Kinematic vs Dynamic Viscosity: Key Differences, Formulas & When to Use Each

If you work in a petroleum testing laboratory, viscosity is likely one of your most frequently measured parameters. Yet the distinction between dynamic viscosity and kinematic viscosity remains a common source of confusion — even among experienced lab professionals.

Getting this distinction right is essential for accurate quality control, regulatory compliance, and meaningful inter-laboratory comparisons. This guide explains the difference, the formulas, the relevant ASTM standards, and how to choose the right testing method for your application.

Dynamic viscosity (denoted by the Greek letter η or μ) measures a fluid’s internal resistance to flow when an external force is applied. It quantifies the shear stress required to move one layer of fluid relative to another at a given shear rate.

Think of it this way: imagine a fluid trapped between two parallel plates. When you push the top plate while keeping the bottom plate stationary, dynamic viscosity is the force that resists that motion. The higher the dynamic viscosity, the more force is needed to make the fluid flow.

Units of Dynamic Viscosity

Unit System	Unit	Abbreviation
SI	Pascal-second	Pa·s
CGS	Poise	P
Common practice	Centipoise	cP (1 cP = 1 mPa·s)

Real-World Examples

• Water at 20°C: ~1 cP — flows freely under applied force
• Olive oil: ~80 cP — noticeable resistance when pumping
• Honey: ~2,000–10,000 cP — difficult to stir without significant effort
• Toothpaste: ~70,000 cP — requires squeezing to flow

Dynamic viscosity is the relevant measurement when fluids are being pumped, stirred, spread, or extruded — any scenario where an external force drives the flow.

Kinematic viscosity (denoted by the Greek letter ν) measures a fluid’s resistance to flow under the influence of gravity alone — with no external force applied. It is defined as the ratio of dynamic viscosity to fluid density.

Kinematic viscosity describes how fast a fluid flows when you simply pour it or let it drain under gravity. It is the standard measurement for lubricants, fuel oils, and hydraulic fluids because these fluids are typically selected for their gravity-fed flow behavior.

Units of Kinematic Viscosity

Unit System	Unit	Abbreviation
SI	Square meter per second	m²/s
CGS	Stokes	St
Common practice	Centistokes	cSt (1 cSt = 1 mm²/s)

Real-World Examples

• Water at 20°C: ~1 cSt — pours instantly under gravity
• SAE 30 engine oil at 100°C: ~11 cSt — flows readily when warm
• Heavy fuel oil at 50°C: ~380 cSt — flows slowly, requires heating

Kinematic viscosity is the standard measurement for lubricating oils, fuel oils, and hydraulic fluids — products classified by ISO 3448, SAE J300, and other international standards.

Property	Dynamic Viscosity	Kinematic Viscosity
Symbol	η (eta) or μ (mu)	ν (nu)
Definition	Internal resistance to flow under applied force	Flow under gravity only
Formula	η = shear stress / shear rate	ν = η / ρ
Common Units	cP (centipoise)	cSt (centistokes)
Force	External force applied	Gravity only
Requires Density?	No	Yes — density must be known to convert
Key ASTM Standard	ASTM D7945	ASTM D445
Instrument Type	Rotational viscometer	Capillary viscometer
Typical Applications	Paints, adhesives, food, polymers, non-Newtonian fluids	Petroleum products, lubricants, fuel oils

The petroleum industry relies on two primary ASTM standards for viscosity measurement:

ASTM D445 — Kinematic Viscosity of Transparent and Opaque Liquids

ASTM D445 is the most widely used standard for kinematic viscosity testing of petroleum products. It measures the time for a fixed volume of liquid to flow under gravity through a calibrated glass capillary viscometer at a controlled temperature. The result is expressed in centistokes (cSt) or mm²/s.

Applicable to: Crude oils, fuel oils, lubricating oils, hydraulic fluids, transformer oils, and other petroleum liquids.

ASTM D7945 — Determination of Dynamic Viscosity

ASTM D7945 specifies the determination of dynamic viscosity using a rotational viscometer. It applies to both Newtonian and non-Newtonian fluids. Importantly, this standard allows calculation of kinematic viscosity by dividing the dynamic viscosity result by the fluid density at the same test temperature.

Applicable to: Petroleum products that exhibit non-Newtonian behavior and applications where shear rate information is needed.

Viscosity is highly temperature-dependent. A change of just 1°C can alter a fluid’s viscosity by 5–10% or more. This is why both ASTM D445 and ASTM D7945 require precise temperature control — typically ±0.01°C at the test temperature.

LabVV kinematic viscometers are equipped with precision temperature control systems that maintain bath stability to ±0.01°C, ensuring compliance with ASTM D445 requirements.

The conversion between dynamic and kinematic viscosity is straightforward when you know the fluid’s density at the test temperature:

To convert dynamic to kinematic:
ν = η / ρ

To convert kinematic to dynamic:
η = ν × ρ

Where ρ is the density at the same temperature as the viscosity measurement (expressed in g/cm³ or kg/m³).

The choice between a kinematic (capillary) viscometer and a dynamic (rotational) viscometer depends on your application:

For laboratories that need both, the LabVV A1014 automatic capillary viscometer supports ASTM D7279, an alternative method that offers high throughput with small sample volumes.

---

Newtonian vs Non-Newtonian Fluids

An important distinction: Newtonian fluids (like water, light oils, and glycerin) have constant viscosity regardless of shear rate. For these fluids, kinematic and dynamic viscosity are simply related by density. Non-Newtonian fluids (like greases, drilling muds, and some crude oils) change viscosity with shear rate — dynamic viscosity measurement is essential for accurate characterization.