How To Cut Foam Board

Boards made of foam are a fantastic alternative to wood for home decorating and other DIY projects. However, if you’ve never worked with the stuff, you might be at a loss on where to start. For example, understanding how to cut foam board is not as easy as it seems.

How To Cut Foam Board

While there are several specialized tools for cutting foam boards, most DIYers get away with regular hardware, with a particular preference for utility and X-Acto knives. However, investing in a professional toolset is wise if you’re planning on larger or more specialized projects. 

Step One: Decide What Type Of Foam Board You Need

Depending on their intended purpose, the materials used in their construction and thicknesses differ between boards. The first step is choosing what type you need and its thickness.

Foam boards consist of 3 layers: 

• A foam (polystyrene or polyurethane), solid styrene, or rigid acrylic core 
• A paper (clay-coated paper, cotton archival paper, or common brown craft paper), plastic (polyurethane/styrene), or aluminum outer layers

These boards come in various thicknesses, including: 

• 0.12” (3mm)
• ⅛” (5mm)
• 0.4” (10mm)
• 0.79” (20mm)

Polyurethane-type boards are some of the most versatile and popular. However, they are available in various sizes and must be cut to the appropriate size.

What you plan on doing with the boards also determines your cutting tool. 

Step Two: Select The Correct Cutting Tool

Once you know what to achieve with the foam, you can better decide what cutting tool will work best.

While there are many different tools, the most popular ones include:

• Hot wire knives (used by crafters and sculptures)
• Table saws (for large blocks)
• Dremel tools (for finer details)
• Vertical-bladed foam cutting saw (the professional’s tool of choice)
• Utility knife (budget option)
• X-Acto knife (utility knife alternative)

Whatever the cutting tool, ensure it is sharp (for blades), clean, and in proper working order before cutting. A dull blade will tear and rip through the foam, rendering it spoiled. If you’re cutting large blocks of foam board, you should know how to use a table saw.

Step Three: Measure And Mark Off Where You Want To Cut

Measuring and marking off how much you need to cut is essential as it allows you to be accurate (which is critical). To borrow an adage from carpentry, “Measure twice and cut once.” Make certain you are happy with the dimensions before you start cutting.

Using a ruler/measuring tape to produce an accurate reading or a template item (like a picture/photo) as a reference point. Next, use a pencil to mark on the foam, and draw a straight line using a ruler/a compass for circles, etc.

This line is where you’ll cut along. 

Step Four: Use A Straight Edge To Ensure You Keep A Straight Line

Once you’ve marked off where you want to cut, using an object with a solid straight edge as a guide would work best. 

Some fantastic tools that work well as guides include:

• Metal rulers
• Set squares
• Speed squares
• T-squares

Once you’re satisfied with where the guide is, you’ll need to ensure it does not move during use, either by holding it firmly, asking another person to hold it, or anchoring it to a solid surface with a clamp.

Step Five: Making The Incision

Once you’re confident of your line, take your blade (most likely a utility knife) and slice through the top layer of the foam board. Avoid pressing too deeply, as you might tear the foam.

Step Six: Start Slicing Through The Top Layer

With the tip of your blade through the first layer, slowly slice down the line, ensuring you don’t cut too deep, and use the guide to cut straight. 

While cutting, try to keep the blade at (most) a 30° angle from the foam (the more horizontal, the better). If your angle is greater, you stand a chance of ripping the foam. It is important to “slice” and not “saw” the board.

You can cut through a thinner board with a sharp blade in one motion, provided you’re experienced enough. It’s valuable to slice through the top layer only on the first pass.

It is important to remember that the foam might be rigid and slick, which results in the knife slipping, or soft and squishy, which leads to tearing/cutting skew.

Step Seven: Slice Through The Next Layer

Once you’ve sliced through once, go back and start the process again. Now you’ll slice through the rest of the foam. 

It is essential that you remain patient during this step. Cutting too deep too quickly may result in tearing the foam.

Repeat this step until you’ve cut through the foam board. The board’s thickness has a determining role in the number of slices, but on average, you’ll make roughly three cuts before you’re through the layers.

Step Eight: Trimming And Neatening

While you shouldn’t need too many adjustments if you are meticulous and patient, foam warps as you cut, so a few pieces often need tidying up. Using your knife or scissors, you can trim away any pieces of foam that bulge or poke out. 

Alternatively, if you need to make slight adjustments, mark off what needs trimming with the pencil and ruler and use your blade with several passes through the foam.

How To Cut Foam Board (Variations)

While small foam board projects are easily completed with a utility or craft knife, you may need to cut larger sections of foam board than is practical or through other materials like aluminum. 

Using Electric Saws To Cut Large Foam Board Piece

A table saw might be necessary to cut a large foam board into smaller, manageable pieces. You could also rip narrow boards using a table saw. Alternatively, you could use a circular saw to make straight cuts on your foam board.

Following the same process of marking off where you want to cut with a pencil and ruler/set square, you’ll place the board on your best table saw and slowly feed it into the blade. 

Cutting slowly and steadily is essential as the blade may rip into the foam and tear it. You’ll need to trim and neaten the foam after you’ve used this saw.

A jigsaw might be the most useful tool to make interesting patterns into your board like cutting a circle.

Once you’ve drawn the desired pattern, place the foam board on a work table and cut along the line. It is wise to ask for assistance to hold the foam steady while you cut to prevent it from hooking and tearing.

Once you’ve finished cutting, you’ll need to trim off/sand down any unruly pieces.

Frequently Asked Questions

Conclusion

Foam boards are renowned for their ease of cutting, but doing it neatly and accurately takes practice, patience, and the correct tools. DIYers often use a utility or X-Acto knife; however, investing in specialized tools is recommended for those undertaking larger or more complicated projects.