Viscosity Converter

Viscosity comes up most often when you're switching between an oil supplier's datasheet (centipoise) and a CFD simulation (pascal-seconds), and the factor isn't memorable enough to do in your head reliably. Off by a factor of a thousand and your simulation models honey when you wanted water.

Enter a value in Pa·s, cP, or P and the other two update. The relationships are fixed: 1 Pa·s = 10 P = 1000 cP. The centipoise is convenient because water at room temperature has a viscosity of almost exactly 1 cP, which makes other fluids easy to mentally compare against (motor oil is roughly 250 cP, honey around 10,000).

Dynamic viscosity only — kinematic viscosity (in centistokes or m²/s) is a different quantity, related but not interchangeable, and divides dynamic viscosity by density.

What dynamic viscosity actually measures is shear stress per unit shear rate — the resistance a fluid offers when one layer slides over another. SI defines this in pascal-seconds (Pa·s), which is force per area times time. The CGS unit is the poise (P), and one centipoise (cP) is a hundredth of that. The factor of 1000 between Pa·s and cP comes from the unit chain (pascals are larger than dynes per cm² by 10, areas convert by 10⁴, time is the same), and although the math is mechanical, almost nobody can rederive it in their head reliably. Hence converters.

A worked example from a CFD setup: a paint datasheet lists viscosity as 350 cP at 25°C. Your simulation expects Pa·s. 350 cP = 0.350 Pa·s. Plug 0.350 into the kinematic-viscosity calculation (divide by density, get m²/s for the simulation), and the flow regime calculation comes out right. Off by a factor of 1000 and the simulation's Reynolds number is wrong by the same factor, which puts you in the wrong flow regime entirely — laminar where it's actually turbulent, or vice versa. The converter is a thirty-second sanity check that prevents three days of debugging.

Limits the converter doesn't address: viscosity is a function of temperature for nearly every fluid, and the relationship is steep. Water drops from about 1 cP at 20°C to about 0.55 cP at 50°C — nearly half. Honey at room temperature is around 10,000 cP; warm it to body temperature and it's a few hundred. If your datasheet's viscosity was measured at one temperature and your application runs at another, unit conversion alone won't make the numbers correct. You need a temperature-viscosity curve for the specific fluid.

Non-Newtonian fluids are a bigger gap. Ketchup, blood, polymer melts, and many slurries don't have a single viscosity value — their viscosity changes with shear rate. The converter handles whatever dynamic-viscosity number you provide, but for non-Newtonian fluids, picking the right shear rate to characterize the fluid is its own problem and a single number is misleading. The cP value on a ketchup datasheet, if there is one, is at a specific shear rate that may not match your application.