The mutual inductance between two planar concentric rings of radii r1 and r2(with r1>r2 ) placed in air is given by
μ0 π r222r1
μ0π r122r2
μ0πr1+r222r1
μ0π r1+r222 r2
μ0i2r1xπ r22 =ϕ = Mi M=μ02r1xπ r22