Ask how much room your quilting setup needs
Backing and batting usually extend beyond the quilt top so there is room to secure the layers and trim them after quilting. The amount depends on how the quilt will be basted and quilted. If you're sending it to a longarm quilter, get their size requirements before you buy or cut either layer.
Add overhang on both sides of each dimension
A 60 × 80-inch quilt top with 4 inches of overhang on every edge needs backing and batting at least 68 × 88 inches. The width grows by 8 inches, and so does the height. Four inches per edge is different from adding 4 inches to the overall size.
Use the controls that match the layer
The backing calculator uses the same overhang on all four edges, then works out the panel joins and yardage. The batting calculator lets you enter a separate allowance for each edge. If your backing needs unequal allowances, work out a larger rectangle that covers them before using the panel plan.
Check the final top, not the planned size
Measure the completed quilt top before cutting the other layers. Piecing, pressing and added borders can make it slightly different from the size on the original plan. Keeping a little planned overhang doesn't replace checking that both layers will cover the actual top.