To determine when fermentation is complete is not a measurement of Alcohol, it is a measurement of Gravity. When you first put a batch into the LBK you should take and record the Original Gravity. After 21 days you take another sample of the beer and check the final gravity, over a period of 3 days if the FG ( final gravity) is the same for 3 days then fermentation is complete and it is time to bottle. To determine ABV (alcohol by volume you subtract the FG from the OG and muttliply by 131. As per Mr.Beer instructions.