Jun 10, 2022 · HDPE sheet expands more in length than in width, due to its molecular structure. The easiest formula to use for an exact expansion / ntraction dimension is as follows: Degrees Fahrenheit: 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".