As mentioned above an on line calvukayir tends to work pretty well.
If you want to do the math yourself...
Not all calculators use these numbers but all I have seen get you pretty close to the same result.
1 pound of honey in enough water to equal 1 gallon by total volume gives you between 0.032 and 0.038 gravity points. Depending upon the amount of water, solids and sugar in the honey. General rule of thumb is to use 0.035.
0.035 times 131.25 equals the amount of alcohol produced if all the sugar in the honey is fermented.
The constant of 131.25 comes from the mol weight of alcohol and CO2 produced. (I would have to look this up to be more accurate in my description.)
Example
3 pounds of honey in 1 gallon total volume of honey and water.
3 X 0.035 = 0.105
0.105 X 131.25 = 13.78 ABV
Assumes all the honey is converted to alcohol.
If read with a hydrometer you Would see 1.105 after mixing the honey and water and 1.000 upon completion of the ferment. 1.000 being the density / specific gravity of water and the 0.105 being the specific gravity of the honey.
This is not exact, the math is not linear the higher the ABV and as you can see there are a few assumptions made. My experience has been it typically gets you within a few tenths of an ABV if measured with a much more precise lab instrument / analysis. Adding fruit or juice also adds sugars and must be accounted for in your calculation if you intend to do so. Some calculators use a general rule of thumb for the amount of sugar provided by the fruit.
