Poise to Stokes Converter
Convert between dynamic viscosity (poise, centipoise, Pa*s) and kinematic viscosity (stokes, centistokes, mm2/s) using the fluid's density. Enter any two of the three quantities and the third is calculated instantly. Switch units on each field to work in the system that suits you best.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity (also called absolute viscosity) measures how much force is needed to make a fluid flow at a given rate. It is the ratio of shear stress to the velocity gradient and has SI units of pascal-seconds (Pa*s). The CGS unit is the poise (P), where 1 P = 0.1 Pa*s. Kinematic viscosity measures how quickly a fluid flows under the influence of gravity - it is the dynamic viscosity divided by the fluid density. The SI unit is square metres per second (m2/s). The CGS unit is the stokes (St), where 1 St = 1 cm2/s = 1e-4 m2/s. In everyday use, centipoise (cP) and centistokes (cSt) are far more common than the base units. Water at 20 degrees C has a dynamic viscosity of about 1.002 cP and a kinematic viscosity of about 1.004 cSt.
The poise to stokes formula
The relationship between the three quantities is simple: nu = eta / rho where nu is kinematic viscosity (St), eta is dynamic viscosity (P), and rho is density (g/cm3). All three quantities use CGS units, so no extra conversion factors are needed. To go the other way: eta = rho x nu For example, a hydraulic oil with a dynamic viscosity of 46 cP (= 0.46 P) and a density of 0.870 g/cm3 has a kinematic viscosity of 0.46 / 0.870 = 0.529 St = 52.9 cSt. Noting the pattern makes the unit chain easy to remember: poise / (g/cm3) = stokes, and 1 cP / (g/cm3) = 1 cSt when density is 1 g/cm3.
Unit conversion quick-reference
Dynamic viscosity unit equivalences: - 1 poise (P) = 100 centipoise (cP) = 100 mPa*s = 0.1 Pa*s = 1 dyne-second/cm2 = 1 g/(cm*s) - 1 centipoise (cP) = 1 mPa*s = 0.01 P = 0.001 Pa*s Kinematic viscosity unit equivalences: - 1 stokes (St) = 100 centistokes (cSt) = 1 cm2/s = 100 mm2/s = 1e-4 m2/s - 1 centistokes (cSt) = 1 mm2/s = 0.01 St = 1e-6 m2/s Note that 1 cP of dynamic viscosity in a fluid of density exactly 1 g/cm3 gives exactly 1 cSt of kinematic viscosity. Water at 20 degrees C is close to this but not exact (density 0.9982 g/cm3, so 1.002 cP / 0.9982 = 1.004 cSt).
Why does temperature matter so much?
Viscosity changes dramatically with temperature. For most liquids, viscosity decreases as temperature rises - think of cold honey versus warm honey. The relationship is approximately exponential for simple liquids and is described by the Arrhenius equation or more accurately the Vogel-Fulcher-Tammann (VFT) equation. For gases, viscosity increases with temperature (the opposite of liquids), because faster-moving molecules collide more often and transfer momentum more effectively. When specifying viscosity, always state the temperature. Most laboratory and industrial standards measure at 20 degrees C, 25 degrees C, 40 degrees C, or 100 degrees C. Engine oil viscosity grades (SAE 10W-40, etc.) are defined at specific temperatures.
Viscosity of common fluids at approximately 20 degrees C
| Fluid | Density (g/cm3) | Dynamic viscosity (cP) | Kinematic viscosity (cSt) |
|---|---|---|---|
| Air | 0.0012 | 0.018 | 15.1 |
| Water | 0.998 | 1.002 | 1.004 |
| Ethanol | 0.789 | 1.20 | 1.52 |
| Diesel fuel | 0.840 | 3.5 | 4.2 |
| Motor oil (SAE 10W) | 0.870 | 65 | 74.7 |
| Motor oil (SAE 30) | 0.880 | 110 | 125 |
| Glycerine | 1.261 | 1412 | 1120 |
| Honey | 1.420 | 10000 | 7040 |
Representative values at 20 degrees C and 1 atm. Actual values vary with temperature and composition.
Frequently asked questions
Can I convert poise to stokes without knowing the density?
No. Dynamic viscosity (poise) and kinematic viscosity (stokes) are fundamentally different quantities linked by the equation nu = eta / rho. Without the fluid density you cannot convert between them. If you only want to convert between different dynamic viscosity units - say, poise to pascal-seconds - no density is needed, because those are all measures of the same quantity.
What is the density of water to use for water-based fluids?
Pure water at 20 degrees C has a density of 0.9982 g/cm3 (998.2 kg/m3). At 25 degrees C it is 0.9971 g/cm3, and at 4 degrees C it reaches its maximum of exactly 1.0000 g/cm3. For most engineering calculations with water at room temperature, using 1.0 g/cm3 introduces less than 0.2% error. Saline solutions and aqueous solutions of other solutes will have higher densities.
How do I convert centipoise (cP) to centistokes (cSt)?
Divide centipoise by the density in g/cm3: cSt = cP / rho (g/cm3). For a fluid with density 1 g/cm3 the two numbers are equal. For a fluid with density 0.85 g/cm3, 100 cP becomes 100 / 0.85 = 117.6 cSt. The calculator handles this automatically when you select cP and cSt as units.
What is a centistoke (cSt) and when is it used?
A centistoke (cSt) equals one square millimetre per second (mm2/s). It is the most widely used unit for kinematic viscosity in industry, especially for lubricating oils, hydraulic fluids and fuel oils. ISO viscosity grades (ISO VG 32, VG 46, VG 100, etc.) are defined by kinematic viscosity at 40 degrees C in cSt. Water at room temperature is about 1 cSt, light diesel around 3-5 cSt, and thick gear oils can exceed 1000 cSt.
Is 1 cP equal to 1 mPa*s?
Yes, exactly. One centipoise equals one millipascal-second: 1 cP = 1 mPa*s. This equivalence makes them interchangeable in practice. The older CGS unit (centipoise) and the SI-derived unit (millipascal-second) appear in different industries and textbooks, but they represent identical physical quantities.
How does kinematic viscosity relate to the ISO VG oil grades?
ISO Viscosity Grade (VG) numbers are the nominal kinematic viscosity in centistokes (cSt) at 40 degrees C, with each grade spanning +/-10% of the nominal value. ISO VG 46 oil, for example, has a kinematic viscosity between 41.4 and 50.6 cSt at 40 degrees C. Knowing the dynamic viscosity at a measured density lets you calculate the kinematic viscosity and identify the appropriate ISO VG grade.