Originally Posted by LyndaOH
The formula is to take the seam allowances off the original size of the pieces and then multiply those sizes by the amount you want the percentage of the original piece you want it. So if you have a pattern that is 100 x 100 and you want it 80 x 80, you want it to be 80% of the original size.
The problem is that these sizes don't always convert to nice even numbers. Ideally you'd try to find measurements that are easy to deal with.
Oops, one step was left off the conversion above. You need to add the saem allowance back on.
It's a three step process:
step 1 subtract 1/2 inch from all dimensions to remove the seam allowance. This accounts for a quarter inch seam on both sides.
Step 2 multiply the dimensions from step 1 with the percent you want to reduce your piece by
Step 3 add the 1/2 inch seam allowance back on
So if you had a rectangle that was supposed to be cut 2.5 X 8.5 and you wanted to make the item 75% of the original size,
Step 1 remove seam allowances, so your 2.5 X 8.5 piece now becomes 2 X 8
Step 2 multiply each dimension by .75 so now you get 1.5 X 6
Step 3 add seam allowance back on, so the final measurement would be 2 X 6.5
Of course, the above example was a nice easy rectangle, and when you applied the percent, a nice round number resulted. This isn't always the case. So do all three steps and then round up to the nearest eighth. Odd shaped pieces, like trapezoids may need to be redrawn with a pencil and paper.