The ratio of the radius of gyration of a thin uniform disc about an axis passing through its centre and normal to its plane to the radius of gyration of the disc about its diameter is : 

\(\sqrt{2}\) : 1

I₁ = MR² / 2

k₁ = \(\sqrt{\frac{I_1}{M}}\) = \(\frac{R}{sqrt{2}}\)

I2 = MR2 / 4

k2 = \(\sqrt{\frac{I_2}{M}}\) = \(\frac{R}{2}\)

k₁ : k₂ = \(\sqrt{2}\) : 1