Dynamic vs Kinematic Viscosity (cP vs cSt)
By Scientific Converters · Updated · Factors from NIST SP 811 and the BIPM SI Brochure
Short answer: Kinematic viscosity equals dynamic viscosity divided by density, ν = μ/ρ; in practical units cSt = cP ÷ (density in g/cm³), so an oil of 90 cP and 0.9 g/cm³ is 100 cSt.
Two kinds of viscosity
Dynamic (absolute) viscosity, μ, is the fluid's resistance to shear: shear stress divided by shear rate. Its SI unit is the pascal-second; product datasheets usually give centipoise (cP), and 1 cP = 1 mPa·s.
Kinematic viscosity, ν, is dynamic viscosity divided by density. Its SI unit is m²/s; oils are graded in centistokes (cSt), and 1 cSt = 1 mm²/s. Gravity-driven viscometers such as ASTM D445 capillary tubes measure kinematic viscosity directly.
Converting between them
ν = μ / ρ
ν [cSt] = μ [cP] ÷ ρ [g/cm³] · ν [m²/s] = μ [Pa·s] ÷ ρ [kg/m³]
The density must be at the same temperature as the viscosity. Because viscosity falls steeply with temperature (several percent per °C for oils), always quote both values with their temperature.
Typical values
| Fluid | μ (cP) | ρ (g/cm³) | ν (cSt) |
|---|---|---|---|
| Air, 20 °C | 0.0181 | 0.0012 | 15.1 cSt |
| Water, 20 °C | 1.002 | 0.9982 | 1 cSt |
| Water, 40 °C | 0.653 | 0.9922 | 0.658 cSt |
| Ethanol, 20 °C | 1.2 | 0.789 | 1.52 cSt |
| ISO VG 46 oil, 40 °C | 40 | 0.87 | 46 cSt |
| Glycerol, 20 °C | 1410 | 1.26 | 1120 cSt |
Property values are rounded literature values for illustration; use measured data for design.
Which one should I use?
- Reynolds number, pipe flow, settling: either, consistently: Re = ρvD/μ = vD/ν.
- Lubricants and fuels: kinematic (ISO VG and SAE data are in cSt).
- Pumps, mixers, coatings, adhesives: dynamic (cP or mPa·s from rotational viscometers).
- Gases: dynamic viscosity varies little with pressure; kinematic viscosity changes with density, so it falls as pressure rises.