If you are looking to increase the amount of alcohol potential of your cider by adding more fermentable sugar, such as apple juice concentrate, check out the chaptalization calculator to see how much sugar you should add to achieve your desired ABV.
Accuracy of the calculator:
There are several variations of this equation that result in slightly different results depending on how much alcohol is in the cider. Specific gravity and alcohol content is not linear, meaning that the calculation for cider is dialed into an accuracy of around 4-6% while the equation for wine is dialed in for around 11-15%.
Because a cider is generally lower than 6% AVB, a basic equation is sufficient for home cider making. If you are making a cider with an ABV closer to wine, say 10-15%, an alternative may provide more accurate results.
ABV equation for cider (4-6% ABV)
source: The Joy of Homebrewing Method by Charlie Papazian, Bee Lee’s Method, Beer Advocate Method
ABV equation for high gravity (10-15% ABV)
%ABV =(76.08 * (OG-FG) / (1.775-OG)) * (FG / 0.794)
source: Ritchie Products Ltd, (Zymurgy, Summer 1995, vol. 18, no. 2) -Michael L. Hall’s article Brew by the Numbers: Add Up What’s in Your Beer, and Designing Great Beers by Daniels
For the propose of home cider making, a slight difference is probably not a big concern however a commercial cider maker may need to conduct an in-depth analysis.