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Viscosity

General

Dynamic and Kinematic Viscosity

A measure of a fluid's resistance to flow, describing the internal friction between fluid layers moving at different speeds.

Definition

Viscosity is a measure of a fluid's internal resistance to flow — essentially, the friction between adjacent layers of fluid as they move at different speeds relative to each other. A high-viscosity fluid like honey or motor oil flows sluggishly and resists deformation, while a low-viscosity fluid like water or gasoline flows easily. Viscosity is fundamental to fluid mechanics, affecting everything from how oil lubricates an engine to how blood flows through arteries to how paint levels out on a wall.

There are two related but distinct measures: dynamic viscosity (also called absolute viscosity), which quantifies the actual shear resistance of the fluid, and kinematic viscosity, which normalizes dynamic viscosity by the fluid's density — making it more directly comparable across fluids of different weights, particularly relevant for gravity-driven flow like oil draining through an engine. The Viscosity Converter handles dynamic viscosity units such as pascal-seconds, poise, and centipoise, while the Kinematic Viscosity Converter handles the density-normalized units like square meters per second and centistokes.

Viscosity is closely related to how efficiently heat and momentum transfer within a fluid, connecting conceptually to Thermal Conductivity, which describes how efficiently heat transfers through a material — both are transport properties that engineers must account for when designing systems involving fluid flow and heat exchange, such as radiators, pipelines, and lubrication systems.

Formula

τ = μ (du/dy)

Where:

  • τ (tau) = shear stress, the force per unit area resisting flow (in pascals)
  • μ (mu) = dynamic viscosity (in pascal-seconds)
  • du/dy = shear rate, the velocity gradient perpendicular to flow direction (in inverse seconds)

Kinematic viscosity: ν = μ / ρ

Where:

  • ν (nu) = kinematic viscosity (in square meters per second)
  • μ = dynamic viscosity (in pascal-seconds)
  • ρ (rho) = fluid density (in kilograms per cubic meter)

Worked Example

Motor oil has a dynamic viscosity of μ = 0.29 Pa·s at 40°C, and a density of ρ = 875 kg/m³ at the same temperature.

Kinematic viscosity: ν = μ / ρ = 0.29 / 875 = 0.000331 m²/s = 331 centistokes (cSt)

This value is close to the specification for an SAE 30 or ISO VG 320 industrial oil at 40°C, illustrating why kinematic viscosity in centistokes is the standard unit referenced on lubricant data sheets. Use the Kinematic Viscosity Converter to convert between square meters per second, centistokes, and other units used across different regional standards.

Key Things to Know

  • Dynamic and kinematic viscosity answer different questions: Dynamic viscosity (pascal-seconds) describes absolute resistance to shear, while kinematic viscosity (square meters per second) divides that by density, making it the standard for comparing gravity-driven flows like oil draining or lubricant performance ratings.
  • Temperature dependence runs opposite for liquids and gases: Liquid viscosity drops sharply as temperature rises, while gas viscosity rises slightly with temperature — an important distinction when selecting lubricants or modeling fluid behavior across operating temperature ranges.
  • Non-Newtonian fluids don't have one fixed viscosity value: Substances like ketchup, blood, and paint change apparent viscosity depending on how much shear force is applied, unlike Newtonian fluids such as water and most oils, whose viscosity stays constant regardless of shear rate at a given temperature.
  • Viscosity pairs with thermal conductivity in engineering design: Systems like radiators, heat exchangers, and engine cooling loops must account for both properties together, since a fluid's ability to flow (viscosity) and its ability to carry heat away (thermal conductivity) jointly determine cooling system performance.
  • Unit conversion errors are common in industry: Because viscosity appears in multiple unit systems — poise, centipoise, pascal-seconds, stokes, centistokes, and SAE/ISO grade numbers — always confirm which measure (dynamic or kinematic) a specification uses before converting, which the Viscosity Converter and Kinematic Viscosity Converter are built to keep separate.

Related Terms

Thermal ConductivityThermal Conductivity (Fourier's Law)

Frequently Asked Questions

Dynamic viscosity measures a fluid's internal resistance to flow in absolute terms, expressed in pascal-seconds or centipoise, while kinematic viscosity divides dynamic viscosity by the fluid's density, expressed in square meters per second or centistokes. Kinematic viscosity is especially useful for comparing how fluids flow under gravity alone, such as in engine oil ratings, since it accounts for how heavy the fluid is per unit volume. Use the Viscosity Converter for dynamic viscosity units and the Kinematic Viscosity Converter when density-normalized values are needed.
Honey has a dynamic viscosity of roughly 2,000 to 10,000 centipoise depending on temperature and moisture content, compared to water's roughly 1 centipoise at room temperature — meaning honey resists flow thousands of times more than water. This high viscosity comes from honey's sugar concentration and the hydrogen bonding between sugar molecules, which creates strong internal friction as layers of fluid try to slide past each other. The Viscosity Converter lets you compare these values across units like centipoise, pascal-seconds, and poise.
For most liquids, viscosity decreases as temperature rises because added thermal energy weakens the intermolecular forces that resist flow — motor oil, for example, can lose over 90% of its viscosity between 0°C and 100°C. This is the opposite behavior of gases, whose viscosity actually increases with temperature due to more frequent molecular collisions. This temperature dependence is why engine oils are rated with viscosity grades like 5W-30, describing performance at both cold start and operating temperatures.
Lubricant selection depends on operating temperature range and load — lower viscosity grades (such as SAE 5W or ISO VG 32) suit cold climates and light loads where the oil must flow easily on startup, while higher viscosity grades (such as SAE 30 or ISO VG 150) suit high-temperature, high-load applications where a thicker film is needed to prevent metal-to-metal contact. Manufacturer specifications should always take priority, but the Kinematic Viscosity Converter helps translate between centistokes and other grading systems referenced in equipment manuals.
Not exactly — thickness is a casual, non-technical description, while viscosity is a precisely defined physical property measured in specific units like pascal-seconds. Two fluids can feel similarly thick to the touch but have very different viscosities depending on temperature and shear rate, especially for non-Newtonian fluids like ketchup or paint whose apparent thickness changes depending on how much force is applied. The Viscosity Converter works with the precise measured values used in engineering and scientific contexts, not subjective thickness descriptions.