I always find it easiest to draw a rough sketch to make sure I am considering all of my needs. The easiest way to determine your fabric needs is to draw the sections.
Note: All examples shown here will be using a 1/2" seam allowance. This example will have the skirt open at all four corners to fit under the bed legs.
The measurement examples are for gathering a medium weight fabric. You may want to allow more if your fabric is very lightweight or if you want more fullness.
- The Platform
The platform is hidden under the mattress and can be any fabric you have on hand or an old sheet cut to the size you need. You can also use left over width of fabric from the skirt fabric, pieced together to create the platform.
To calculate the size of the platform: Use the length of the frame or box spring by the width of the frame or box spring and add 1" to each measurement (1" equals 1/2" to each side if you are going to use a 1/2" seam allowance).
- The Long Sides: The height is the measurement from the floor to the top of the box spring or frame PLUS 1/2" for a seam allowance to attach the skirt to the platform and a hem allowance. The hem allowance is your choice. The type of fabric will guide you to deciding if a baby hem will be sufficient or if the hem needs more fabric to hang nicely.
The width is the length of the frame or box spring times 1.5 to allow enough fabric to gather, plus 1" for hemming the sides of the section.
- The End Sections The height is the same as the long side sections.
The width is the width of the frame or box spring times 1.5 to allow for gathering plus 1" to hem the sides of the section.
Using the height measurements, determine the width of the fabric you will need. For example, if the height measurement is 34" it would be a huge waste of fabric to buy 54" wide fabric, unless you want to use the cross wise grain and seam the fabric to create long enough sections of fabric. I prefer to use the length wise grain parallel to the floor.Everything You Need to Understand Fabric Grain and Grainlines
Add all of the width measurements to calculate the yardage you will need (inches divided by 36 to determine yards)
Tip: Consider flat sheets as a source of fabric. The wide measurements of sheets makes them an economical choice to create a custom bed-skirt... and you can usually find a color that will match your comforter. Tips to Use Sheets as Fabric
Always Pre-Shrink Your fabric (or sheets) before cutting out any parts.