Flow Cup Viscosity Converter

Convert flow-cup efflux time to kinematic viscosity and, when density is provided, dynamic viscosity, using the equations in ISO 2431:2011. Select the flow cup used for your measurement.

Active Conversion Equation — ISO 2431 Cup No. 4

ν = A·t − B/t  |  ν in mm²/s (cSt), t in seconds, 30 s ≤ t ≤ 100 s

Select ISO 2431 Flow Cup

No. 3
3 mm orifice
30 – 100 s
Low viscosity
No. 4
4 mm orifice
30 – 100 s
Most common
No. 5
5 mm orifice
30 – 100 s
Medium viscosity
No. 6
6 mm orifice
30 – 100 s
High viscosity
Valid range: 30 – 100 s for Cup No. 4
Optional — required for dynamic viscosity
Enter efflux time to check measurement validity

Kinematic Viscosity

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Dynamic Viscosity

Enter density

Practical Note

Select a flow cup that gives an efflux time between 30 and 100 seconds (ISO 2431 §7.2.1). Condition the sample and cup to 23.0 ± 0.5 °C before testing. This method is only reproducible for Newtonian or near-Newtonian liquids — if your material contains thixotropic or flow-arresting additives, treat the result as a consistency check, not a true viscosity, and verify with a rotational viscometer (ISO 3219).

Conversion equations are taken directly from ISO 2431:2011, Table 1 (ν = A·t − B/t, valid for 30 s ≤ t ≤ 100 s per cup).

Dynamic viscosity (cP) = Kinematic viscosity (cSt) × Density (g/cm³)

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