Things You'll Need
Furniture cushion foam
Framing square or straightedge
Electric carving knife
There are a number of "quick and easy" tricks to restoring the firmness of old couches and cushions, but most won't last long and never actually improve the comfort and support of the cushions. The fact is, a flattened, misshapen, or mushy cushion simply has worn-out foam, and the best way to bring back the cushion's firmness and comfort is by replacing the foam. This is easier and less expensive than you might think, and you'll be glad you spent the time to do the job properly -- every time you sit down on your couch.
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Measure the Cushion and Purchase Materials
Remove the foam inserts from cushions. The foam may be covered in synthetic or cotton batting -- typically a 1-inch or thinner padding -- and a layer of netting to keep the batting in place.
Remove the netting and batting from the foam, if applicable. Measure the width, length, and thickness of the foam. Be sure to measure the largest dimensions, to account for compressed or worn areas.
Measure the cushion cover if the foam is badly misshapen. Spread out the cover and measure between the seams of each sewn section. If the cover has welting cord or piping along the edges, measure between the insides of the welting. Add 1 inch to each dimension to find the final foam size. For example, if the cover measures 24 inches wide, 32 inches long, and 5 inches deep, your foam should be 25 by 33 by 6 inches.
Purchase new foam from a foam supplier or upholsterer. When choosing density and firmness, consult with the foam supplier, or follow this rule of thumb: for 4- to 5-inch-thick foam, choose 25/35 (density/firmness); for foam 6 to 8 inches thick, choose 22/30. Also buy enough 1-inch Dacron batting to cover all faces of each piece of foam after it's cut to size.
Cut the New Foam and Batting
Mark the foam to your measured dimensions, using a framing square or straightedge to ensure straight lines and square corners. Alternatively, place the old foam atop the new foam and trace around it to mark the cutting lines.
Cut the new foam piece to size using an electric carving knife. You can leave the foam flat on a work surface during cutting, provided you don't mind scratching the surface. Hold the knife straight throughout the cuts to create straight lines and vertical side edges.
Place the cut foam piece onto a strip of batting so their edges are flush. Wrap the batting all the way around the foam so it meets itself at one of the rear corners. Trim the batting to this length with scissors. Reserve all leftover batting for covering the two remaining sides of each foam piece.
Secure the Batting and Complete the Cushion
Fasten the batting to itself along the rear corner of the foam, using staples spaced about 4 inches apart. Exact spacing isn't important, but avoid using too many staples; the goal is to keep the batting from coming loose inside the cover.
Cover each side edge of the foam with batting and trim it slightly smaller than the foam surface. Staple the batting in place along one long edge, then stretch it slightly and staple the opposing edge before stapling it at the ends. Repeat with the other side edge of the foam.
Trim off any excess or loose bits of batting. Carefully insert the cushion into its cover, making sure it extends fully into the corners and that the edges of the foam are aligned with cover seams, and zip closed.
A sagging couch may be exacerbated by a worn support system under the cushions. If your cushions need more firmness after you've replaced the foam, you can add a "sofa saver" panel (available through home accessory retailers) or a piece of plywood under the cushions. For a more effective and permanent repair, have the understructure restored or replaced by an upholsterer or other furniture restoration professional.