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.