Should You Prewash Quilt Backing Fabric?

Start with the fabric maker’s care instructions and your quilter’s advice. If you decide to prewash, plan for changes in both length and width before cutting.

Make the washing decision for the whole quilt

Consider how you prepared the quilt top and how the finished quilt will be laundered. There is no single shrinkage percentage that fits every cotton fabric, finish, washing temperature, and drying method. A labeled width alone cannot tell you its usable width after washing.

Robert Kaufman’s fabric prewashing guidance recommends considering the intended laundering conditions, using mild detergent, and separating light and dark fabrics. Follow any more specific care instructions supplied with your fabric.

What the calculator’s percentage means

The optional “Prewash shrinkage” field adds a percentage to the planned purchase length. Entering 5 means base length × 1.05. It is an extra-yardage allowance, not a measurement of your fabric and not an exact compensation formula.

For the default throw plan, two 80-inch panels need 160 inches before this allowance. Adding 5% makes the shopping length 168 inches. The rounded purchase becomes 4¾ yards or 4.3 meters. The cut list still calls for two 80-inch panels because that is the size needed after preparation.

See the 5% allowance example

An allowance is different from exact compensation

If you know that fabric loses exactly 5% of its original length, the arithmetic for a 160-inch final length is 160 ÷ 0.95, or about 168.42 inches before washing. Simply adding 5% gives 168 inches. Those happen to round to the same eighth-yard purchase in this example, but that will not always be true.

Use a measured fabric-specific estimate when exact compensation matters, then allow for squaring and the shop’s selling increments. The calculator intentionally follows the simpler percentage-addition method shown beside its input.

Do not overlook width shrinkage

The percentage field does not reduce usable width. Edit that field separately to the width you expect after washing. With ½-inch seam allowances, two 40-inch panels cover 79 inches; two 39-inch panels cover 77 inches. A planned backing width of 78 inches fits the first pair but needs another panel with the second pair.

If your fabric is already washed, pressed, and measured, enter that actual usable width. Leave the percentage at zero unless you intentionally want another purchase allowance. Before cutting, recheck both dimensions and make sure uneven ends, print matching, and trimming are covered.