The fin limits the sail size.
The FSC calculates the maximum sail size depending on the altitude, the air temperature & humidity of a windsurf spot, For the current location, the app can query those data itself (preview).
> Disclaimer. Windsurfing can be a dangerous sport. I am not responsible for any damage or injury resulting from the use of this website and/or the Fin to Sail Size Calculator (abbreviated FSC).
By using the FSC you agree to the disclaimer.
The FSC App is based on an Excel spreadsheet
The light blue fields are only preset and can be overwritten (by double-clicking) with your own data. Altitude and outside temperature affect the sail size, your weight affects the wind speed. The top left field is used to select the wind units:
0 = m/s, 1 = Bft, 2 = kts, 3 = km/h, 4 = mph.
(On smartphones, the pre-installed browser seems to be the best choice for using the spreadsheet. Otherwise, the FSC App is still available).