1"=25.4mm
Your seatpost should be 22.2mm.
Insert it into your seat tube and check the diameter of your seat tube.
It should be 1" or 25.4mm or both!!
If your seatclamp is used or it might even be a bit squashed then remove the bolt(s) and try wedging something in the gap to widen the clamp when you drop it down onto the seat tube. Once its seated then remove the wedge.
check that the top of the seat tube isn't splayed out due to someone trying to remove a post and bent the top edge of the tube with the split in it!
The next size up for seatclamps is 26.2mm (I think)