Percentiles

Calculate the $p \in (0,1)$ percentile based on $L_p = p(n+1)$:

=PERCENTILE.EXC(data,p)

Note that Excel also provides =PERCENTILE.INC(data,p) where $p \in [0,1]$ (i.e. the 0 or 1 are valid values for $p$.)

Quartiles

Calculate the $k \in [1,2,3,4]$ quartile:

=QUARTILE.EXC(data,k)

Demo