COBOL to Java Migration: A Practical Cost and Risk Guide for UK Enterprises

Java has become the default migration target for UK enterprises moving away from COBOL mainframe systems, owing to its mature ecosystem, strong static typing, and large developer talent pool. A critical technical requirement in such migrations is mapping COBOL's packed-decimal COMP-3 fields to Java's BigDecimal class, as using primitive float or double types introduces rounding errors in financial calculations. Enterprises can choose from three main migration approaches — automated conversion, parallel rewrite, or incremental strangler-fig migration — with most organisations opting for a hybrid strategy. A mid-sized COBOL-to-Java migration typically costs between £200,000 and £800,000 and takes one to two years to complete. The biggest risks involve accurately translating the data access layer and undocumented business logic that has accumulated over decades of mainframe operation.