Calculating abv

[damo1988]

Firstly hopefully this is the right section.

Ok so I brewed some cider and took readings as followed:

OG = 1.060
FG = 1.010

I used the abv calculator at brewers friend and the readings give me 6.56% abv.

Now I back sweetened it with a litre of the original apple juice, so it will no longer be at 6.56%.

Can I work out the new abv by simply taking another hydrometer reading and using that with the OG?

Many thanks,

Damian

 

[Hello]

Wen you back sweeten you're not adding alcohol but volume. So you'd figure your ABV will be slightly lower if you increased volume. I believe the formula is:
(original volume X ABV) = new ABV
________________________________
(original volume) + (backsweetened volume)

So it's the top formula divided into the bottom in case that wasn't clear the way it posted.

 

[Ayuir]

To clarify the formula a little:

new ABV = (original ABV) x (original volume / new volume)


For example:
new ABV = (6.56)x(20/21) = 6.24

This is using 20l (5gal) as the original volume with one liter added.

Of course this all assumes fermentation is complete and you don't get a little extra after bottling/kegging from some overzealous yeast

 

[damo1988]

Thanks for the replies,

so going off that formula:

I had 3 litres of 6.56 abv and topped up to 4.5 litres.

my new abv would be 4.37%.

working to

( original vol (3l) x abv (6.56%) = 19.68)
___________________________________

(original vol (3l) + added vol (1.5l) = 4.5l = 4.373%)

does this sound correct?

Many thanks