shorthand grain to ABV% calculation in metric

[aceman]

Howdi friends

If like me you are often devising possible grain bills when you should be doing other things (new recipe window open at work perhaps?) this shorthand calculation can help (let’s say, when you’re away from your desk in a (yawn) meeting):

where grain% = dry fine basis % - moisture %

grain% x brewhouse efficiency% x yeast attenuation% x 5

= ABV per kg in 10 litres

eg targetting 6% ABV with vienna malt in a 20 litre batch with T-58 and sloppy brewhouse efficiency gives:

.0755 x .7 x .7 x 5 = 1.85% per kg in 10 l or half that in 20 litres 0.925
so you need 5/.925 = 5.4kg

feel free to inform me of holes in the back of my envelope there

 

[J A]

Seems to me that since it requires having access to the malt spec sheet for every malt on the bill it sort of defeats the purpose of being a handy, back-of-the-envelope calculation.
I'm pretty satisfied knowing that 2lbs per gallon gives me an average 6% beer with my system and estimating from there. I can use a recipe calculator for all the rest.

 

[aceman]

oops I guess I meant 6/.925 = ~6.5kg
@JA yes that is a much easier method
in metric hmm.. I guess it's 6.5kg per 20 litres! (I'm gonna need a bigger pot...)
re: grain DFB% ... I think this is the figure that starts me thinking when I'm considering something new, like using dark crystal so yes I guess I just remember it while the idea ticks away ....

 

[J A]

Aren't you using the recipe calculator? It does a great job of getting you just the ABV you're shooting for once you know your system and have the settings dialed in.

 

[aceman]

yes I've been using it - however I myself am the biggest unknown variable, only got about 50% efficiency last batch so working on that (first time I've tried BIAB - much simpler but still some learnings to discover yet). Aiming for 60% next batch but not yet dialled in - so might get anywhere from 4% to 7% ABV who knows.
fyi dark crystal is only 1.5% different to vienna so hardly necessary to adjust, for "back of the envelope" at least.
I think I need to add the Joe White values into BF ...

 

[aceman]

hmm okay to add Joe White I need to calculate these fangled "points per gallon" figures hmmm so
DFB% x 46.21 seems to be the magic number

 

[Hawkbox]

I just pick whatever looks close and then eyeball the recipe later. It generally ends up pretty close.