I am working on a Celtic Knot quilt that has 2 colors per 15.5" block on a white background. I plan on a 2 1/2 inch sashing around the blocks.... are there any set guideline for how wide to make the boarders that will go around this center panel?

some one told me once that borders should be in proportion to the block size...so, if a 12" block borders should be 3", 4" or 6" to look proportionate. i'm thinking for a 15" block anything smaller than 3" would look too small, so i would use either 3" or 5" borders. but remember there are no set in stone rules, use what looks best to you.

I'm putting 12" blocks together with a 1" border so there are no set rules. Use what you like for the look you are going for.
Very nice blocks.

1 or 2 inch sashings. around the outside of the blocks too. then a 6 inch or more border.

I try to use a number that the blocks are divisible by for the sashing & borders. For 15" blocks, I'd use a 1.5 or 3 inch sashing & 3 &/or 5" borders.

Love the blocks by the way!

sounds like this will be an inner border with other blocks or borders around it...Why not make the first border the same size as the sashing, then increase the other borders?