How this instrument works
Dynamic viscosity, measured in poise, describes how much a fluid resists being sheared — thick honey pushes back harder than water does. Divide that resistance by the fluid's density and you get kinematic viscosity, the rate at which momentum spreads through the fluid on its own, independent of how heavy the fluid is. The formula is one line: ν = μ ⁄ ρ. In the CGS system the units line up exactly — a poise (g ⁄ cm·s) divided by g ⁄ cm³ leaves cm² ⁄ s, which is defined as one stokes.
Both units predate the SI system and carry the names of nineteenth-century fluid dynamicists: the poise honors Jean Léonard Marie Poiseuille, who measured how liquids move through narrow tubes, and the stokes honors George Gabriel Stokes, whose drag law describes a sphere settling through a viscous fluid. A viscometer often reports the dynamic figure directly, while pipe-flow and Reynolds-number work call for the kinematic one, so this conversion sits at exactly that handoff.
Because the conversion divides by density, two fluids with identical dynamic viscosity can land far apart in kinematic terms — a light hydraulic oil near 0.85 g ⁄ cm³ reads noticeably higher in stokes than water does for the same poise figure. Water is the deceptive case: at room temperature it sits near 1 centipoise and 1 g ⁄ cm³, so its two readings (in centipoise and centistokes) look almost identical — a coincidence of water's own density, not a rule that holds for any other liquid on a datasheet.
- Enter the fluid's dynamic viscosity in the Dynamic viscosity, poise field — read it straight off a viscometer, or convert from centipoise by dividing by 100.
- Enter the fluid's density in the Density, g/cm³ field; water is 1, and most lubricating oils fall between 0.80 and 0.95.
- The instrument divides the two automatically and reads out Kinematic viscosity, stokes with no further steps.
- For a lubricant certificate quoted in centistokes, multiply the stokes result by 100 before comparing the two figures.
Worked example — 1 poise fluid at 1 g/cm³ density
Take a fluid with a dynamic viscosity of exactly 1 poise and a density of exactly 1 g ⁄ cm³ — water's behavior scaled up a hundredfold from its everyday centipoise reading, kept in whole poise here for a round figure. The formula does the rest: ν = μ ⁄ ρ = 1 ⁄ 1 = 1.0 stokes. Nothing rounds away in that line; every value is exact.
This is why water sits at the top of every viscosity table: at room temperature it measures almost exactly 1 centipoise and 1 g ⁄ cm³, so its kinematic reading comes out to almost exactly 1 centistokes — a tidy coincidence the CGS unit pair was partly built around. Motor oil, syrup, or brine will not repeat that trick, because their densities pull the poise and stokes numbers apart in one direction or the other.
Questions
Why divide dynamic viscosity by density instead of using it directly?
Because dynamic viscosity (poise) measures the force needed to shear a fluid, while formulas such as the Reynolds number need the fluid's momentum diffusivity instead, which already has mass divided out. Dropping ρ from the equation leaves kinematic viscosity, ν = μ ⁄ ρ, in units of area over time — cm² ⁄ s, or stokes — rather than force over area over speed.
Is a stokes the same size as a centipoise?
No, and the names are easy to mix up. A poise, and its hundredth the centipoise, is a dynamic-viscosity unit; a stokes, and its hundredth the centistokes, is a kinematic-viscosity unit. The two only produce numerically close readings for fluids near water's density, 1 g ⁄ cm³, where dividing by ρ barely changes the figure.
Why do water's poise and stokes readings look identical?
Because water's density sits almost exactly at 1 g ⁄ cm³, and dividing by 1 does not change a number. At room temperature water measures about 1 centipoise dynamic viscosity and about 1 centistokes kinematic viscosity — but that near-equality traces back to water's own density and is not a general link between the two units.
How do centistokes relate to the stokes this instrument returns?
One stokes equals 100 centistokes, the same 1-to-100 pattern as poise to centipoise. Most industrial viscosity grades, including ISO 3448 and SAE gear-oil classes, are quoted in centistokes at a stated temperature, so multiply this instrument's stokes result by 100 before comparing it against a lubricant datasheet.
What happens if the density field is set to zero?
The instrument blocks it: dividing by zero density is undefined, so the field requires a value greater than zero before it returns a kinematic-viscosity figure. Every real fluid has positive density, so this check only fires if the field was left blank or mistyped.
Where does kinematic viscosity actually get used?
Chiefly in the Reynolds number, Re = vD ⁄ ν, which predicts whether flow through a pipe or around an object stays smooth (laminar) or turns chaotic (turbulent). Because that formula wants ν rather than μ, an engineer starting from a dynamic-viscosity viscometer reading routinely runs this exact division before the Reynolds-number step.