how to figure out gravity /abv ,when the recipe includes lactose

[Soulshine2]

I've been brewing a few years, this last batch however was my first use of lactose (UNfermentable milk sugar) . As in ,it was an Oatmeal Choc Milk Stout I dubbed "Dark Side of the Moon" since I brewed it Dec 12th ,the day of the final full moon of the year.
It was a 6 gallon batch with 1 full pound of lactose . I had an AG Oatmeal stout recipe kit that I added a few other (chocolate ) malts for extra flavors ,an extra 1/2 lb of oats, and 10 oz of dextrose(corn sugar) ,for a boost of abv. but the use of lactose and its effect on the gravity and actual alcohol has me a little baffled. I know the addition of it naturally raises the gravity but since it is unfermentable will contribute nothing to the alcoholic end of it. My OG was 1.078 with a final gravity of 1.030 .
I bottled yesterday and it smelled and tasted incredible . It should be carbed/conditioned and ready to drink by Jan 2nd.

 

[bucketnative]

The ABV is typically determined using the change in gravity. In this recipe, the lctose is basically a spectator, and it is present in both your PG and FG measurements. The usual formula for this estimation is:

ABV = 131.25 (OG - FG)

So, in your case: 131.25 (1.078 - 1.030) = 6.3 % ABV

 

[Jag75]

I wonder if the OP is talking about true readings of fermentable gravity . Lactose isnt fermentable so when he took his OG some of those wont be fermentable . I think 1# of lactose is 1.043 gravity in 1 gallon . I'm not sure but wouldn't that be about 7 points for 6 gallons ? If that's the case then actual fermentable OG would be 1.071 . If finished at 1.030 it would be 5.38 abv. But the yeast may take it down into mid to high 20's?

 

[bucketnative]

What matters is the change in gravity. When lactose is not included in a recipe, the OG measurement still includes some degree of unfermentables, and we don't subtract those from the OG when determining ABV. So, why should we treat the lactose any differently?

If you're going to subtract it from the OG reading, then you need to subtract from the FG reading as well, because the same amount of lactose is present there, too.

131.25 (1.071 - 1.024) = 6.3 % ABV

 

[Saunassa]

I have become accustomed to most of my beers finishing between 1.008 and 1.012. The first time brewing a stout using lactose and seeing fg of 1.024 after 3 weeks kind of freaked me thinking I had a stalled fermentation. Maybe this is what OP was thinking too?

 

[Soulshine2]

ok , so that all makes sense now...since the lactose is NOT fermentable , treat it as if there isnt any and just calculate the difference as usual. Not sure why that didnt occur to me before.