How to Cut the Foam for Your Cover
There is no denying foam is a versatile material with tons of applications. If you are working with polyurethane foam for the first time in creating a custom project, you may need a little help. There are a few things to know about how to cut polyurethane foam. Let's take a close look in detail at how to cut foam cushion the right away for the best end result.
Supplies Needed to Complete This Task
Before you learn the best way to cut polyurethane foam, you need to gather the right supplies for the task. You will need the sheet of foam you ordered, the modified cushion measurements made during the How to Measure step, a tape measure, a long straight edge like a yardstick and a sharp carving knife or electric knife to cut your custom foam cushions. To make complete cuts, make sure the blade of your cutting tool is longer than the foam is thick.
Plotting Your Cushions
One of the most important elements is to make sure you have straight lines. Before we get into how to cut foam straight, you should plot the outline the right way. Using the adjusted measurements you created during the How to Measure step, plot out the dimensions of a square or rectangle on your foam that represents the greatest length and width of each cushion. This can be done easily by cutting out sections of newspaper to the cushion's greatest dimensions. Putting these templates on the foam will give you an easy way to visually assess how much room to leave between cushions without running out of room. It will also ensure you orient your cushion in the right way; cushions may fit on the foam side-by-side, but they may not if turned and stacked end-over-end. When these are situated, make a small mark on the edge of the foam to mark the rough boundary for each cushion.
Marking the Foam - Part One
With the modified measurements you recorded during the How to Measure step, begin to mark the shape of your foam within its designated boundary area. When possible, use the outer edges of the foam sheet as one of the edges for your cushion to reduce the number of cuts that need to be made. For square or rectangular cushions, this means you can have a cushion with two sides already squarely machine-cut. Do not plot out cushions to share a cutting edge. This strategy may reduce the number of cuts you have to make, but a single mistake can ruin both cushions, while leaving buffer room around each can make a mistake salvageable.
With the edge of the foam facing you serving as the cushion's rear edge, begin to mark the cushion's dimensions using a straight edge, as seen to the right. Make tick marks with a marker, spaced apart at a distance you feel comfortable with. Do not draw complete lines just yet, in case a mistake in measurement was made.
Marking the Foam - Part Two
Check that the marks you made are straight, within each cushion's boundary area, and that they match your measurements. When these things are confirmed, connect the dots using the straight edge. Be careful to not tear into the foam with the marker tip. These lines will be what you cut along as you form the cushion.
Cutting the Foam
Before you cut foam, always make sure your work surface is flat, stable and won't be damaged by cutting through the foam with the knife. If you choose to use a carving knife, use a gentle back-and-forth sawing motion and let the blade work through the sheet. 
Do not compress the foam and try to make quick cuts through the condensed foam. A softer foam will compress more easily during cutting. Minimizing pressure during cutting will result in cleaner edges. Medium and firm foams can resist pressure slightly better during cutting, but this is not a reason to apply extra force. The less the foam is compressed during cutting, the more control the user has over making straight cuts and flush edges.
The picture to the right illustrates the ease with which a sharp blade can cut the foam without compressing it. If you use an electric knife, do not force the blade through the foam. The slower you pass the knife through the foam, the more control you maintain. A good way to think of it is to cut the foam like it's freshly-baked bread. Forcing the knife tears through the foam, leaving a sloppy cut. With either method, start cutting from an outside edge of the sheet.
Previous: How to Measure Your Cushion
To adhere two pieces of foam to create your cushion, go to How to Glue Foam Together
To wrap the foam in polyester fiberfill, go to How to Apply Fiberfill Wrap to Your Cushion
To use a foam-only form for your cushion, skip ahead to How to Stuff Your Cushion
