First of all, the fin limits the size of a sail.

Based on a fin size, the FSC (Fin to Sail Size Calculator) calculates a suitable sail size depending on elevation, outside temperature and relative humidity. Higher values for elevation, temperature or humidity result in larger sails, lower values in smaller sails. The effect of humidity is low. As a reference, an elevation of 0 m above sea level, an outside temperature of 15°C and a humidity of 55% are assumed.

The results are based on the assumption of a passive sailing technique on a half-wind course with a take-off speed of 4 m/s and a planing threshold of 5 m/s.

This would also categorise the target group of the FSC. It is the recreational wind surfer with planing respectively take-off experience.
 
With an active sailing style (e.g. by pumping), smaller material (sail, fin/foil) can be chosen, which would have the advantage of increasing the upper wind limit, which would put you on the safer side, especially in gusty wind conditions. 

Please always pay attention to this:

Windsurfing and windsurf foiling can be dangerous sports!

Due to the large number of sail types optimized for specific use, the FSC can only serve as a rough guide. The results are not recommendations. I designed the FSC for my own use. In fin mode the focus is on the upper wind range, in foil mode on the lower wind range. Whether these fit regardless of sail type or other hard-to-specify fin or foil characteristics is up to you.

The use of the results and information is at the user's own risk. Please note the  disclaimer.

The online version of the FSC is designed to be self-explanatory through simplified calculation and operation. Only estimated fin areas are used. These are oriented more towards slalom fins. If you want to know more precisely, you should work with the full version (app or Excel). If necessary, it is also possible to fall back on the estimated fin area.

The default settings of the light blue fields of the full version can be overwritten with your own values.

Some input fields have multiple entries, but should be explained by themselves.

Mode
The selection is made via the Front Wing Lever field
Fin  to Sail Size Calculator:  Front Wing lever arm = 0;
Sail to sail calculator: see tip 3.
Foil to Sail Size Calculator:  Front Wing lever arm > 1 < 60.

INPUT FIELDS

Front Wing Arm (cm)
If not known, the lever arm of the front wing can be easily determined. See tip 2 below. 

Fin / FW Span width  (cm)
In foil mode, the field for the fin length is filled with the span of the front wing.

Fin/FW (cm²) 
The fin area is preset with an estimated value in the corresponding mode and should be replaced by its real size (see also tip 1 below). But be careful, once overwritten, no matter in which mode, they are no longer available until a reset (Set button in the app - top right in the menu).  In foil mode, the area of the front wing should always be known. 

As a sail-sail calculator, see tip 3.

Elevation (m)
Altitude of the surfing spot above sea level.

Air Temperature (°C)
Outside temperature at the surf spot.

Humidity (%)
Relative humidity at the surf spot.

Wind Unit 
The wind unit is selected in the top left-hand field of the EXCEL table or in the correspondingly labelled field of the app, provided this option is enabled:

Wind speed:
0 = m/s,
2 = kts,
3 = km/h.
4 = mph.

Wind force:
1 = Bft.

Sailor (kg)
In fin mode, your own body weight only affects the calculation of the wind speed.

FOIL+ Board + Sail (kg)
In addition to your own body weight, it is very important to use a realistic value. Does not play a role in fin mode.


OUTPUT FIELDS

Aspect Ratio
Is the aspect ratio (λ - Lambda) of the fin or front wing, the ratio of the length/span to the average width. The higher the value, the slimmer the fin or front wing. (See also explanation of formula (4) in FSC_Physik.pdf, currently only available in German.)

Sail Size (m²)
The result of the calculated sail size.

Wind
How to evaluate wind speed in relation to sail size is outlined in the description.

Geostrophic Wind Calculator ( GWC )

In the geostrophic wind calculator on the weather page, i.e. in the high-altitude wind calculator without taking local effects into account, the sail calculator of the FSC full version, which is also hidden, is integrated. In contrast, the size of a reference sail (m²) under standard conditions (0 m above sea level, 15°C, 1013 hPa) is expected in the input field for wind units, regardless of fin or foil. In this mode, only altitude and outside temperature are relevant. As far as the input device allows, both are preset for the current location.

Tips.

1. If you don't know the area of your fin, you can draw its outline on a sheet of paper, draw parallel lines starting 0.5 cm below the fin head in 1 cm increments and add up their lengths. The result is very close to the real fin surface.

2 The length of the lever arm can be measured along the fuselage from the centre of the mast to the centre of the front wing.

3. If you only want to know the dependence of a certain sail size on the altitude and/or the outside temperature - and very slightly on the humidity - you can activate this mode by setting Wing Lever Arm = 1 and Fin/FW Span = 1 and entering the corresponding sail size in m² in the Area field. 


Translated with the help of www.DeepL.com/Translator (free version)

April 15, 2025 As the continuation of the FSC as an app is not assured, the recently published online version will remain available until further notice.
The wind animation on the weather page can now be provided with the positional accuracy of the device used.