cSt to SSU viscosity converter (Saybolt seconds)

Introduction

Older oil data sheets, pump curves and hydraulic manuals give viscosity in Saybolt Universal Seconds (SUS or SSU), the time 60 mL of oil took to run through a standard orifice. Modern specifications use centistokes (cSt, the same as mm²/s). The two are not proportional at low viscosity, so a simple factor does not work: 20 cSt is 97.8 SUS, but 2 cSt is not a tenth of that.

This calculator converts in both directions with the official ASTM D2161 equation at 100 °F, corrects for other test temperatures, and gives the dynamic viscosity in centipoise for a given density. For heavy oils it also gives the Saybolt Furol seconds at 122 °F. It is for engineers working with lubricants, fuels, hydraulic fluids and pump selection.

Formulas

Saybolt Universal seconds at 100 °F from kinematic viscosity ν in cSt (ASTM D2161):

SUS = 4.6324 ν + (1.0 + 0.03264 ν) / ((3930.2 + 262.7 ν + 23.97 ν² + 1.646 ν³) × 10⁻⁵)

At another test temperature t in °F:

SUS_t = SUS_100 × (1 + 0.000061 (t − 100))

Saybolt Furol seconds at 122 °F, and dynamic viscosity:

SFS = 0.4717 ν + 13 924 / (ν² − 72.59 ν + 6 816)
μ (cP) = ν (cSt) × ρ (g/cm³)

• ν — kinematic viscosity at the test temperature, mm²/s (cSt)
• Above 75 cSt the relation is linear: SUS = 4.632 × cSt at 100 °F

Assumptions and limitations

• The equation is the one ASTM D2161 gives for computer conversion, fitted to the original experimental data. It applies from 32 SUS (about 1.8 cSt) upward.
• The kinematic viscosity must be at the same temperature as the Saybolt time. Viscosity itself changes strongly with temperature; this page converts units, it does not correct a viscosity to another temperature.
• Saybolt Furol seconds apply to heavy oils, roughly above 25 SFS. Below that the Universal scale is used.
• Dynamic viscosity needs the density at the same temperature.

Worked example

A hydraulic oil has a kinematic viscosity of 20 cSt at 100 °F and a density of 870 kg/m³. What is it in Saybolt seconds and in centipoise?

1. Linear term: 4.6324 × 20 = 92.65.
2. Correction term: (1.0 + 0.03264 × 20) / ((3930.2 + 5254 + 9588 + 13168) × 10⁻⁵) = 1.6528 / 0.3194 = 5.17.
3. Saybolt Universal seconds: 92.65 + 5.17 = 97.82.
4. Dynamic viscosity: 20 × 0.870 = 17.40 cP.

ASTM D2161's table lists 97.8 SUS for 20.00 cSt at 100 °F.

FAQ

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