Bidicalc is an experimental spreadsheet tool that allows users to manipulate outputs directly and see related inputs adjust automatically. Typically, spreadsheets only update outputs when inputs change. Bidicalc flips this approach, letting users change a calculated result and have the inputs recalibrated to match it. This concept raises several questions, such as whether it can handle complex formulas, how to manage multiple solutions, and whether it can work with chains of formulas. The tool distinguishes between variables, constants, text entries, and formulas. Variables are simple numbers that the solver can adjust. Constants, indicated by a hash (#), remain unchanged. Text entries require quotes, while formulas can be input similarly to traditional spreadsheets. The backward solver, which is the core of bidicalc, aims to find inputs that yield the desired output, addressing challenges like underdetermined problems where multiple solutions exist. Bidicalc is built with TypeScript and is open-source, allowing for modification and sharing under the AGPL license. The author, Victor Poughon, acknowledges that while the current version is functional, it has limitations. Future improvements include adding domain restrictions for variables, enhancing the solver's capabilities, and refining user experience. The tool is not intended for critical applications, like engineering calculations, but is a fun exploration of math and spreadsheets.