Memory foam provides a stiffer, more comfortable surface to sleep on than many other bedding materials. It's available in different thicknesses and widths for almost any application and can be cut to size with a variety of common tools. Getting a straight, clean cut is important, but cutting memory foam may invalidate your warranty. You also have to exercise caution when using scissors or knives for your DIY projects.
Get a Clean Edge
The most common issue when cutting foam is getting a clean edge. Use the wrong tool or get too aggressive with the right tool, and the foam becomes jagged and rough. Use the right tool with care, and memory foam cuts cleanly and smoothly.
Carving knives have serrated blades, as opposed to knives with smooth blades. The tiny points of the serrated tips are the key to cutting memory foam clean and straight. Use short, gentle back and forth motions to saw through the foam, and avoid compressing it.
Utility or craft knives, because they are surgically sharp, can be used to cut memory foam. The secret is to make several light passes. Instead of sawing through the foam as you would with a serrated blade, establish an initial shallow cut-line, and then deepen it until you've cut through the foam. Use caution when cutting foam with any type of knife or tool. Gloves, breathing and eye protection are advisable.
Thinner memory foam, such as a thin topper less than about 3/4 inch thick, can be cut with heavy-duty scissors. Use short chops, employing only about a 1/4 inch of the blades where they meet near the handles. If you make a mistake, or you're cutting odd shapes or forms, it's possible to glue foam back together. Use spray adhesive designed for foam. Spray both pieces, stick them together but be careful because they bond on contact. You won't even see the splice if it's done right.
In addition to knives or other cutters, tools such as a drywall square, straightedge, and a marking pens help you get lines and cuts accurate.
Drywall squares are ideal for cutting long, straight pieces of foam. Measure and place the implement on the foam, squaring it. Place your knee on the square to it to keep it steady. Use the side of the square as a guide, sliding a utility knife or carving knife along the edge for a nice, clean cut.
Plywood or Hardboard
If you don't have a drywall square, use a strip of 1/4-by-3-inch-wide hardboard or plywood as a straightedge. Place your hand or knee on it, and slide the cutter along the edge.
Freehand cutting is also an option, and depending on the application, such as angled cuts or odd shapes, can be easier than using a guide. Use whatever straightedge or template you have available to draw a black line on the foam, and follow the line freehand to cut foam.
Secure If Needed
Large, or mattress-sized, pieces of memory foam typically have enough friction to keep them in place when cutting. For smaller pieces -- or when large pieces slip around -- it's fine to place strips of plywood or other heavy objects along the edges, or use an assistant to help you hold it in place when cutting it. Add clamps to the strips if desired.