This calculator will help you to compute the V Parameter/Number (also known as Normalized Frequency) of A Step-index Optical Fiber.
V$_{cutoff}$=2.405
$V$= $\frac{2 \pi}{\lambda}$aNA
$\lambda_{cutoff}$=$\frac{2 \pi a NA}{V_{cutoff}}$
Wavelength of signal $\lambda$; Core radius of the Fiber a; Numerical aperture NA; V parameter; Cutoff Wavelength $\lambda$;