I'm making a changing mat that has 4x4 (2-7/8" ea) pinwheel blocks sewn around a 12-1/2 x 20-1/2 rectangle. I want to add in between a 1-1/2" strip. How much larger do I make the pinwheel blocks to accommodate this and how the h* do I figure it out?

When all else fails, cut the true size pieces out of construction paper, or even newspaper and put the quilt together like a paper puzzle. Have extra paper, scissors and tape on hand. Not only do you end up with a visual reproduction but with a pattern. Then ALL surprises (mistakes) will be with paper, not cloth. For those of us who can visualize better than we can work out math problems this elementary method works wonderfully well.

For this, and masses of other uses, I went to my local newspaper office where each morning they put out the paper ends not used.
They sell them for 5 dollars or so, depending on how much is left on each roll. I got one that should last me the rest of my life but school teacher DD wanted it. I had to go and get myself another one, it is so good to use for a lot of other things and it's pure white so no ink messes.

You could extend the 1 1/2 strip out into the pinwheels. The corner pinwheels would have the strip on each side. That way you won't need to re-figure anything.

Great idea on using a pattern. All these years of sewing and I didn't think of a pattern! honestly...

C.quilts - that's a good idea too and would give it a bit more character. Thanks!

Right now I'm ripping the pinwheels off. I think the colors are too dark which is why I added the strip so while I'm at it I'll change the colors.

I'm so disappointed I'm not even going to post a "before" pic. Actually, I won't even take a "before" pic!

When I have a problem of this type I always draw it out on graft paper and then cut templates for the pieces.
To me only advanced math students could figure it out and I'm not sure anyone could give a formulae without doing the paper work (drawing or cutting paper) first.

Agreed. Have to understand the question/problem before figuring out the answer.

Great minds at work here. I've just picked up a couple good ideas. Thank you.

I could also reduce the size of the center rectangle and make up the difference with the 1-1/2" strip! aha!

Another word. You might want to reconsider the 1.5" strip. If you are going to do additional rows of pinwheel blocks make the strip a width so the blocks can be in even inch increments. Off the top of my head a 1.5" might make them a very odd measurement and aggrivating to cut.