Kinematic Viscosity Converter
Same value in every unit
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Read the temperature before you read the number
A kinematic viscosity quoted without a temperature is close to useless. Oils thin sharply as they warm: a mid-range mineral oil typically falls by a factor of five to ten between 40 °C and 100 °C. So “46 cSt” only carries meaning as “46 cSt at 40 °C”. Converting the unit is the easy part; matching the reference temperature is what decides whether two figures are comparable at all.
cSt and mm²/s are the same number
One centistoke is exactly one square millimetre per second, so moving between those two entries changes the label and nothing else. The stoke is the CGS unit (1 St = 1 cm²/s = 100 cSt) and is too large for everyday fluids — water sits near 0.01 St, which is why the centi- prefix stuck. The SI base unit is larger still: 1 m²/s is one million cSt. The imperial entries are coarse too — 1 ft²/s is 92,903 cSt and 1 in²/s is 645.16 cSt, scales you meet in fluid-mechanics texts rather than on a lubricant datasheet.
What lubricant grades actually measure
ISO VG grades are kinematic viscosity at 40 °C: ISO VG 46 is nominally 46 mm²/s there, with a ±10 % band under ISO 3448. SAE engine-oil grades use 100 °C instead — the 40 in 10W-40 is a band of 12.5 to just under 16.3 mm²/s at 100 °C under SAE J300. The 10W is set by cold-temperature dynamic viscosity limits in mPa·s (cold-cranking simulator and pumping viscosity), so no kinematic table applies to it. Because the two systems are specified at different temperatures, ISO VG and SAE grades cannot be lined up directly.
Dynamic viscosity needs the density
Kinematic viscosity is dynamic viscosity divided by density, so no unit table can take you from cSt to cP or Pa·s on its own. Given a density it is simple arithmetic: centipoise = centistokes × density in g/cm³. Engine oil usually sits near 0.85–0.9 g/cm³, but take the value from your own datasheet, at the same temperature as the viscosity.
Useful anchors:
- Water at 20 °C: about 1.00 mm²/s (1 cSt), density about 0.998 g/cm³
- Water at 100 °C: about 0.29 mm²/s
- Air at 20 °C and atmospheric pressure: about 15 mm²/s — roughly fifteen times water's kinematic viscosity, even though air is far less viscous in the dynamic sense, because its density is about 1/800 of water's
- ISO VG 46 hydraulic oil: 41.4 to 50.6 mm²/s at 40 °C, by definition of the grade
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Frequently Asked Questions
Yes — one centistoke is exactly one square millimetre per second, so the number never changes when you switch between them. cSt is the older CGS-derived label and still dominates lubricant datasheets, while mm²/s is the SI form used in ISO specifications. If one sheet quotes 46 cSt and another quotes 46 mm²/s at the same temperature, they are describing the same fluid.
It is a kinematic viscosity band at 100 °C set by SAE J300: an SAE 40 oil must sit between 12.5 and just under 16.3 mm²/s at that temperature. J300 also imposes a high-temperature high-shear minimum at 150 °C, but that is a dynamic figure in mPa·s. The 10W is governed by cold-temperature dynamic viscosity limits, also in mPa·s, from the cold-cranking simulator and the pumping-viscosity test — so a kinematic unit table cannot handle it. Only the second number belongs on this page.
Multiply by density. Dynamic viscosity equals kinematic viscosity times density, and in these units it is tidy: centipoise = centistokes × density in g/cm³. Water at 20 °C is about 1.00 cSt and about 0.998 g/cm³, so it works out very close to 1.0 cP. A unit converter cannot make this jump on its own, because it has no way of knowing your fluid or its temperature — and density itself shifts with temperature, so both figures must come from the same condition.
About 1.00 mm²/s (1 cSt) at 20 °C, which is the one figure in this field worth memorising because it anchors everything else. Near 100 °C water falls to roughly 0.29 mm²/s — a drop of about three and a half times across 80 °C. That is mild next to oils, where a mid-range grade commonly loses a factor of five to ten between 40 °C and 100 °C.
It means the oil's kinematic viscosity at 40 °C is nominally 46 mm²/s, with ISO 3448 allowing ±10 %, so 41.4 to 50.6 mm²/s. The grade number is defined only at 40 °C, and the same oil will be far thinner at 100 °C. This is why ISO VG grades and SAE engine-oil grades cannot be compared directly — they are specified at different temperatures.
No, and no single fixed factor would be correct. Saybolt Universal Seconds is an efflux time through a specific orifice, and its relationship to centistokes is non-linear; ASTM D2161 gives the equations and tables, both referenced to the same test temperature. Above roughly 75 cSt the ratio settles near 4.63 SUS per cSt, but below that it climbs — about 4.9 at 20 cSt and about 5.9 at 10 cSt — so use the published relation rather than a multiplier.