This calculator will help you to compute the Mode-field diameter of the fundamental mode in a step-index fiber.
$M F D$= $2a(0.65 + \frac{1.619}{V^{3/2}}+\frac{2.879}{V^6})$
$V$= $\frac{2 \pi}{\lambda} a$NA
Wavelength of signal $\lambda$; Core radius of the Fiber a; Numerical aperture NA;