Viscosity Converter
Convert centipoise, pascal seconds, centistokes and stokes — handling dynamic and kinematic viscosity as the two different quantities they are, bridged only by density.
Viscosity Converter
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What this result does not account for
- Crossing between dynamic and kinematic viscosity requires a density and is refused without one.
- Viscosity is strongly temperature dependent; no figure means anything without its temperature.
- Non-Newtonian fluids are not described by a single viscosity value at all.
- Density itself varies with temperature, so the bridge should use density at the same temperature as the viscosity.
In short: Viscosity comes in two kinds. Dynamic (Pa·s, poise) is resistance to shear. Kinematic (m²/s, stokes) is that divided by density: ν = μ / ρ. Crossing between them requires a density — no factor exists. Two exact identities help: 1 cP = 1 mPa·s and 1 cSt = 1 mm²/s. Air has the lowest dynamic viscosity here but 15× water's kinematic, because it is barely dense at all.
Formula
Dynamic viscosity μ is measured in pascal seconds and kinematic viscosity ν in square metres per second. Within each family the conversion is a simple factor; between them it is a division by density, which is why this converter asks for one rather than pretending a factor exists.
Worked Example
- Identify the quantity. Centipoise is dynamic; centistokes is kinematic.
- Convert within the family if you can. 1 cP × 0.001 = 0.001 Pa·s.
- If crossing, get a density. Water at 20°C is about 998.2 kg/m³.
- Divide. 0.0010016 ÷ 998.2 = 1.0034×10-6 m²/s.
- Read it in shop units. That is 1.0034 cSt — near but not equal to the 1.0016 cP it came from.
Air is the case that settles it. Air at 20°C has a dynamic viscosity of about 0.018 cP, the lowest of any common fluid, yet its kinematic viscosity is roughly 15 cSt — fifteen times that of water. Nothing about air is thick; it is simply 830 times less dense, and dividing a small number by a very small number gives a large one. Anyone treating kinematic viscosity as a rescaled dynamic viscosity will get this exactly backwards.
Strengths & Limits Of This Model
Where this engine is strong
- Treats dynamic and kinematic viscosity as genuinely separate quantities.
- Refuses an impossible conversion instead of inventing a factor.
- Reports both families at once whenever a density makes it valid.
- States the exact cP to mPa·s and cSt to mm²/s identities.
Where it stops
- Cannot supply the density or the temperature for you.
- Says nothing about shear-rate dependence in non-Newtonian fluids.
Practical Use Cases
Lubricant selection
ISO VG grades are kinematic viscosity in centistokes at 40°C, while many datasheets give dynamic viscosity in centipoise. Comparing them needs the oil's density at the same temperature.
Coatings, inks and adhesives
Paint and ink specifications are usually dynamic, in centipoise, because what matters is the force needed to move the fluid through a nozzle or under a blade.
Pipeline and pump hydraulics
Reynolds number calculations use kinematic viscosity, so a dynamic figure from a datasheet must be divided by density first — see the Flow Rate Converter for the volumetric side.
Legacy and imperial documentation
The reyn and pound-force second per square foot appear in older American bearing and tribology literature and need translating into SI.
Methodology & Editorial Standards
This converter treats dynamic and kinematic viscosity as the separate physical quantities they are. Dynamic viscosity routes through the pascal second, kinematic viscosity through the square metre per second, and conversion between the two families is performed only when a positive fluid density is supplied, using ν = μ / ρ. Requesting a cross-family conversion without a density returns an explanation rather than a number, because no factor can bridge them: the same 100 cP is 100 cSt in water but about 111 cSt in a light oil. Two identities are exact rather than approximate, one centipoise being precisely one millipascal second and one centistokes precisely one square millimetre per second. Imperial units are derived from exact definitions, so the reyn is the exact pound-force divided by the square of the exact inch. Reference fluid comparisons are orientation only, and note that highly viscous materials are often non-Newtonian, meaning a single viscosity figure does not fully describe them. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Viscosity Converter — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity measures a fluid's internal resistance to shear when a force is applied, in pascal seconds or centipoise. Kinematic viscosity is that value divided by density, in square metres per second or centistokes, and describes how the fluid flows under gravity alone. They answer different questions and are related only through density.
How do I convert centipoise to centistokes?
Divide by the specific gravity, or equivalently divide the dynamic viscosity in pascal seconds by the density in kilograms per cubic metre. So 30 cP in a fluid of specific gravity 0.85 is 35.29 cSt. Without knowing the density the conversion is impossible — there is no universal factor.
Is 1 cP the same as 1 cSt?
Only for a fluid whose density is exactly 1 g/cm³, which water very nearly is. Water at 20°C is about 1.0016 cP and 1.0034 cSt, close enough that the two are constantly conflated. For any other fluid they diverge in proportion to how far its density sits from that of water.
Why does air have a high kinematic viscosity?
Because kinematic viscosity divides by density, and air is about 830 times less dense than water. Air's dynamic viscosity is tiny, roughly 0.018 cP, but dividing by such a small density gives about 15 cSt, fifteen times water's. It is the clearest demonstration that the two quantities are not interchangeable.
Is 1 cP exactly 1 mPa·s?
Yes, exactly. The poise is 0.1 pascal seconds by definition, so a centipoise is 0.001 pascal seconds, which is one millipascal second. Likewise one centistokes is exactly one square millimetre per second. Both identities are definitional, not approximations.
What is a reyn?
The imperial unit of dynamic viscosity, one pound-force second per square inch, equal to 6,894.757 pascal seconds. It is an enormous unit, so tribology literature normally uses microreyns. It appears mainly in older American bearing design texts.
What do ISO VG grades mean?
ISO viscosity grades state the nominal kinematic viscosity in centistokes at 40°C. An ISO VG 46 oil has about 46 cSt at that temperature, within a permitted band. Because the grade is kinematic, comparing it with a dynamic figure from a datasheet requires the oil's density.
Does temperature affect viscosity?
Enormously, and far more than most unit differences. Water's viscosity roughly halves between 20 and 50°C, and oils change by orders of magnitude across their working range. Any viscosity figure is meaningless without the temperature at which it was measured.
What is a non-Newtonian fluid?
One whose viscosity changes with the shear rate applied, rather than staying constant. Ketchup, paint, blood and many polymer solutions behave this way. For these materials a single viscosity number does not describe the fluid, and conversion of that number, while arithmetically valid, carries limited meaning.
Which viscosity do I need for a Reynolds number?
Kinematic, if you use the form with velocity, length and ν. If you use the form with density, velocity, length and μ, you need dynamic. Both forms are equivalent because the density cancels; the mistake to avoid is mixing a dynamic value into the kinematic form.