When sofa seat cushions begin to sag and droop, replacing the foam inside will refresh the look from old to new. Sofa cushions are made of springs or a board and springs wrapped with foam. Some foam cushions are also wrapped with down to create a different look and feel.
Measure the length, width and depth of the cushion.
Remove the cushion fabric from around the seat. Many sofas' cushion fabric can be unzipped and peeled off. Others may need to be dunthreaded to remove the cushion guts. Use the thread puller to unthread one side of the cushion. That will allow removing the cushion foam.
Disassemble the cushion guts. If the foam is wrapped with down, remove the down wrapping. Unwrap the foam from around the board or springs.
Measure the foam length, width and depth. Be careful to measure the thickest depth of the foam. The sagging side of the foam is compressed and will not provide accurate measurements. Foam may be designed to fold around the springboard or be two large pieces of foam. Take note of how the cushion was assembled.
Obtain new foam from an upholstery supplier in the measurements of the old foam.
Reassemble the cushion. Wrap the foam around the spring or board. Stretch the down wrapper around the foam if necessary. Pull the fabric back on over the cushion. Zip the cushion or sew up the unthreaded side of the cushion.