Before you begin cutting and piecing a king-sized (and any size) quilt top, you must calculate yardage. If you skip this important step, you may not have enough fabric to finish your quilted design. Calculating yardage does take a little math, but the few minutes you'll spend crunching numbers will ensure that later you won't have to search for more fabric to complete your quilt top.
Choose a quilt pattern or design your own quilt pattern. You can't begin to calculate yardage for your king quilt until you know what design you intend to use.
Determine your finished quilt's total size. A standard king quilt measures about 107 inches in length and 108 inches in width. However, if you want the quilt to hang over the sides of the bed, add a few inches to the quilt width, and if you want to tuck the quilt under your pillow when making the bed, add a few inches to the quilt length.
Figure out how many quilt blocks you need to make in order to make a quilt in the desired size. For example, if your pattern uses 12-inch blocks, your quilt must measure nine blocks across and nine blocks long in order to approximate a standard king quilt size.
Reference your quilt pattern's supplies list to determine the number of fabric units you need for each of the quilt block's colors and how big each fabric unit needs to measure.
Multiply the number of fabric units needed for each quilt block by the number of blocks in the quilt. For a standard king with 12-inch blocks, you will multiply the fabric units needed for each quilt block by 81. Calculate the number of units needed one color at a time, and note your totals.
Determine how many fabric units you can fit into a yard of fabric. Calculating one color at a time, and working under the assumption that your fabric will measure 42 inches across (this is the width of a standard fabric yard minus an inch at each end), divide 42 by the width required for each fabric unit. Round down to the nearest whole number. This is the number of fabric units you can fit across a single yard. Repeat for each color.
Divide the total number of a single color's units necessary to complete the quilt top by the number of units you can fit across each yard, once again calculating for one color at a time. This will give you the number of strips you must cut across the length of your fabric.
Multiply the number of strips you must cut by the height of the fabric unit. This will tell you how many yards you need for each color. For example, if you need 20 strips of fabric that measure 4 inches high, you need 80 inches of fabric. Repeat this process for every color.
Divide the number of inches of fabric you need for each color by 36 to determine how many yards you must purchase in each color.
Even if you're certain you'll have enough fabric to complete your quilt design, get a little more than you think you'll need. This will ensure that you have enough fabric to make up for any margin of error.
Check your numbers against an online yardage calculator (see Resources) to make sure your math is correct.