Calco-Rechner

The "Calco" app from the Federal Office of Customs and Border Security BAZG enables you to determine the alcohol strength of spirits, expressed as a percentage by volume. The prerequisite is the use of an alcoholometer, which provides you with the necessary starting information.
In "Calco" you have access to the "Decrease" and "Reduction" functions of spirits.
You can use the "Decrease" function to calculate how many percent by volume your measured spirit has, based on the reference temperature of 20°C.
You can use the "Reduction" function to calculate the amount of water that you need to add to a spirit in order to achieve the drinking level you want.