Last updated: 2026-06-30

Dynamic viscosity

Dynamic viscosity (also called absolute viscosity) is a measure of a fluid's internal resistance to flow. When you push a layer of fluid sideways relative to an adjacent layer, the viscosity tells you how much force that relative motion requires per unit area. High-viscosity fluids like honey resist flow; low-viscosity fluids like water flow easily.

The SI unit of dynamic viscosity is the pascal-second (Pa·s), equal to one newton-second per square metre (N·s/m²) or one kilogram per metre-second (kg/(m·s)).

Unit systems

SI units — Pa·s and mPa·s

The pascal-second is the standard SI unit. For most aqueous solutions and light oils the pascal-second is inconveniently large, so the millipascal-second (mPa·s) is often preferred. Water at 20 °C has η ≈ 1.002 mPa·s — a value close to 1, which makes mPa·s a comfortable scale for dilute liquids.

CGS units — poise (P) and centipoise (cP)

The poise is the CGS unit of dynamic viscosity, defined as g/(cm·s) and named after Jean-Louis Marie Poiseuille. Because the poise is too large for typical lab measurements (water at 20 °C is only ≈ 0.01 P), the centipoise (cP) — one hundredth of a poise — became the de facto standard in chemistry, pharmaceutical sciences, and the petroleum industry.

A key identity: 1 cP = 1 mPa·s exactly. This means any measurement in centipoise is numerically identical to the same value in millipascal-seconds, which is why both units remain in active use: cP in legacy and industry contexts, mPa·s in modern SI reporting.

Conversion table

Unit	Symbol	Relation to Pa·s
Pascal-second	Pa·s	1 Pa·s
Millipascal-second	mPa·s	0.001 Pa·s
Poise	P	0.1 Pa·s
Centipoise	cP	0.001 Pa·s

Key relationships:

• 1 Pa·s = 10 P = 1000 cP = 1000 mPa·s
• 1 P = 100 cP = 0.1 Pa·s
• 1 cP = 1 mPa·s = 0.001 Pa·s

Common fluid viscosities

Fluid	Temperature	Viscosity
Air	20 °C	≈ 0.018 cP
Water	20 °C	≈ 1.002 cP
Water	100 °C	≈ 0.282 cP
Whole blood	37 °C	≈ 3–4 cP
Olive oil	20 °C	≈ 84 cP
Honey	20 °C	≈ 2000–10 000 cP
Glycerol	20 °C	≈ 1412 cP

Dynamic vs kinematic viscosity

Dynamic viscosity η measures resistance to shear stress — it is purely a fluid property. Kinematic viscosity ν accounts for the fluid's inertia by dividing dynamic viscosity by density:

ν = η / ρ

Kinematic viscosity uses SI units of m²/s or the CGS unit stokes (St), where 1 St = 10⁻⁴ m²/s. For water at 20 °C: η ≈ 1.002 mPa·s, ρ ≈ 998 kg/m³, so ν ≈ 1.004 × 10⁻⁶ m²/s ≈ 1.004 centistokes (cSt).

Choosing which type to use depends on context: fluid mechanics and pipe-flow calculations often use dynamic viscosity; kinematic viscosity is preferred for gravity-driven flow (e.g. lubrication, die swell) and in standardized ASTM oil-testing methods.

How to use this converter

Enter a value in any field — Pa·s, mPa·s, P, or cP — and the other three update automatically. For example:

• Motor oil rated SAE 10W at 40 °C has η ≈ 40 cP → enter 40 in the cP field to read 0.04 Pa·s and 0.4 P.
• A pharmaceutical solution measured at 5.3 mPa·s → enter 5.3 in the mPa·s field to confirm 5.3 cP and 0.053 P.

Frequently Asked Questions (FAQ)

How do I convert centipoise (cP) to pascal-seconds (Pa·s)?

Divide the centipoise value by 1000. The exact relationship is 1 Pa·s = 1000 cP, so 1 cP = 0.001 Pa·s. For example, water at 20 °C has a dynamic viscosity of about 1.002 cP, which equals 0.001002 Pa·s. Conveniently, 1 mPa·s = 1 cP exactly, so millipascal-seconds and centipoise are interchangeable.

What is the viscosity of water in common units?

At 20 °C (68 °F), water has a dynamic viscosity of approximately 1.002 cP (1.002 mPa·s, 0.001002 Pa·s, 0.01002 P). At 100 °C it falls to about 0.282 cP. A viscosity near 1 cP is the practical benchmark for low-viscosity fluids — water at room temperature is the reference.

What is the relationship between poise and centipoise?

1 poise (P) = 100 centipoise (cP) and 1 poise = 0.1 Pa·s. The poise is the CGS unit of dynamic viscosity, named after Jean-Louis Marie Poiseuille. Because poise is too large for most practical measurements (water at 20 °C is only 0.01002 P), the centipoise became the standard lab unit and remains common in chemical and petroleum industries.

What is the difference between dynamic and kinematic viscosity?

Dynamic viscosity (absolute viscosity) measures a fluid's internal resistance to flow and has SI units of Pa·s (kg/(m·s)). Kinematic viscosity is dynamic viscosity divided by the fluid's density, with SI units of m²/s or the CGS unit stokes (St). For water at 20 °C, η ≈ 1.002 mPa·s and ρ ≈ 998 kg/m³, giving kinematic viscosity ν ≈ 1.004 × 10⁻⁶ m²/s (≈ 1.004 cSt).