Viscosity Converter (Pa·s, cP, cSt, Stokes)
Convert dynamic viscosity — Pa·s, poise, centipoise, reyn — and kinematic viscosity — m²/s, stokes, centistokes — with a density field to cross over.
Pa·s
0.001
mPa·s
1
µPa·s
1,000
P (poise)
0.01
cP (centipoise)
1
lbf·s/ft²
2.0885e-5
lb/(ft·s)
0.00067197
reyn (lbf·s/in²)
1.4504e-7
1 cP is exactly 1 mPa·s and 1 cSt is exactly 1 mm²/s, so those rows always agree; the centi- units were sized so water at 20 °C reads near 1.
Cross to the other family
mm²/s
1.0017962
Defaults to water at 20 °C (998.207 kg/m³). Use the density of your own fluid at its working temperature — not a handbook value for 4 °C.
How to use
- Choose which quantity you actually have: dynamic viscosity in the pascal-second and poise family, or kinematic viscosity in the square-metre-per-second and stokes family. A datasheet that names a temperature and gives cSt means kinematic.
- Type the value and pick its unit. Every other unit in the same family updates at once, and each card copies the full-precision number rather than the rounded display.
- To move between the families, set the density of your fluid at the working temperature — the preset buttons cover water, air, engine oil and glycerol at 20 °C — and read the cross-conversion card below.
- Check the two identity rows as a sanity test: cP and mPa·s must match, as must cSt and mm²/s. If your source disagrees, it is quoting the other family.
About this tool
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Viscosity comes in two kinds that share a name and little else. Dynamic (absolute) viscosity μ measures resistance to shear directly and is quoted in pascal-seconds, or in the CGS poise that the lubricant trade never abandoned. Kinematic viscosity ν is that same figure divided by density, which makes it an area per time: square metres per second, or stokes. This converter keeps the two families on separate lists on purpose. No constant turns a centipoise into a centistokes, and a converter that offers one has quietly guessed at a density you were supposed to supply.
The gap between the two is larger than it sounds. Air is about 55 times less viscous than water in the dynamic sense — 18.25 µPa·s against 1.0016 mPa·s at 20 °C — and yet roughly 15 times more viscous in the kinematic sense, 15.16 mm²/s against 1.0034 mm²/s, because dividing by a density of 1.2 instead of 998 more than reverses the ranking. Pick the wrong family and the error is not a few percent; it can invert which of two fluids you call thicker.
Two pairs of rows always read identically, and that is history rather than a bug: the centipoise was sized so water at 20 °C comes out near 1, which makes 1 cP exactly 1 mPa·s and 1 cSt exactly 1 mm²/s. The imperial rows are derived from the defining constants rather than typed in as decimals, so lbf·s/ft² lands on 47.880259 Pa·s and the reyn — one pound-force second per square inch — on 6894.7573 Pa·s, the same number as one psi.
Two things this page deliberately cannot do. It has no temperature input, because viscosity falls steeply as a liquid warms and a figure with no temperature attached is not a measurement — an ISO VG 46 oil is 46 cSt at 40 °C within a ±10% band, 41.4 to 50.6, and nothing like that at 100 °C. And it cannot reach Saybolt Universal Seconds, which is an efflux time read off an empirical table rather than a unit with a conversion factor, and is not defined at all below about 1.8 cSt.
Frequently asked questions
How do I convert centipoise to centistokes?
Only through density: ν = μ / ρ. One centipoise is 0.001 Pa·s, so divide by the density in kg/m³ and read the answer in m²/s, then multiply by a million for mm²/s. For water at 20 °C, 1.0016 cP over 998.207 kg/m³ gives 1.0034 cSt — close to a one-to-one swap only because water is the fluid the units were built around. For an engine oil at 876 kg/m³ the same arithmetic scales by 1/0.876, and for air it scales by 1/0.0012.
Why is 1 cP the same as 1 mPa·s?
Because the poise is 0.1 Pa·s, so a hundredth of it is 0.001 Pa·s, which is a millipascal-second exactly. The coincidence is deliberate: Poiseuille-era workers chose a scale on which water at room temperature reads about 1. The same choice gives 1 cSt = 1 mm²/s, since a stokes is 10⁻⁴ m²/s.
Which is the bigger unit, a poise or a pascal-second?
The pascal-second, by a factor of ten — 1 P = 0.1 Pa·s. This trips people up because CGS units are usually the smaller ones and here the SI unit is the large one, so a figure in poise looks ten times more impressive than the same viscosity in SI. Glycerol at 20 °C is 1.412 Pa·s, which is 14.12 P, 1412 cP and about 1120 cSt.
What is a reyn, and why does it match psi?
A reyn is one pound-force second per square inch, the dynamic viscosity unit used in bearing and lubrication engineering. Because a second is dimensionless in the conversion, the numeric factor is exactly that of one psi in pascals: 6894.7572931683 Pa·s. It is a large unit — most oils sit in the micro-reyn range.
Can I enter Saybolt seconds or an SAE grade?
No, and that is not an omission. Saybolt Universal Seconds is the time a fixed volume takes to drain through a standard orifice; the relationship to centistokes is an empirical table with no closed form, and it is undefined below roughly 1.8 cSt. SAE and ISO VG grades are bands at a stated temperature rather than values, so a grade cannot be converted — look up the measured cSt for the specific oil first, then convert that.
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