I have a 3" memory foam mattress topper, queen size. I want to cut it down to full size. I have no likely tools. (I'm a not-handy apartment renter). I have heard that an electric carving knife will cut polyurethane foam, but that would be the cheap, lightweight stuff, not 6 lb. density memory foam. I need to be able to do this in the apartment – it's way too heavy and awkward to take anywhere and anyway I don't have a car.

Any ideas? Thanks!

 

Why not test cut on the section that will be cut away and thrown out. Try using one of those carving knives that never get dull. Or the electric knife as you mentioned in your post. How about a good quality tile knife? I think trial and error is the procedure needed here.

 

Unless you buy a foam cutter (which is pricey), an electric knife would be best.
Draw a straight line using tailor chalk or invisible pen or similar and follow the line while you cut. Just try to hold the knife straight while you cut, rather than on an angle, or you'll end up cutting the foam angled in at the bottom.
If you could slip a board underneath, at the chalk line, you can follow the bottom of the foam along the board to get an even cut.

 

Will the memory foam compress over time if you put weight on it? If you can lay a board along where you want to cut and have several friends stand on it until the foam compresses down. Then use a new, sharp box cutter blade to cut using the board as a guide. Try to cut in one pass and one motion as much as possible because as you cut the cut foam will expand up blocking your access. If cutting becomes difficult get a fresh blade. I often hold the bare blade by it's very tip with a pair of Vice grip pliers to have the most blade exposed as possible. And of course be very careful. You are working with a very sharp exposed blade and apply a good bit of pressure.

Other than the other suggestions you've received a box cutter can do it although you'll get a somewhat messy cut. Mark the line where you want to cut. Wear a heavy glove on your non knife hand. Use a box cutter handle for safety and cut the foam along the line as deep as you can go in one pass. Then use your other hand, a spatula or any other inanimate object to push the foam away opening up the cut made on your first pass. Stick the box cutter down into the slit and make another pass. Eventually you will make your way through.

 

The electric carving knife is the best. As mentioned, the biggest problem is holding the knife straight up and down to avoid a tapered cut.

 

Maybe two boards and C clamps. Compress the foam between the two boards with the boards directly on top of each other, edges aligned. Align your electric knife so it follows the edges of the board on both sides. I'd suggest two stud length 2x4s. Plenty of edge thickness to keep the knife square.