Maybe a box cutter?

A serrated knife. Do it outside. :)

Unless you have a heated wire cutter?

Use an electric knife

Electric knife the only way to go!!!!

I was advised to use an electric knife.

Another vote for electric knife

What ever you use do not force it through the material. If you do you may end up with a strange edge.

Let the weight of he blade do the work. Do not press down.

If you've got an electric knife, it'll cut through almost like butter. Mark your foam and cut just behind the mark. And go slow : ) Even a very old electric knife will work.

Electric carving knife, like the ones your parents or grandparents used to carve turkeys and roasts. Go lightly and easily and you'll be amazed at how well it cuts upholstery foam. Goodwill is your friend if you don't have one.

Electric carving knives are very similar to what our upholstery department used, but if you do not have that, you can use a long serrated bread knife and saw through it. I use long strokes when I cut it, to avoid jagged edges. I don't have an electric carving knife, and so I just use a bread knife to cut foam.

Electric knife.

My first thought was laying a board across it on the cutting line and putting pressure on it with your knees, then cut with a sharp knife. It seems like you may get a better cut if the foam is squished down...? I have used scissors on foam before but never that thick.

Definitely an electric knife. Check you tube videos!

Definitely do not squish the foam down. Ask me how I know.

Good luck with your project.

If you do not have an electric knife or a bread knife, you can cut it with a very sharp knife, but it has to be fairly long - at least an 8" blade, as you need to make long strokes when you cut it. And definitely do not squish the foam down! That only works when you are cutting very thin foam with scissors.

Hmm. Learn something new everyday. I totally would've gone box cutter had it been my foam, never having asked. Electric knife. Cool!

Wow!!! I learned a LOT! Don't squish it, who knew?? I was planning on flattening that thing as much as I could.

I'm going to throw out a request on Freecycle for the use of an electric knife! Thanks, everyone!!