Ampere’s circuital law states that line integral of magnetic field forming a closed loop around the current(i) carrying wire, in the plane normal to the current, is equal to the μ_o times the net current passing through the close loop.

 Closed line integral of B.dl = μ₀i

 Here μ_o = permeability of free space = 4π×10^-15N/A²

 This is a simple tool for evaluating the magnetic field at a point due to  the current flowing.