Indoor applications experience little to no expansion as the temperature is constant. For outdoor ambient temperature applications, the rule of thumb is that a 4 x 8 sheet will expand as much as 1/2" in length from a hot summer day to sub-zero winter temperatures in cold climates. HDPE sheet expands more in length than in width, due to its molecular structure.
The easiest formula to use for an exact expansion / contraction dimension is as follows:
0.00011 x degree variance(°F) x length in inches
Example: 0.00011 x 30°F temperature change x 96" long sheet = 0.3168" or approximately 5/16"
0.00019 x degree variance(°C) x length in inches
Example: 0.0001 x 16.7°F temperature change x 96" long sheet = 0.3173" or approximately 5/16".
While these numbers may seem high, expansion can be easily negated by using correct installation techniques.