Accurate estimation of project costs is an essential prerequisite to making a reengineering project. Existing systems are usually reengineered because it is cheaper to reengineer them than to redevelop or to replace them. However, to make this decision, management must know what the reengineering will cost. This contribution describes an eight step tool supported process for calculating the time and the costs required to reengineer an existing system. The process is derived from the author?s 20 year experience in estimating reengineering projects and has been validated by several real life field experiments in which it has been refined and calibrated.