calibrated orifice. Still common on older pump curves and legacy lubricant specs in North America.

The Saybolt scale deserves a special note. It is not a linear unit, so there is no single multiplication factor between SSU and centistokes. This converter uses the standard ASTM D2161 correlation, which maps SSU to cSt with a pair of empirical formulas, one for readings under 100 seconds and one above. The scale bottoms out around 32 SSU, roughly 1.8 cSt, which is why very thin fluids simply cannot be expressed in Saybolt seconds.

Where this bites in practice

A pump datasheet rated in SSU being matched against a fluid spec in cP is the classic trap. Do the conversion in two explicit steps: SSU to cSt with the correlation, then cSt to cP with the actual fluid density at operating temperature. And that last phrase matters more than any unit conversion, because viscosity itself changes enormously with temperature. A 320 cSt gear oil at 40 deg C might be 25 cSt at 100 deg C. No unit converter can fix a viscosity quoted at the wrong temperature, so always carry the temperature along with the number, whatever units you use.

Related Calculators and Reference

See also the Permeability Converter for where that viscosity actually gets used, converting permeability into hydraulic conductivity.

• Drilling Hydraulics Calculator uses plastic viscosity in cP directly in friction loss equations.
• Settling Velocity Calculator shows viscosity controlling how fast particles fall through fluid.
• Mud Weight Converter for the density side of the cP to cSt bridge.
• Volume Unit Converter for barrels, gallons, liters, and cubic meters.
• Reynolds Number Calculator: whether pipe, duct or plate flow is laminar or turbulent, and the critical velocity.
• External reference: ASTM D2161, the standard practice for converting kinematic viscosity to Saybolt Universal seconds.