The Scherrer Equation to calculate the Crystallite size is given by

Crystallite size =  (k x λ) / (F. W. H. M. x cos θ)

and the Lattice Strain is given by:

Lattice Strain = F. W. H. M. / ( 4 x tan θ)


where
k is the Scherrer’s constant (Shape factor)
λ is the X-Ray Wavelength (Here input in nanometers)
F.W.H.M. is the Full Width at Half Maxima (Here input in degrees)
2-θ  is the Peak position in XRD graph (Here input in degrees)