Recommended to be in the 30 cu ft plus for stiffness - and 4 to 6 inches thick - for a couch - and another recommendation said -
Fair Quality: 0.8LB – 1.2lb Density.
Good-Quality: 1.2LB – 1.09lb Density.
High-Quality: 2.0LB – 3.0lb Density.
Not sure that anybody can get their "numbers" to match from one site to another. And my thoughts are that do you want a "sitting" couch - or one that will be slept on for a bed - like a "gaucho sofa" - some people prefer a softer bed and others a firmer bed. Do some research on line and throw your dart !?
Your post regarding Amazon foam - looks a bit flimsy/soft ?? Years ago I made a bed for a UHaul van I rented for traveling thru Alaska to photograph wildlife - and I bought a 4 inch foam mattress at Fred Meyers which was just about perfect for me. Only like $ 30-$40 bucks as I recall - dimensions like 28 x 70 ? I'm sure pricing as skyrocketed since 2005 !
Sounds like Russ ^^ knows his stuff. I have a 'Foam and Fabric Outlet' Mom and Pop store in my city that custom cuts any kind of foam. Great place to get automotive upholstery too. If you are near a fair sized town see if you can find a similar store. These guys cut tons of custom foam and know just what to recommend.
There is no replacement for getting your hands on what you are interested in before you buy.
As usual, I agree one hundred percent with Scotty. Mill End Fabrics here in Reno carries a mind boggling variety of foam in different thicknesses and densities. Being able to try before you buy has prevented a lot of missteps for numerous projects.
Ended up with 4" 1.88/45lb foam for the bottom and 2" 1.88/45 lb foam for the back and its just perfect.
When my cusions arive I may overstuff them with some 1/2" high density yoga mat foam I have kicking around here to firm them up a little bit more and provide that end-of-travel cushion
