1. Linear expansivity is the fractional increase in length of a specimen of a solid, per unit rise in temperature. If a specimen increases in length from l₁ to l₂ when its temperature is raised θ°‎, then the expansivity (α‎) is given by:

This relationship assumes that α‎ is independent of temperature. This is not, in general, the case and a more accurate relationship is:

where a, b, and c are constants.

2. Superficial expansivity is the fractional increase in area of a solid surface caused by unit rise in temperature, i.e.

where β‎ is the superficial expansivity. To a good approximation β=2α‎.

3. Volume expansivity is the fractional increase in volume of a solid, liquid, or gas per unit rise in temperature, i.e.

where γ‎ is the cubic expansivity and γ=3α‎. For liquids, the expansivity observed directly is called the apparent expansivity as the container will also have expanded with the rise in temperature. The absolute expansivity is the apparent expansivity plus the volume expansivity of the container. For the expansion of gases, see Charles’ law.

http://www.kayelaby.npl.co.uk/general_physics/2_3/2_3_5.html Values of the expansivity of selected liquids and solids at the NPL website