• Welcome to the new forum! We upgraded our forum software with a host of new boards, capabilities and features. It is also more secure.
    Jump in and join the conversation! You can learn more about the upgrade and new features here.

Gravity Readings: The Boil


Grandmaster Brewer
Oct 8, 2011
Reaction score
Re: http://www.beersmith.com/forum/index.php/topic,9508.msg39762.html#msg39762

In the gravity readings thread, I said that I would describe the various parts of the process and how I use gravity
readings to manage each stage of the brewing process.  In the post above, I described the mash, and the sparge.  Next up
is the boil.  This post will be much shorter because, the boil is much simpler than the mash.

Coming out of the mash, there are several possibilities. Which one depends on what choice you make in terms of adjusting
the mash.  The exact situation doesn't really matter though.  There is really only one thing that matters:

Do you have the predicted pre-boil wort sitting in your kettle or not? 

If so, then you can simply proceed with the boil as described below.  You will only need to manage the boil-off rate to
ensure that your boil lasts as long as it is supposed to.  If the wort in the kettle doesn't match the pre-boil
predictions (gavity or volume high/low), then you need to make some adjustments to your hop schedule. 

Hop Adjustments

There are lots of ways to do this.  The most accurate way is probably to plug the data back into beersmith and let it
calculate the IBUs for each addition, and adjust the quantities up or down as needed to get the IBU contributions back to
the base recipe.  If you follow this approach, its probably easiest to create an extract recipe with the actual pre-boil
SG and volume.  Then add the hops and adjust as needed.  THIS IS NOT WHAT I DO.

What I do is linearly scale my hop additions based on the SG/volume variation from the recipe.  Given that BS2 has already
done the complex math to calculate the IBU controbitions on the base recipe, the error that I will introduce from a +/-
10% linear scaling factor is really minimal.  I can do linear scaling in my head most of the time. 

The first step is to calculate what the final cooled kettle volume will be using the point system.  My brewsheet contains
a copy of the following table that I fill out as the boil progresses:

|_______ |_______|__Meas_ |_Corr. V_______|_ Points____|_________ |_Target V (room temp)_ |_ Post-Boil V_____|
| Time__  |_SG__    | _Volume | (Meas V / 1.04)|_ (SG * CV) |_Target SG |_(Points / TargetSG)___| (Target V * 1.04)_|
| Pre-boil_|_______|_______|_____________ |__________|_________ |____________________|______________ |
| Boiling__|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +15 min_|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +30 min_|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +45 min_|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +60 min_|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +75 min_|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +90 min_|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +105 min|_______|_______|_____________ |__________|_________ |____________________|______________ |
| +120 min|_______|_______|_____________ |__________|_________ |____________________|______________ |

(Asside: The table formatting features of SMF are SORELY lacking.  I gave up and just did it ascii as seen above.)

So, (target V) is the parameter of interest.  I then scale my hop additions by the ratio of predicted targetV to actual
targetV.  The formula looks like this:

Actual_Hops = Recipe_Hops * Actual_Volume/Predicted_Volume

The Boil
The table above kind of spoils the plot for this, but there still a little left to the story.  So, I always do an extended
boil.  Notionally, its planned around 90 minutes.  In reality, its generally close to that...but, might fluctuate 5-10
minutes one way or the other...as needed to hit my exact post-boil gravity.

Boil-off-rate is an important parameter for beer flavor and color.  if you boil too quickly you will get significant
melanoidin formation during the boil.  These are not the same as kettle carmelization which occurs at much higher sugar
concentrations and temperatures.  Melanoidins are like the brown crust that forms on a steak---and have a similar flavor
profile.  If you boil a straw colored beer too fast, it will develop these meaty type flavors.  Its not good.  15% / hour
is a good round number for the upper limit on boil-off-rate.  15% is a reasonably vigorous boil.  There will be
significant crown to the wort, and the current outward will also be rapid.  There will be a little bit of foam formation,
and the occasional jump of the wort....but, it should not be covered over in foam...and it should not be continuously

If you use base-malts with an SRM below 3.0 then DMS will form during the boil---that's just a fact of life.  However, DMS
is driven off as a gas during the boil if it is vigorous enough.  For most homebrew shaped kettles that means a boil-off-
rate no less than about 12%.  For most kettles, 12% looks more like a simmer than a "boil".  What you see is just a
crowning of the wort surface and an outward current from the crown.  There's very little to zero foaming, bubbling,
jumping etc.  Its just a gentle "boil". 

Why did I bother describing this?  Because, it puts limits on how much room you have to speed up or slow down the boil as
you progress. 

So, how does this work?  if you've made beer more than once, then you know about where to set the burner control to get a
decent boil-off.  You also probably have some data on what the boil-off-rate (gallons per hour) will be for that setting. 
Since the first row of the table was completed during the hops section above, you already know the necessary total boil-
off.  Simply divide the needed total-boil-off by your expected boil-off-rate.  This gives you the number of hours (1 to 2
hours).  I target a boil-off of 1 gallon per hour.  So, if I have 1.5 gallons I need to boil for 1.5 hours. 

Once the boil actually starts, I fill in the second row of the table and wait for 15 minutes.  Then I fill in the second
row of the table.  As I have noted before, I use a refractometer for measuring SG.  I take a small sample (25 ml) and cool
it down below 120F or so.  Just so it isn't evaporating so quickly.  At this point, I have three measurments on the total
points and thus target post-boil-volume, and I have my first measurment on the boil-off-rate.  Then I adjust my burner
accordingly. Also from this I can predict how long the boil is going to ACTUALLY last.  Once I know that, I can determine
when I'm exactly 60 minutes away from the end of the boil (for my first potential Hops addition). 

Every 15 minutes I take another measurment of the SG and volumes and fill in the next row of the table.  Then again I
adjust the burner, as needed to continue targetting my desired boil-off duration, and target post-boil volume. 

Using this approach (and the mash approach described previously) I always end the boil with exactly what I expected (SG, IBUs, and volume).  Volume is the only thing that varies by any measureable amount, but even that is never more than a quart (and THAT would be a BAD day). 


Beer Lover

Grandmaster Brewer
Nov 3, 2014
Reaction score
Hi Tom,
Great post.  Good information.
I should have read this post earlier.



New Forum Member
Nov 19, 2015
Reaction score
I am sure I'm not alone in suggesting that the Beersmith Final Gravity estimates do not take into account the fermentability of the grains.  Carapils and crystal malts count the same as base malts in the calculation of FG. So, those figures are too low.
It would seem easy to put in the fermentability factor. Of course, all gravity estimates are estimates subject to a lot of things but I see no reason to build into the program an inaccuracy factor.
Otherwise I have no complaints.