How to Cut Foam to Make the Perfect Camping Mattress


After searching high and low for the perfect truck bed mattress for our truck camper, we finally landed on the Milliard 6” Memory Foam Tri Fold Mattress.

It was exactly what we needed, but there was one problem.

It was about 3” too long, and (of course), didn’t have the corner section cut out like we needed for our propane tank storage!

memory foam camping mattress cut and modified to fit odd space

We know many other DIY campers need an odd sized mattress like this, or pieces of foam cushion that get puzzled together to create a sleeping space.

So if you’re finding that you might need to modify foam cushions or a foam mattress and are feeling hesitant to jump in, we’ve got you covered!

It is pretty easy to do and I bet you already own all the needed supplies.

**Special note for those cutting a tri folding mattress!

Your three pieces of foam fold up like an accordion. Depending on your application, this could really matter.

We needed the modified section of foam (which is nearest our tailgate) to be the one that the is able to be folded up on top of the middle section of the mattress (NOT the section that could only fold down and underneath the middle section of the mattress).

Be sure to plan this out before you mark and cut a certain section of the tri fold foam! See images below for clarification**

Supplies Needed to Cut Foam:

  • Measuring tape

  • Sharpie marker

  • Straight edge (could even be a piece of cardboard)

  • Long serrated bread knife

  • Seam ripper and basic sewing kit (if your foam is encased in fabric)

Step 1: Remove Any Fabric

After measuring how much foam needs to be cut to make the mattress fit in its space, remove any fabric encasement from your memory foam mattress.

(For the Milliard mattress, I unzipped the top fitted sheet and removed the foam section that needed to be altered. I then had to use a seam ripper on the layer of fabric that covers the individual piece of foam to open up a slit just big enough to pull the foam out. This left me with one of the trifold pieces loose.)

Step 2: Measure and Mark Your Foam

Measure, then use you sharpie to mark what foam you need to cut off. Use a straight edge to help keep everything exact.

I would recommend marking on all four sides of the foam (top, bottom, and sides). This will help when cutting.

diy cutting memory foam mattress mark with sharpie and cut with knife

*Special note: If dealing with foam that is topped with memory foam, don’t forget to be paying attention to which side is “up” (the memory foam side!) as you are marking what to cut off. For us, this didn’t matter when just cutting the length off, but did matter when cutting the corner out.*

Step 3: Prep Your Area

Find a way to prop the foam up off the floor or table surface so that your knife doesn’t damage anything.

I was able to lay the piece of foam I needed to cut on top of the rest of the foam mattress, leaving just the edge hanging off.

Step 4: Cut Your Foam Mattress!

Work slowly and basically just saw the thing apart!

Having all four sides of the foam marked will help you keep your knife at a 90* angle against the mattress top, resulting in a cleaner cut. It’s easy to just peek underneath to see if your blade is coming through on the line you marked along the bottom, and then adjust.

If you aren’t recovering with any fabric or encasement, then you’re done with your foam cutting project!

Wasn’t so bad, right?!

Step 5: Recover the Foam With Fabric (keep scrolling for a no sew option)

Recovering individual pieces of foam using a sewing machine shouldn’t be too challenging.

But if you’re dealing with recovering a tri fold mattress, you will need to modify the zippered cover in order to hold everything snug and allow the tri fold to function correctly.

Otherwise, you’d be trying to sleep on something like this and your foam could slip out from underneath you!

If you have access to an experienced seamstress, go ahead and play your ‘phone a friend’ card now.

My mom worked for quite a while in order to make this snug again, but we wouldn’t recommend this approach unless you have the extra time and experience.

Check out her finished product! Whoop!

No Sew Option for Recovering Foam

If sewing isn’t an option for you, consider using safety pins to tighten the cover up against the foam. When trying this out, we just gathered the excess material up, folded it over, and pinned it back (being sure not to puncture the mattress).

This may not be ideal, but certainly seemed like it could be good enough - especially if you were mindful of the pins when transporting the mattress around, and if you typically have an additional sheet over it anyway!

Ta-da! Done!

In case you’re wondering, yes, it was a little painful to be cutting a really nice, brand new mattress into such a weird shape, forever changing its original design.

But having a tri fold memory foam truck bed mattress that fits perfectly in our truck shell camper has been a game changer!

If you want to learn more about this specific mattress and why we think it’s the bomb, we’ve got a post all about it.

tri fold memory foam camping mattress in back of truck bed camper cut to fit

Hopefully these tips on how to cut foam can help you in your pursuit of finding a great camping mattress, whatever your setup may be!

As always, thanks for reading! Don’t forget to SUBSCRIBE for more blog updates!

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