Cyclohexane is considered a benchmark in determining ring strain in cycloalkanes and it is commonly accepted that there is little to no strain energy.  In comparison, smaller cycloalkanes are much higher in energy due to increased strain.  Cyclopropane is analogous to a triangle and thus has bond angles of 60°, much lower than the preferred 109.5° of an sp hybridized carbon.  Furthermore, the hydrogens in cyclopropane are eclipsed.  Cyclobutane experiences similar strain, with bond angles of approximately 88° (it isn't completely planar) and eclipsed hydrogens.  The strain energy of cyclopropane and cyclobutane are 27.5 and 26.3 kcal mol, respectively. Cyclopentane experiences much less strain, mainly due to torsional strain from eclipsed hydrogens: its preferred conformations interconvert by a process called pseudorotation.
Ring strain can be considerably higher in bicyclic systems.  For example, bicyclobutane, CH, is noted for being one of the most strained compounds that is isolatable on a large scale; its strain energy is estimated at 63.9 kcal mol (267 kJ mol).