Unit of Viscosity: Pascal-Second, Poise and Other Units

The unit of viscosity helps us measure how easily or how difficult a fluid flows. We come across viscosity in our daily lives more often than we realize. For example, water flows quickly when poured from a glass, while honey takes much longer to flow from a spoon. This difference happens because different fluids have different viscosities. Fluids with high viscosity flow slowly, whereas fluids with low viscosity flow more easily. Knowing the unit of viscosity helps us compare the flow behavior of different liquids and understand their use in everyday life as well as in industries. This article explains the unit of viscosity, its SI unit, CGS unit, and common conversions in a simple way.

Table of Contents

• What is Viscosity
• SI Unit of Viscosity: Pascal-Second (Pa·s)
• CGS Unit of Viscosity: Poise
• Other Common Viscosity Units

What is Viscosity

Viscosity is the property of a fluid that describes its resistance to flow. In simple words, it is the internal friction between the molecules of a liquid or gas that slows down its movement. Have you ever noticed that honey pours much more slowly than water? This happens because honey has a higher viscosity, while water has a lower viscosity. Generally, liquids that appear thicker have higher viscosity and flow more slowly. Viscosity is represented by the Greek symbol η (eta), although the symbol μ (mu) is also commonly used. It is often referred to as dynamic viscosity or absolute viscosity.

Interestingly! There is another related term called kinematic viscosity. While dynamic viscosity measures an internal resistance to the flow of fluids, kinematic viscosity takes the density of the fluid into account. In simple terms, kinematic viscosity is the ratio of dynamic viscosity to density.

SI Unit of Viscosity: Pascal-Second (Pa·s)

The SI unit of viscosity is pascal-second (Pa·s). It is the standard unit used to measure viscosity in the International System of Units (SI). The unit is named after the French scientist Blaise Pascal, who made important contributions to the study of fluids and pressure.

One pascal-second is defined as the viscosity of a fluid when a shear stress of one pascal produces a velocity gradient of one reciprocal second (s⁻¹). Moving ahead, let’s look at some common examples of viscosity in everyday life.

Examples of Viscosity

Fluid	Approximate Viscosity (Pa·s)	Flow Behavior
Water	0.001 Pa·s	Flows very easily
Motor Oil	0.1 Pa·s	Flows slower than water
Honey	10 Pa·s	Flows very slowly

CGS Unit of Viscosity: Poise

The CGS system stands for centimeter-gram-second. In this system, length is measured in centimeters, mass in grams, and time in seconds. The CGS unit of viscosity is called poise (P). It is named after the French scientist Jean Léonard Marie Poiseuille.

One poise is the viscosity of a fluid when a shear stress of one dyne per square centimeter produces a velocity gradient of one reciprocal second (s⁻¹). In simple words, it is the amount of viscosity needed for a fluid to flow under a given force. Interestingly, the poise is a fairly large unit. The viscosity of water at room temperature is about 0.01 poise, while the viscosity of honey is around 1000 poise.

Moving further, although the CGS system is still used in some areas of science, the SI system is more commonly used today. In the SI system, the unit of viscosity is pascal-second (Pa·s).

Other Common Units of Viscosity

Apart from the SI unit (pascal-second) and the CGS unit (poise), some other units are also used to measure viscosity in different industries and countries. Interestingly, different fields may use different viscosity units depending on their requirements.

Reyn

The Reyn is a British unit used to measure dynamic viscosity. Although it is not commonly used today, it may still be found in some older engineering references.

Saybolt Universal Second (SUS)

Earlier, viscosity was often measured using a Saybolt viscometer, a device that measures how long a liquid takes to flow through a small opening. The result was expressed in Saybolt Universal Seconds (SUS).

You may also come across the abbreviations:

• SUV: Saybolt Universal Viscosity
• SSU: Saybolt Seconds Universal

These units were widely used for petroleum products such as lubricating oils and fuels.

Some Other Dynamic Viscosity Units

Unit	Symbol
Kilogram-force second per square metre	kgf·s m⁻²
Poundal second per square foot	pdl·s ft⁻²
Pound per foot hour	lb (ft·h)⁻¹
Pound per foot second	lb (ft·s)⁻¹
Pound-force second per square foot	lbf·s ft⁻²
Pound-force second per square inch (Reyn)	lbf·s in⁻²
Slug per foot-second	slug (ft·s)⁻¹

Till now, we have learned that viscosity tells us how easily or slowly a fluid flows. The SI unit of viscosity is pascal-second (Pa·s), while the CGS unit of viscosity is poise (P). These units are useful for measuring and comparing the flow behavior of different liquids and gases.