I have a 12 pattern but need the finshed block to be 12.5
 

I need to increase the finished block to be 12.5 but my pattern's finished block is 12. What do I need to add to each individual peice to get it to end up to measure 12.5? 1/4 seams.

My first response is to clarify the 12.5 finished block..never heard of one that size before. If, in fact, it is 12.5 finished, it would be easier to give you ideas if we knew what the block was - can you provide a photo or give a name? with some blocks you can just add the extra half inch but to others you need some really small corrections to all the pieces.

It is impossible to answer that question without knowing which block it is.

Does a 1/2"on each block make that much difference? If you need the quilt a certain size you can put on wider borders.

blocks very seldom (finish) as a 1/2"....the 1/2" is the seam allowance before being joined with other blocks/sewn into the quilt...we would need to know the block to figure out the math for it- but (basically) to enlarge (or decrease) the size of a block you first subtract the seam allowance amount- then increase or decrease as needed- then add the seam allowance back in---an example...if you wanted to double the size of a block with pieces that are 2 1/2" to start- you would subtract the seam allowance (now 2") double it---to 4" then add the seam allowance back on- so cut 4 1/2".

I agree with ktbb, never heard of a block finished size 12 1/2 Are you sure that is what you are looking for.

It depends a lot on what block it is, how many seams and if there is a center element, etc. For example for a log cabin you could enlarge the center block 1/2 inch and then adjust the length, but not width, of the remaining logs. For a 9 patch I personally would enlarge each block by 1/8 inch and then fudge it so that you used a 12 3/8" block but that is probably cheating.
I've drafted blocks that were 12 3/8" finished for a special project but agree that 12 1/2" finished is an odd size.

I agree with the other posters. 12.5" block is a very odd size. Depending on the block itself, you might wind up adding 1/8" or 1/16" or 0.0023467" to the pieces, which would make it difficult to cut. That's why we would need to either see the block or have the name.
If you are going with the 12.5" size to fit a particular space or bed, I would consider adding an additional row of blocks, a border on the whole quilt or add a narrow sashing (1/2"), or increase the sashing than you planned by 1/2".

I have the book 501 Rotary Cut Quilt Blocks and there are several patterns in it that finish at 12 1/2, some 13 1/2 and even 12 1/4

Can you add sashing to make the block 12 1/2"? That would be easier if there are a lot of pieces in the block.
Sue