Consider a circular loop of wire of radius r carrying current I. To calculate the magnetic field at the center O. The entire loop can be divided into a large number of small current elements.
Consider a current element dl of the loop. According to Biot savart law, The magnetic files at the center O due to the element is
The magnetic field due to all such current elements will point into the plane of paper at centre O. Hence the total magentic field at the centre O is
