How to calculate ABV

[roxy002]

Sorry if this sounds like a daft question, but I know you can work the calculation of ABV, by doing the sum with the starting and finishing gravity. But what about if you carry on adding sugar after the previous lot you added has been fermented? This this change everything?

 

[Dunfie]

You will still need to know you OG so that you have something to work from.

Off the top of my head, sucrose has an ldk value of 360. This means that if you dissolved 1kg of sugar in 1l of water you would have a solution with a gravity of 1360.

Put another way by doing simple arithmetic, if you were fermenting 5l of wine and you added 100g of sugar you would be adding around 7.2 gravity points which roughly equates to an extra 1%abv (ish).

So if you measure you OG and then add the gravity points when you add sugar you will be able to calculate you final abv.

 

[evanvine]

Dunfie is spot on!
Original Gravity and Final Gravity are the ones that matter.
If you don't add any extra sugar, ABV is calculated by subtracting the figures after the decimal point of the FG from the figures after the decimal point of the OG and dividing by ( some use 7.46, others use 7.36, mid point is 7.41).

EG:-
OG = 1.044
FG = 1.002
44 - 2 = 42
42 / 7.41 = 5.668 or 5.7% ABV.

Now you are wanting to add extra sugar during the ferment!
No problem as long as you know your volume.
Dunfie rightly says that 1 kg of sugar will add 360 gravity points to 1 litre.
IE 100g sugar will add 36 points to 1 ltr.
Assuming you have a volume of 4.5ltrs, every 100g of sugar will add (36/4.5 = 8) points to your ferment, which you have to add to your OG.
Thus 100g sugar will increase your OG to 1.052.
You then use the above calculation with the new figure.

If I've got it wrong it's cos I've just finished work and am now suppin!

 

[Ceejay]

This is something I was getting confused about the other day - because you've added volume to the liquid, do you not have to take the new volume into account? :wha:

 

[evanvine]

Yes you do!
If you start with say 4ltrs at 1.040 and add 1 ltr, making a total of 5ltrs.
The corrected SG will be 4/5 of the initial gravity, 1.032.

If you are talking about the vol of sugar added, techinically yes, practically no.

 

[Ceejay]

I see! Thanks for clearing that up - it was making me go cross-eyed the other day! :thumb:

 

[graysalchemy]

You could just simply use the calculator in the menu bar top left :thumb: :thumb:

 

[ano]

This is something I was getting confused about the other day - because you've added volume to the liquid, do you not have to take the new volume into account? :wha:

as evanvine said, but just to reiterate, if you add sugar it dissolves so the volume is not increased by a significant amount.

 

[Moley]

if you add sugar it dissolves so the volume is not increased by a significant amount.

Volume increases by half a pint per pound of sugar, so if you are only adding a few ounces, are only concerned about ABV as an approximation to the nearest one percent or so and don't want to get into any heavy maths, ignore it.

 

[ano]

Yeah Van der Waals radii and all the chemistry I wanted to avoid. I guess it depends upon your view of significant (given below average hydrometers, temperature corrections often ignored etc there are plenty of other places for errors to creep in). I gave up calculating abv to any more than 1 s.f. a while ago but I realise I may be in the minority.