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Loop entropy is the entropy lost upon bringing together two residues of a polymer within a prescribed distance. For a single loop, the entropy varies logarithmically with the number of residues N {\displaystyle N} in the loop
where k B {\displaystyle k_{B}} is the Boltzmann constant and α {\displaystyle \alpha } is a coefficient that depends on the properties of the polymer. This entropy formula corresponds to a power-law distribution P ∼ N − α {\displaystyle P\sim N^{-\alpha }} for the probability of the residues contacting.
The loop entropy may also vary with the position of the contacting residues. Residues near the ends of the polymer are more likely to contact than those in the middle , primarily due to excluded volume effects.