A square loop of side 2a, and carrying current I is kept in XZ...

Net force acting on wire $AB$ & $CD$ are zero

${F}_{1} = ILB = I \times \left(2a\right) \times \left(\dfrac{{\mu}_{0}I}{2\pi\sqrt{{a}^{2} + {b}^{2}}}\right) = \dfrac{{\mu}_{0}{I}^{2}a}{\pi\sqrt{{a}^{2} + {b}^{2}}}$

$τ = {F}_{1}cos \theta \times 2a = \dfrac{{\mu}_{0}{I}^{2}a}{\pi\sqrt{{a}^{2} + {b}^{2}}} \times \dfrac{b}{\sqrt{{a}^{2} + {b}^{2}}} \times 2a$

$τ = \dfrac{2{\mu}_{0}{I}^{2}{a}^{2}b}{\pi\left({a}^{2} + {b}^{2}\right)} \approx \dfrac{2{\mu}_{0}{I}^{2}{a}^{2}}{\pib}, \left(a ≪ b\right)$