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The SI unit of volume is the cubic meter (m 3), which is a derived unit. Liter (L) is a special name for the cubic decimeter (dm 3). The symbol for the liter is uppercase letter “ell” (L) is preferred to avoid the risk of confusion between the lowercase letter “ell” (l) and the number one (1).
Now to answer this question whether litre is a SI unit or not. The answer is no litre is not an SI unit. The SI unit of volume is said to be cubic meters. It is defined as 1 d m 3 but in computations expressed in litres. But litre is commonly used unit to volume.
litre (plural litres) The metric unit of fluid measure, equal to one cubic decimetre. Symbol: L, l, or ℓ. You should be able to fill four cups with one litre of water. (informal) A measure of volume equivalent to a litre. The litre is not an SI unit but is accepted for use with SI units.
As a unit accepted for use with SI, the liter can also be used with SI prefixes. Examples include milliliters, deciliters, decaliters, kiloliters, and more. Adding a prefix to the base unit (liter) indicates a unit that is related to the liter by a power of 10. Using the examples mentioned: