This volume contains the proceedings of F-IDE 2021, the sixth international workshop on Formal Integrated Development Environment, which was held online on May 24-25, 2021, as part of NFM'21, the 13th NASA Formal Methods Symposium. High levels of safety, security and privacy standards require the use of formal methods to specify and develop compliant software (sub)systems. Any standard comes with an assessment process, which requires a complete documentation of the application in order to ease the justification of design choices and the review of code and proofs. Thus tools are needed for handling specifications, program constructs and verification artifacts. The aim of the F-IDE workshop is to provide a forum for presenting and discussing research efforts as well as experience returns on design, development and usage of formal IDE aiming at making formal methods more accessible for both specialists and non-specialists.