Since we typically pronounce the unit of length "mm" as a single word (millimeter), it takes the article "a".

Since we typically pronounce the unit of length "mm" as a single word (millimeter), it takes the article "a". The rule for using a or an is based on pronunciation, not spelling, so we say "a mm".

A mm or millimeter is a metric unit used to measure the length of small objects.

Remember, we put "a" before words that begin with a consonant sound, including mm or millimeter.

What is a mm on a ruler?

Similarly, we say a mL (milliliter).

A mL of liquid is a very tiny quantity.