Calculating ABV

[Androshen]

I am having problems calculating my ABV ...
I had been using the formula (OG - FG) X 105 = ABV but my friend ( who are more experienced drinkers all tell me that the ABV is considerably higher than what I have calculated.
I have since checked online and found the following formulas ... also for determining ABV from the SG readings:

(OG - SG) X 131

(OG - SG) / 7.36

(OG - SG) / .75

(OG - SG) X 129

(OG - SG) / 7.69

Why are there so many different formulas and which one is correct?

 

[CandleWineProject]

Um, temperature is a constant variable.

 

[MedsenFey]

The (OG-FG)*105 gives an estimate of alcohol by weight. If you take that same number and multiply by 1.2, you'll have an estimate of ABV.

The multiple versions of these formulas are all different simplifications of the same process described on howstuffworks.com, and some simplify more than others. All of them try to fit a linear formula to non-linear data so they all have some degree of error. Of course, even if you use the same OG and FG numbers, you may have different levels of alcohol in the batch depending on how much the yeast used to form biomass and other products like glycerin, and depending on how much alcohol was entrained with the CO2 or evaporated during fermentation (which are temp dependent). Even with the same OG and FG numbers you can easily be +/- 1/2% ABV, so all these formulas are estimates that will hopefully get you within 1% ABV of the actual number. Even the TTB allows the actual amount and what is label to be off by 1%.

If you want to be exact you need a cash still or some specialized equipment for measuring alcohol.

Personally, I use the (OG-FG)*131. It is easy to remember and in this case, it really is close enough for government work. lol.