[Context and motivation] Stakeholder requirements are notoriously informal, vague, ambiguous and often unattainable. The requirements engineering problem is to formalize these requirements and then transform them through a systematic process into a formal specification that can be handed over to designers for downstream development. [Question/problem] This paper proposes a framework for transforming informal requirements to formal ones, and then to a specification. [Principal ideas/results] The framework consists of an ontology of requirements, a formal requirements modeling language for representing both functional and non-functional requirements, as well as a rich set of refinement operators whereby requirements are incrementally transformed into a formal, practically satisfiable and measurable specification. [Contributions] Our proposal includes a systematic, tool-supported methodology for conducting this transformation. For evaluation, we have applied our framework to a public requirements dataset. The results of our evaluation suggest that our ontology and modeling language are adequate for capturing requirements, and our methodology is effective in handling requirements in practice.