Adding a fruit juice after Primary Fermentation

[bken620]

I'm not asking whether this is a good idea or not but more of an if I did this, what would the alcohol content be...

So say I had a beer start at 1.069 and then ferment down to 1.017, that would give me about a 7.2% beer. If I then pour the beer over a fruit juice and it brings it up to 1.030 and then ferments back down to around 1.015 which is about 2% alcohol, would I then have a 9.2% (7.2% + 2%) beer or something else?

 

[BlueHouseBrewhaus]

I think the correct way to calculate it is to adjust the OG as if you added the juice before you pitched. So, if the juice raised your gravity 13 points (1.030 - 1.017), add the 13 points to your OG and calculate the ABV comparing your adjusted OG (1.069 + .013 = 1.082) with the FG after the juice has fully fermented (1.015). This gives you 8.8% ABV.

 

[MindenMan]

I agree, calculate your ABV as if all of the sugars were introduced at the beginning of fermentation. The number may not be as exact this way, but it is very close, and widely accepted.

 

[bken620]

Thanks to everyone who replied, I will do as you people say and calculate the difference.