stripline length

Hi guys

im designing hairpin filter. one of the book that i have refered written the length of the quaterwave rsonators as:
The length of the hairpin resonators is found to be 860 mils by
using the effective dielectric constant, ε
eff = 1.88, for 30-mils wide lines.

what is the formula to calculate the length of the haipin resonator?

thanks guys
-danesh-

Hiar pin resonators are two lambda/4 line length connected in series forms lambda/2.
so it is caluclated by using lambda/(2*sqrt(1.88 ))