How to Cut Foam Core With 45 Degree Sides

Sharp craft knives can be used to make smooth cuts in foam core.
careful! image by Josef F Stuefer from

Foam core — a craft material made from thin layers of plastic foam and laminated paper — serves as a handy mounting board for posters, graphic designs, photos and prints. Artists and hobbyists often cut foam core into square or rectangular shapes which feature 45-degree angles on four sides. Though rigid, foam core can be cut rather easily. You only need a handful of supplies and a few minutes to make 45-degree cuts in foam core.

Sharp craft knives can be used to make smooth cuts in foam core.
careful! image by Josef F Stuefer from

Things You'll Need

  • Craft Knife
  • Angle Ruler Or T Square
  • Pencil

Position the foam core flat on a stable and solid flat surface. Place a mat under the foam core to avoid damaging the surface below. Make sure that the glossy, paper side of the foam core board is facing up.

Sketch out the shape of the cut you'd like to make -- including any sides with 45-degree angles -- in the foam core, lightly marking the board with a pencil.

Place an angle ruler or T square atop the foam board. If using an angle ruler, make a 45-degree angle and lock the ruler. Position the 45-degree angled joint of the ruler or T square over the sketch of the 45-degree side angle.

Press the angle or T square firmly and cut along one edge with a craft knife. For the first cut, penetrate only the top paper layer of the foam core. Cut against the ruler rather than trying to follow along its side.

Cut into the initial groove, keeping the ruler or T square in place as guide. Penetrate the foam layer of the board this time.

Cut into the line once again to slice through the bottom piece of paper.

Repeat the cutting process along the ruler or T square's other edge to complete your 45-degree angle cut.


  • Thin metal rulers work best when cutting foam core.

    Use a sharp blade for the cleanest cut. Replace your craft knife's blade after every 10 or so foam core cuts.


  • Work slowly and exercise caution when using sharp craft knives.