There are four areas that are covered when grading memory foam. Those four areas are ILD rating, weight, resilience and tensility.
The ILD rating, or Indentation Load Deflection, determines how firm the memory foam material is. The measurement for ILD ratings is determined through how much, usually in percentage (%), indentation is created through the weight pressure in pounds (Lbs). If an ILD rating is higher then that normally means that the memory foam is much firmer than those with a lower ILD rating.
The amount of chemicals used in the production of the memory foam material and polyurethane foam determined the weight of the foam. Since the chemicals used in the manufacturing process determine the weight, the more chemicals that are used in the visco elastic memory foam production process means a much more visco-elastic mattress.
This measurement determined how ‘bouncy’ the mattress is. When taking resilience in consideration when looking for a memory foam mattress it is important to keep in mind that even though the more resilient mattress is more durable (tested using a steel ball dropped three feet onto memory foam mattress), this would also mean there is more pressure upward towards your body on the mattress. H.R. foam is an example of highly resilient foam.
Tensility of memory foam is the determination of how much the foam can be stretched apart before rupturing. This factor is determined in percentages. However, under most circumstances, memory foam mattresses are not stretched any more that that of the downward pressure of the weight on top of the mattress. So there is little or no need to focus too much on this value when purchasing a memory foam mattress for home use.