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Poiseuille is an SI-derived unit for absolute viscosity and dynamic viscosity, named after the French physicist Jean Léonard Marie Poiseuille (1797 – 1869). Its symbol is PI. (The second letter in the abbreviation is capital "eye" not lower case "ell".)

In practice the unit has never been widely accepted and some international standards bodies do not include the Poiseuille in their list of units. The third edition of the IUPAC Green Book, for example, lists Pa·s as the SI-unit for dynamic viscosity, and does not mention the Poiseuille.

The equivalent cgs unit, the poise, abbreviated, P, is most widely used when reporting viscosity measurements.

1 PI = 1 Pa ⋅ s = 1 N ⋅ s/m = 10 dyn ⋅ s/cm = 10 P