cP · cSt · SUS

Viscosity converter: cP to cSt and SUS

Match the data sheet viscosity to the pump curve's unit before ordering.

Last updated 2026-09-27. Math runs in your browser, no data leaves your computer.

Data sheets quote dynamic viscosity in cP or mPa·s, while pump curves and older specs often use cSt or SUS. Enter the figure you have and the density from the SDS, and the tool fills in the rest.

Take it from the technical data sheet with its measurement temperature, because viscosity changes sharply with heat.

cP and mPa·s are dynamic viscosity; cSt, mm²/s, and SUS are kinematic, and density bridges the two groups.

Take it from SDS Section 9 at or near the viscosity temperature, since cSt = cP / SG and the density has to match.

Used only for the SUS row, which ASTM D2161 defines at 100 °F with a small correction at other temperatures.

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General guidance only. Verify against the cited primary sources before you commit to a shipment, declaration, or contract.

Dynamic, kinematic, and Saybolt viscosity

Dynamic viscosity is the resistance to shear, reported in cP or mPa·s, and kinematic viscosity is that resistance divided by density, reported in cSt or mm²/s. NIST SP 811 Appendix B fixes the units exactly: 1 cP is 10⁻³ Pa·s and 1 cSt is 10⁻⁶ m²/s, so cP equals mPa·s and cSt equals mm²/s with no factor at all. With density in g/mL, cSt = cP / SG.

Saybolt Universal Seconds are the time a sample takes to drain through a standard orifice, and ASTM D2161 is the standard practice for converting kinematic viscosity into them. At 100 °F the equation is SUS = 4.6324ν + (1.0 + 0.03264ν) / ((3930.2 + 262.7ν + 23.97ν² + 1.646ν³) × 10⁻⁵), and at any other temperature t in °F the result is multiplied by 1 + 0.000061 (t − 100).

To go from SUS back to cSt the tool solves the same equation numerically, since it has no closed inverse. We checked the equation against a published D2161 conversion table: 4, 10, 20, and 50 cSt come out at 39.2, 58.8, 97.8, and 232.6 SUS at 100 °F, and 10 cSt reads 59.2 SUS at 210 °F.

Worked example: a 120 cP oil and a pump rated in SUS

The booking. A transfer pump is rated for liquids up to 600 SUS at 100 °F, and the supplier's data sheet lists the oil blend you want to move at 120 cP at 100 °F. Treating cP and cSt as the same number, 120 cSt converts to 556 SUS, under the rating, so the order goes in and looks fine on paper.

The failure. The blend has an SDS relative density of 0.85, so its kinematic viscosity is 120 / 0.85 = 141.2 cSt, which the D2161 equation turns into 654 SUS. That is 9 percent over what the pump is rated for, and the pump runs slow, hot, and short of flow. The decision in front of you is plain: change the grade, or change the pump.

The fix. Run the density through the tool before either order goes out. A grade the supplier lists at 100 cP gives 100 / 0.85 = 117.6 cSt and 545 SUS, inside the 600 SUS rating, and the transfer runs at the flow the pump was bought for.

Density on the SDS given in lb/gal or kg/m³ rather than SG? The specific gravity converter turns it into the figure this tool needs at /tools/chemicals/specific-gravity-converter.

Density itself shifts with temperature, and the cP to cSt step needs it at the viscosity temperature. The density at temperature calculator corrects it at /tools/chemicals/density-at-temperature-calculator.

Frequently asked

How do I convert cP to cSt?

Divide by the density in g/mL: cSt = cP / SG. Centipoise measures dynamic viscosity and centistokes measure kinematic viscosity, and density is the link between them. A liquid at 120 cP with SG 0.85 is 120 / 0.85 = 141.2 cSt, while in water near SG 1.00 the two numbers are almost the same.

Is cP the same as mPa·s, and cSt the same as mm²/s?

Yes, each pair is exactly equal. NIST SP 811 Appendix B lists 1 centipoise as 1.0 × 10⁻³ Pa·s, which is 1 mPa·s, and 1 centistokes as 1.0 × 10⁻⁶ m²/s, which is 1 mm²/s. Technical data sheets use the old and SI names interchangeably, so no conversion is needed between each pair.

How do I convert cSt to SUS?

Use the ASTM D2161 equation, not a flat multiplier. At 100 °F, SUS = 4.6324ν + (1.0 + 0.03264ν) / ((3930.2 + 262.7ν + 23.97ν² + 1.646ν³) × 10⁻⁵), with ν in cSt. At another temperature t in °F, multiply that result by 1 + 0.000061 (t − 100). For thick liquids the first term dominates and SUS works out close to 4.63 times the cSt, so 100 cSt at 100 °F is 463.5 SUS.

Can I convert a viscosity at one temperature to another temperature?

Not with this tool, and not without data on the liquid itself. The SUS temperature term in ASTM D2161 only converts cSt measured at a temperature into SUS at that same temperature. To move a viscosity between temperatures you need the supplier's viscosity curve or two measured points, which a good technical data sheet provides.

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