Predicting Gravity

I know a lot of you out there aren't too concerened with this and would rather shoot from the hip rather than mess around with the details but I love this stuff. For me its part of what makes brewing so fun.

What I'm trying to do is predict my gravity/eff before I boil off so I can tell if I need to add any DME to get to my target gravity. I've taking readings of each sparge, adjusting for temperature, weighting them based on volume and then using this equation to predict the final gravity:

SG2=V1/V2 x SG1

My results are all over the place and I can't figure out why. Any ideas?

Assuming the "2" s are post-boil, and the "1" s are pre-boil, that should work, but you need to express the gravity as points, not the measurement (ie don't use 1.040, use 40)

Ex: Preboil 7 gallons at 1.040, postboil = 5.5 gallons

Postboil gravity = (7 * 40) / 5.5 = 51, or 1.051

thanks...i'm gonna use this too, with my partial mash i've been "guess-timating" with promash

adding to my spreadsheet now...

alright, peoples...i just created a spreadsheet using this formula. it also has my ABV and attenuation calculator on there. i tested them against promash and it seems to be working great!

Here it is: Brewing Calculator

And here's how it works:



obviously, putting in the OG and SG will give you ABV and attenuation

for predicting how much extract to add:

1. Enter the Preboil Gallons, Preboil Gravity and Target Gallons.
This will give you your Projected Gravity at your Target Gallons.

2. Enter the Extract Gravity (so you can enter any LME or DME) and enter your Target OG.
This will give you the exact amount of the Extract Addition you need to reach your Target OG, rounded to two decimals.

i made this for Partial Mash, but I'm sure it will work for all-grain.

Please feel free to check it out and use it...let me know if there are any problems or you feel that it could use some improvements!

No but really its not working for me for some reason. Maybe I should mix them all together and then take a reading rather than measureing each running. I like to start my boil while the second sparge is going in so I don't have to wait. I'm always looking for ways to shorten the process and thats not going to be one. Oh well, might just have to live with it.

Yep, you've got to take a COMBINED breboil reading. Slow down and enjoy the process, there's no rush. More time to drink beer anyway.

There's a great book called "Designing Great Beers" by Ray Daniels that has a full chapter on hitting your target gravity. The main thing you need to keep in mind is that Total Gravity (beg. of boil) = Total Gravity (end of boil).

When working on gravity adjustments, it is easiest to think in terms of total gravity. Say you're brewing 6 gallons of bock beer for which you want an OG of 1.068. Multiplying that (6x68), you find that you need a total gravity of 408 GU.

If you perform a mash that gives you a total gravity of 355 GU, you are 53 GU short of where you need to be. To find out the weight of extract needed, divide the difference between your target gravity and the actual total gravity by the extract/pound value for the type of extract you will use.

extract(lbs) = (target GU - mash GU) / (Extract/lb. value)

Generally, the extract potential for dry extract is 1.045 ( 45 GU) per pound. If you want to calculate the amound of dry extract needed to bring your 6 gallon batch of bock to desired gravity, follow this formula:

Extract(lbs) = (408 GU - 355 GU) / (45 GU/lb.)
= 53 GU / 45 GU/lb.
= 1.2 lbs.

Also since hydrometers are calibrated for 60 deg. F. so remember to adjust your readings for your temperature. 100 degree wort will require to add 0.006 to your reading.

Cheers,
Gary