I am making a quilt, I want to feature a pc of fabric 18" x 20 " in the center and have pinwheels going around this, then a border and then binding, my question is do you leave the center fabric whole (its Motocycles) or do you make squares the same size as the pin wheels, it is a baby girl quilt? Hope this makes sense.
Any suggestions?

you could certainly leave the piece whole and have the boarders be pinwheels and sashing. Could also do piano keys boarder, then pinwheels then outer boarder. show a picture when you finish.

It depends on which math you want to do. You will want the pinwheel blocks to have a dimension that will be even with the sides of your center piece. Currently I don't think it will work because 18" and 20" do not have a same divisor unless you are going to do 2" pinwheel blocks. If you made it an 18" square center then you could use 6" pinwheel blocks on each side.

Does that make sense?

Good luck.

How big are your pinwheels? How big is the print on the piece of fabric? Can you capture a whole motorcycle if you cut the fabric down? How big is the repeat? Picture of your focus fabric with something that shows scale, like a ruler, would help a whole bunch with this type of question.

good point, OP would have to put a little sashing strips on her FQ to get it up to a nice evenly divisble size and add her necessary SA as well. Or she needs to cut it down a smidge.

oh, i can cut the center pc down, the pinwheels are 5-1/2, i haven't cut the center down, there is a 1/2 yd of fabric.there are 90 pin wheels,9 designs 10 of each one i can, i thought the sash could be about 6", again help