There are models for estimation of the effort required in software projects that present limitations when they are used in early stages of the service life of development. In this article a revision of the existing models is presented and a new model able of estimating effort in the first stages of the development is described, whose entrances depend on both historical of projects made and on the experience of similar projects. The model exposed in this article was the result of an applied research project done between Universidad de Antioquia and Orbitel S.A., with the objective of considering the effort required by the analysts of the Management of Computer science for the creation and operation of solutions. Considering the historical available in Orbitel, our model gives precise estimations from the statistical point of view. Additionally, the proposed model can be simulated with a tool oriented to the Web.