how much is the value of magnetism at the centre of bar magnet ?

Hi,

The magnetic field at the centre of the poles of a bar magnet is strongest and the field at midpoint of a bar magnet is weakest. The strongest field is in the center of the N pole and center of the S pole. The weakest field is halfway along the length of the bar, in the center of the wider side.

 Hope this helps,

Good luck.