How to calculate ring strain energy?
Typically, ring strain energies (RSE) are calculated as the difference of the heat of formation of cyclic starting materials and less cyclic products in homodesmic reactions.

For cyclopropane, the RSE would be calculated using the following reaction:

$$\ce{(CH2)3 + 3 CH3-CH3 -&gt; 3 CH3-CH2-CH3}$$