Calculation of average crystallite size:

By subtracting, we find

But θ₁ and θ₂ are nearly equal to θ_B , so that

where k is a constant representing shape factor. (k is 0.89 for spherical, 0.94 for cubic and 0.9 for unknown size particles).
This equation is known as Scherrer's formula for calculating the average crystallite size using XRD peaks. This equation not applicable for grains larger than 0.1 - 0.2 μm.