Viscosity calculator and converter

The centipoise is the practical unit of dynamic viscosity: 1 cP = 0.001 Pa·s = 1 mPa·s, per NIST SP 811. So cP and mPa·s are interchangeable: 350 cP is 350 mPa·s or 0.35 Pa·s. For reference, water at 20 °C has 1.0016 cP according to NIST.

cSt is the centistokes, the practical unit of kinematic viscosity: 1 cSt = 1 mm²/s = 0.000001 m²/s. On the data sheet, record the cSt value together with the measurement temperature: without it, the number cannot be used to select a pump, because viscosity changes with temperature.

Multiply the kinematic viscosity in cSt by the density in g/cm³ at the same temperature: cP = cSt × ρ. Example: 100 cSt with a density of 0.90 g/cm³ gives 90 cP. The other way round, cSt = cP ÷ ρ. Without the density at the same temperature, cSt and cP cannot be treated as equivalent; they are only close when the density is near 1 g/cm³, as with water.

According to the NIST Chemistry WebBook (IAPWS 2008 formulation, at 1 atm), water has 1.7911 cP at 0.01 °C, 1.0016 cP at 20 °C, 0.65273 cP at 40 °C, 0.46604 cP at 60 °C and 0.35405 cP at 80 °C. In kinematic viscosity, that is 1.003 cSt at 20 °C. The table on this page lists the values every 10 °C up to boiling.

Dynamic viscosity expresses resistance to flow; kinematic viscosity is dynamic viscosity divided by density. With cP, cSt and g/mL, the relation is cP = cSt × density. Saybolt Universal expresses a measurement in seconds found in fluid documents. Conversion must respect the reference temperature and applicable method.

ASTM D2161 provides the tables and equations to convert kinematic viscosity in mm²/s (cSt) to Saybolt Universal seconds (SSU) at the same temperature, and also to Saybolt Furol. Identify whether the value is Universal or Furol and state the original temperature. Unit conversion does not estimate viscosity at another temperature. The FBE selection table is in SSU: send the value you have, and engineering does the conversion.

Viscosity affects losses, power and suction conditions. Centrifugal pumps may require correction of the water performance curve; in positive displacement pumps, viscosity also changes internal leakage and power. Compare models at the same flow, pressure and temperature. Choosing FBCN, FBE or FBEI depends on the process, without a universal viscosity cutoff.

Yes. For water, according to NIST, viscosity drops from 1.7911 cP at 0.01 °C to 0.28166 cP at 99.974 °C, more than six times lower. Use the fluid data sheet for startup and operating conditions; unit conversion does not predict that variation. For Saybolt conversion, retain temperature and identify Universal or Furol and the applicable method.

The FBE table lists up to 100,000 SSU for 3- to 6-inch sizes, with speed linked to the viscosity range. Smaller sizes have their own limits. This does not validate every liquid: composition, solids, temperature, suction, pressure and material compatibility must also be checked by engineering.