Convert centipoise, pascal-second, stokes, and SAE grades for fluids and oils.
Every viscosity number on an oil bottle hides a fork in the road: cP (centipoise) is dynamic viscosity, how much force the fluid resists shear with; cSt (centistokes) is kinematic viscosity, how readily it flows on its own. The two coincide only at density 1 g/cm3, which is why water, about 1.002 cP and 1.004 cSt at 20 °C, looks identical on both scales. This tool converts across both families - cP, Pa·s, stokes, cSt - and maps a 100 °C kinematic value onto the SAE J300 engine-oil grade bands. What most people miss: cP-to-cSt needs density, and an SAE grade is a band plus cold-test criteria.
A concrete input and expected output from the current implementation.
Input
10 cSt at 100 °C, fluid density 0.90 g/cm3
Expected output
9 cP = 0.009 Pa·s = 0.09 P; 10 cSt = 0.1 St; SAE 30 at 100 °C (J300 band 9.3-12.5 cSt)
Kinematic viscosity times density gives dynamic viscosity: 10 x 0.90 = 9 cP, and 9 cP = 9 mPa·s = 0.009 Pa·s. A 100 °C kinematic value of 10 cSt lies inside the SAE 30 band (9.3 to 12.5 cSt), so the tool reports that single-grade classification.
Dynamic viscosity (cP, P, Pa·s) is the internal friction a fluid opposes when sheared; kinematic viscosity (cSt, St) is dynamic viscosity divided by density, measured from how fast fluid drains through a capillary tube. Within each family the factors are fixed - 1 Pa·s = 1000 cP = 10 P, and 1 St = 100 cSt - but moving between families needs a density you supply, applied as cSt = cP / density in g/cm3.
SAE J300 defines engine-oil grades by kinematic viscosity at 100 °C, with bands 5.6-9.3 (20), 9.3-12.5 (30), 12.5-16.3 (40), 16.3-21.9 (50) and 21.9-26.1 cSt (60). W grades (0W-25W) add only a 100 °C minimum (3.8-9.3 cSt); their real limits are low-temperature cranking and pumpability tests. The tool therefore reports the single grade band a number fits - it cannot certify a multigrade like 5W-30.
cP is dynamic viscosity - the force needed to shear the fluid - while cSt is kinematic viscosity, dynamic viscosity divided by density, which is what a gravity drain test measures. The numbers coincide only at density 1 g/cm3, which is why water's ~1.002 cP and ~1.004 cSt at 20 °C look interchangeable. For oil at 0.85-0.9 g/cm3, cSt runs 10-15% higher than cP.
Only into a range, and only at 100 °C. SAE 30 means 9.3-12.5 cSt at 100 °C, so the honest answer is a band, not a number, and you still need density to move from cSt to cP. Below 100 °C the SAE number says nothing about viscosity, and the W half of a multigrade depends on cold cranking and pumpability tests the tool cannot reproduce.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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