What is the process for estimating and planning custom software projects?

Software project estimation combines historical data, decomposition, and uncertainty modeling. Process: (1) Requirements decomposition: break features into user stories, then into technical tasks. (2) Relative estimation: use story points or T-shirt sizes (S/M/L/XL) rather than hours because relative comparison is more accurate than absolute time estimation. (3) Velocity calibration: after 2-3 sprints, measure actual velocity to calibrate future estimates. (4) Risk assessment: identify technical unknowns, third-party dependencies, and requirements ambiguity, and add contingency buffers. Estimation techniques: Planning Poker for team consensus, reference class forecasting (compare to similar past projects), three-point estimation (optimistic, most likely, pessimistic) combined using PERT formula (O + 4M + P) / 6. Common estimation failures: anchoring bias (first estimate influences all subsequent), planning fallacy (systematic underestimation), scope creep (requirements expanding without timeline adjustment), and ignoring non-development work (testing, documentation, deployment, code review). Agile approach: estimate in relative complexity, measure velocity empirically, plan in iterations (2-week sprints), and re-estimate remaining work regularly. Project planning: define MVP scope clearly, create a prioritized backlog, identify technical spikes for unknowns, plan for architecture and infrastructure setup in sprint 0, and communicate estimates as ranges rather than single numbers.