Calculating a new mattress

I’m planning to DIY a new memory foam mattress, and have a pretty good idea of what I want the end result to feel like, and how to put metrics to it. 

I’ll spare you all the details, but for me memory foam is far more comfortable than latex or innerspring. With a semi-waveless waterbed being a distant 2nd. 

I understand that memory foam characteristics are measured by two distinct properties:

• Density - the weight of the material per cubic-foot, which denotes the quality, and thus longevity of the foam. Heavier is better. 
• Firmness - Indentation Load Deflection, or ILD. This is done by taking a cut of foam no smaller than 24" x 24" x 4" and compressing it 1 inch, a 25 percent compression test, with a 50 square inch circular foot , measuring in pounds the force needed to compress the foam. The force needed is stated in a number reflecting the number of pounds needed to compress the foam 1 inch. The greater the number, the firmer the foam and the lower the number, the softer the foam.

I’m a 5’6” 165lb side-sleeper, which I’m estimating as 3 sq-ft of surface area while sleeping. 

I presently have pressure-point pain from my ankles, knees, hips, ribs, shoulder, elbow & wrists that causes me to flip sides about every 20-30 minutes all night long. I feel 3”-4” of submersion into the mattress would solve this. 

So 165lbs/432 sq-in = 0.382lbs/sq-in. That value multiplied by the 50 sq-in foot used in the IDL measurement equals 19.1 lb - I’ll round up to 20lbs. That means an IDL value of 5-6 would give the desired result. Of course, this doesn’t take into account the curvature of my body, and IDL values seem to only go down to the 9-10 range, so that’s realistically what I’ll be dealing with. 

Am I on the right track so far?

Ok, but so far I’ve been ignoring any base level(s) of foam, and their IDL values - and this is where I get stuck. 

I presume the top layer will distribute my weight over a larger area than the ~3 sq-ft above, correct? I don’t think I can calculate it though, can I? So I don’t really know how much deflection any base layers will provide, right?

The ASTM standard has 2 different ILD measurements - 25% and 65%.  25% is what most people are talking about, but make sure you are reading the right now.

A number of <10 sounds insanely low.  Like crazy.  Aircraft seats (on business class seats that you sleep on) are still in the ~50ILD range.  I would think even a super-soft 'topper' would still be in the ~20 ILD range, but I may be wrong there.

Also note that resilience is in an important property of memory foam.  There is a wide range between standard polyurethane foam and memory foams.  A lot of people like to call just any polyurethane foam 'memory foam'.

(I design aircraft seats and specialize in foam/comfort system)

Also note that you won't have an even distribution across your contact area.  0.3 average Psi is probably on the way low side.  If I see this when I'm on my work computer tomorrow I can give you a more realistic number, but I would imagine around 1psi for back sleep and slightly over that for side sleep.

In reply to ProDarwin :

Awesome info, thanks!

I did read that ILD values don’t give a 100% true comparison between different producers of memory foam, but I’ve not seen many who actually provide any details. 

I suffer similar problems with seating - I have a bony ass. I can’t sit on unpadded surfaces for more than about 5-minutes, and after driving to/from IL(about 12-hours) I can barely walk. 

I’ve been talking to a rep from this place, since they seem to be the primary place for fools like me to get a cheap custom mattress. I think their 4” 5lb 14lb ILD memory foam on top of a 35lb ILD base will work for my side. I need to get SWMBO more involved on what she wants for her half, but I’m thinking their 4” latex pad on the same base would work well. She doesn’t like the heat or submersion of memory foam, but still wants something fairly plush.