That which is requisite (or required) is so in the judgment of the person requiring it, but may not be so absolutely; thus, the requisite is more a matter of personal feeling than the indispensable.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

needed Of course, a license is needed to operate a crane.
necessary Close observation is necessary for the experiment.
requisite I won't be able to go travelling until the requisite visa fees are paid.
mandatory English is a mandatory language in Czech schools.
compulsory In Algeria, it's not compulsory to wear the hijab.

Popularity over time

Word	1850 → 2019	Trend
required		falling
needed		rising
necessary		falling
requisite		falling
needful		steady
mandatory		steady

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: due¹
6 letters: needed²
7 letters: needful²
9 letters: necessary⁴, essential³, mandatory⁴, requisite³
10 letters: compulsory⁴, obligatory⁵

The small number after each word is its number of syllables (needed² = 2 syllables).