The web-app gcwCi helps to estimate the cuticular conductance to water of a leaf surface using normal gas exchange parameters, requiring as an input a known CO2 concentration within the leaf (ci). The gcwCi web-app was designed to be used along with the Red-Light technique presented by Marquez, Stuart-Williams, Farquhar & Busch (2021) but it can be used under any experimental set up where the ci is known. It is handy to estimate total cuticular conductance accurately with the most common setup of gas exchange instruments, i.e. a chamber mixing the adaxial and abaxial gases, permitting routine use of this application.