Dynamic viscosity

cSt to centipoise

Centistokes measure how fast the oil flows under gravity. Centipoise measure the drag the oil offers to a moving part. The connection is the density at the same temperature.

μ (cP) = ν (cSt) × ρ (g/cm³)

That identity is exact in these units: 1 cSt = 1 mm²/s = 10⁻⁶ m²/s, and 1 g/cm³ = 1000 kg/m³, so the product is millipascal-seconds. One centipoise is one millipascal-second. ASTM D445 is the kinematic method. ASTM D7042 measures dynamic viscosity and density together and calculates the kinematic result.

Worked numbers

20 cSt at a density of 0.870 g/cm³ is 17.4 cP. The converter’s default density, 0.88 g/cm³, turns 220 cSt into 193.6 cP. Water near 20°C is about 1 cP and about 1 cSt because its density is about 1 g/cm³. A mineral oil is not water, so the two numbers diverge.

0.88 g/cm³ is a fair mineral-oil density near 15°C. Oil expands as it warms, so the density at 100°C is lower. Using the 15°C density at 100°C overstates the centipoise a little. For a film-thickness argument, use the density at the viscosity temperature, from the data sheet or from ASTM D1298 / D4052.

Which number a gearbox actually feels

ISO VG, SAE J306, and the Walther chart are all kinematic. The shear in a bearing or a gear mesh is a dynamic-viscosity problem, but the catalogs you are matching are written in cSt. Convert to centipoise when a pump curve, a damper, or a cold-cranking limit asks for cP or mPa·s. Leave the oil in cSt when you are picking an ISO VG or an SAE grade.

Run a value in the converter. Grade boundaries are on the ISO VG chart and the SAE gear-oil page.

Questions

Is centipoise the same as mPa·s?

Yes. 1 cP = 1 mPa·s = 0.001 Pa·s.

Why does the converter default to 0.88 g/cm³?

It is a typical density for a mineral lubricating oil around 15°C. Synthetics, synthetics with esters, and phosphate esters sit elsewhere. Type the data-sheet density over the default.