Density and Viscosity Converter (cP, cSt, SSU, API)

Dynamic versus kinematic viscosity

Dynamic viscosity (μ) is a fluid's resistance to shear, in Pa·s or cP. Kinematic viscosity (ν) is the same thing divided by density: ν = μ / ρ, in m²/s or cSt. Reynolds number calculations use the kinematic value and pressure drop calculations the dynamic one. The density used in the conversion has to be at the same temperature.

SSU and cSt

Older oil data sheets give viscosity in Saybolt Universal seconds (SSU or SUS): the time a set volume of oil takes to run through a standard orifice. ASTM D2161 relates that time to kinematic viscosity. Above 75 cSt, SSU = 4.632 × cSt holds, and the relation curves for thinner oils. For example 10 cSt is about 58.8 SSU and 100 cSt about 463 SSU.

API gravity and Baumé

Petroleum products quote density as API gravity: °API = 141.5 / SG − 131.5, where SG is the specific gravity at 15.56 °C. Water is 10 °API and lighter products score higher. A crude with a specific gravity of 0.85 is about 35 °API. The Baumé scale used in the chemical industry has two separate formulas, for liquids lighter and heavier than water.

Limits

Viscosity varies strongly with temperature, so check the temperature each figure refers to. The SSU conversion does not hold below 1.81 cSt. API and Baumé conversions are defined at 15.56 °C, and a density measured at another temperature has to be corrected first.