Foam rubber is used to make cushions for chairs, mattress pads and various craft projects. The thickness of foam rubber ranges from less than an inch to more than 12 inches. The size of the piece you need and the thickness determines the method of cutting the foam rubber. Another factor in choosing the cutting method is the texture of the foam rubber. There are industrial type cutters for extensive foam rubber work, but for home use, a craft knife or serrated blade works well.
Measure the general size needed for the project, such as the measurements of a seat cushion or pillow. Due to the pliable nature of foam rubber, make the measurements slightly larger than the size you need. For special shapes, transfer the shape to a sheet of cardboard.
Mark the measurements on the foam rubber using a magic marker. For an accurate shape, make small dotted lines instead of drawing a straight line on the foam rubber. If you draw a line across the surface of the foam, the marker pulls the foam out of shape, distorting the accuracy of the shape.
Place a piece of cardboard on the foam rubber. Push down on the cardboard with a firm hand, slightly compressing the foam. The compression keeps the foam in place as the knife cuts.
Slice the foam rubber using a craft knife or utility knife. Make a shallow cut from one end to the other of the marked line. Increase the depth of the cut with another pass of the knife. Continue cutting with shallow cuts -- until you completely cut through.
Attempt a dry fitting with the foam rubber, once you have completed all the cuts. If the piece is too big, shave off a small section using the same technique. Check for size again, and repeat the shaving process -- until the foam rubber fits in the project.
Use scissors for thin pieces of foam rubber. For extremely thick pieces of foam rubber, use a serrated knife.