The effect of magnetic shielding effectiveness has been investigated for apertures on metallic enclosure and has been calculated as a function of enclosure dimensions, aperture dimensions position within the enclosure. The calculation magnetic shielding depends upon the frequency and polarization of the applied field, the dimensions of the enclosure and the apertures, the number of apertures, and the position within the enclosure. Analytical formulation confirms that long thin apertures are worse than round or square apertures of the same area. For a typical sized enclosure, the theory predicts that doubling the length of a slot reduces magnetic shielding effectiveness and by about 12 dB, while doubling the width only reduces magnetic shielding effectiveness and by about 2 dB. Calculations using the new formulation show that doubling the number of apertures reduces magnetic shielding effectiveness and by about 6 dB. However, dividing a long slot into two shorter ones increases magnetic shielding effectiveness and by about 6 dB.