i want to divide a square field into grids

Question

in my project user enters length and width of a field or farm in meter and i want to devide that farm into same sized grid but i dont know how to do it and i dont know the actual formula for that so cplease help me with that.

 

 

 

as per the above layout i want to devide farm into blocks using height and width given by the user so please help me.