The coefficient of thermal expansion is a measure of the fractional change in size of an object per degree change in temperature at constant pressure. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure, such that lower coefficients describe lower propensity for change in size. The coefficient of thermal expansion is a material property that is indicative of the extent to which a material expands upon heating. There are several types of coefficients of thermal expansion, including volumetric, area, and linear, and the choice of coefficient depends on the particular application and which dimensions are considered important. For isotropic materials, the area and volumetric thermal expansion coefficient are, respectively, approximately twice and three times larger than the linear thermal expansion coefficient. The coefficient of thermal expansion must be considered in components that use a mixture of materials such as heat exchangers with mild steel shells and austenitic grade tubes. The coefficient of thermal expansion is unique to each material and is based on their other physical properties. The higher a coefficient of thermal expansion a material has, the more it will expand in reaction to being heated.