Kinematic Viscosity Converter

Kinematic viscosity describes how readily a fluid flows under its own weight. It’s the fluid’s dynamic viscosity divided by its density, which is why the units work out to area per time, m²/s in SI. Where dynamic viscosity answers “how hard is this to stir,” kinematic viscosity answers “how fast will it run downhill.”

Key conversions:

• 1 m²/s = 1,000,000 mm²/s
• 1 stokes (St) = 0.0001 m²/s = 100 mm²/s
• 1 centistokes (cSt) = 0.000001 m²/s = 1 mm²/s

Where common fluids land:

• Water at 20°C: about 1 cSt
• Motor oil (SAE 30): about 100 cSt
• Honey: 2,000-10,000 cSt

The stokes and centistokes come from the cgs system, named after George Stokes, and the centistokes survives mostly because water sits at almost exactly 1 cSt. Industrial oil grading lives here too: the ISO VG number on a lubricant is just its kinematic viscosity in cSt at 40°C.

The division by density is the part that trips people. Two fluids can share the same dynamic viscosity yet have very different kinematic viscosity if their densities differ. Mercury is the classic example. Its dynamic viscosity is about 1.5 cP, half again as much as water, yet it is 13.5 times denser, so its kinematic viscosity works out to roughly 0.11 cSt, about a ninth of water’s. Pour the two and the mercury runs faster. Stir them and the mercury is stiffer. Always check which viscosity a spec is quoting before you compare two fluids.

The density box above exists for exactly that conversion: cP = cSt x specific gravity. Leave it at 1.0 and the two figures come out identical, which is true for water and misleading for everything else. A typical mineral oil sits near 0.88, so a 46 cSt hydraulic oil is about 40 cP. Glycerine is 1.26 the other way.

ISO viscosity grades, the numbers stamped on every drum of industrial lubricant, are simply kinematic viscosity in centistokes at 40°C, with a tolerance of plus or minus 10%:

One thing that grade does not tell you is how the oil behaves when hot. Viscosity falls steeply with temperature, and how steeply varies by base stock, which is what the viscosity index measures separately. Two oils can share an ISO VG number at 40°C and be nothing alike at 100°C.