The configuration factor (C-factor) of a resin composite restoration is defined as the ratio of:

C-factor = number of bonded surfaces divided by number of unbonded (free) surfaces. A high C-factor (Class I = 5) means more bonded walls and less free surface to relieve polymerization shrinkage stress, increasing the risk of debonding and marginal gaps.Other options: Reversing the ratio is wrong; filler/resin ratio and cavity dimensions describe other properties, not the C-factor.Key exam takeaway: Higher C-factor = higher shrinkage stress; Class I cavities have the highest C-factor. Use incremental layering to reduce stress.