magzzzz2 :
all you have to do is calculate the fabric you need for one square of section of your quilt and multiply then add for borders.
multiply the size numbers for the pattern you HAVE
eg., 60 x 80 = 4800
multiply the size numbers for the size you WANT
eg., 90 x 108 = 9720
divide the second number by the first to get you factor
(what you WANT divided by what you HAVE)
eg., 9720/4800 = 2.025 if the factor
(If scaling up, the factor will be greater than one, if scaling down, it will be less than one.)
Multiply each yardage component by your factor
eg., pattern calls 3.5 yds of fabric A
3.5 x 2.025 = 7.0875
Round up to the nearest 1/4 yd for 7.25 yds of fabric A
If the project has smallish pieces, seam allowances can take up substantial fabric
In those cases, I add another 1/2 yard and get 7.75 yds.
(This approach doesn't work on bargello type quilts. Those things can be 50% seam allowance.
Mischele Hart says they're one big seam allowance.)
This formula is great for scaling borders up or down to keep them proportional. If the pattern called for 6" borders,
6 x 2.025 = 12.15. Round up to 12.25 for borders of a size that are the same proportion as the original quilt.
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