Context: The current trend for software construction consists in developing software through increments, which is recommended by recognized methodologies such as Rational Unified Process or Agile Methodologies. Although Cosmic Functional Size Measurement Method is one of the most known techniques to estimate effort and cost in software projects, there is little evidence that indicates how to adapt it to non-waterfall lifecycles, such as incremental ones. Goal: This paper presents an approach called Incremental CFP that allows estimating effort for each increment using Cosmic Function Points (CFP) and COCOMO's effort adjustment factor (EAF). Method: In order to validate this new approach, we employed data from two different projects in which undergraduate students had to work originally with IFPUG function points. Results: The results were considered as promising given that the difference between the estimated effort and the real effort was lower than 30% for the second increment. Conclusions: With a minimum difference between the estimated effort and the real effort, it was possible to establish that the proposed technique is acceptable and predictive. However, more study cases should be performed before generalizing its applicability for different contexts.