Calculator
Centistokes converter
Enter a kinematic viscosity and its temperature. You get Saybolt Universal seconds, centipoise, the ISO VG window, and the approximate SAE grade. Two data-sheet points give the viscosity at any other temperature, and the viscosity index.
Charts of commercial oils are on oilviscositychart.com. Gear geometry is on gearcalculators.com.
Measured viscosity
Results appear here.
Viscosity at another temperature
Two points fix the ASTM D341 line. The first load uses the ISO VG 220 midpoint at 40°C and a VI 95 estimate at 100°C. Replace both with the data sheet.
The extrapolated viscosity and viscosity index appear here.
SUS by ASTM D2161. Centipoise from density. ISO VG by ISO 3448 at 40°C. SAE gear grades by SAE J306 (2019) and engine grades by SAE J300, both at 100°C. The temperature curve is ASTM D341. Viscosity index is ASTM D2270.
For a gearbox, start with how the viscosity gets chosen, then ask for a grade.
Reference
- cSt to SUSASTM D2161, including the temperature factor.
- cSt to centipoiseKinematic viscosity times density.
- ISO VG chartEvery ISO 3448 window at 40°C, plus a typical 100°C.
- ISO VG 220The grade at 40°C, and what it does when the sump heats up.
- ISO VG 320One step heavier, and when that step matters.
- ISO VG 460Slow gears, hot plants, and a lot of worm drives.
- SAE gear gradesSAE J306 against centistokes, and why SAE 90 is not an engine oil.
- Viscosity indexWhat ASTM D2270 is actually counting.
- Industrial gearbox oilThe AGMA 9005 pitch-line-velocity approach, in plain steps.
- Worm gear oilWhy a worm box wants a heavier oil, and often a different chemistry.
Questions
Is a centistoke the same as mm²/s?
Yes. 1 cSt = 1 mm²/s. Both are kinematic viscosity.
Does converting cSt to SUS also change the temperature?
No. ASTM D2161 changes the unit at the temperature you already have. A small factor accounts for the Saybolt tube at that temperature. To move the oil to a different temperature, use the two-point Walther calculation.
Why is there sometimes no ISO VG?
ISO 3448 defines each grade as a ±10% window at 40°C, and the windows do not touch. A viscosity can sit in a gap. Away from 40°C the grade is not defined at all, even when the number falls inside a window.
Is this a substitute for a viscosity measurement?
No. The page does arithmetic on the numbers you type, using the standards named above. A specification still wants a measurement by ASTM D445 or D7042.