A theoretical model was developed for the correlation between the applied amount of light and the depth of cure of composite resin. The hardening depths of different composite resins were determined by means of ISO 4049. The depth of cure increases nonlinear with rising dose of light and decreases with rising time of radiation. With duplication of exposure time a constant growth of depth of cure can be measured. The depth of cure can be described by the equation d = A+B (ln(D×k) / ln2), with A representing the depth of cure achieved with 10 J/cm² dose of light and B representing the increase of depth of cure in mm with duplication the dose of light. Clinical relevance: several thin layers require a shorter exposure time than one thick layer. We propose that the manufacturers publish the for their composite applicable constants A and B and calibrate the scales of the light curing units in J/cm². Keywords: composite, depth of cure, dose of light, layering technique