If this is your first visit to Homebrew Notes, read THIS first.
Homebrew Notebook Page: Summary Homebrew Note – Correcting Gravity and/or Volume After Mash
Status: Ready to Use!
Source(s): This Homebrew Note was developed using three Brew Your Own Magazine articles – Hitting Target Original Gravity and Volume, Wort Boiling, and Extract for All-Grainers.
Contents
Why This is Important and/or Interesting
Consistency is one of the primary goals of homebrewing. You want to brew the beers you like the same way every time and you want your process to be dialed in enough that if you make a change to your system or process, you know that change is what produced differences in the final beer – not other sources of unintended process variation. This is a primary reason that homebrewers take so many measurements throughout the homebrewing process. They want to make sure their process is improving, i.e. becoming more replicable and consistent.
Two measurements used to monitor and verify accuracy are pre-boil gravity and volume. With a few simple calculations, you can measure your actual gravity and volume against a target gravity and volume. When you hit those targets, you can feel confident that you are far more likely to hit your final gravity and volume targets as well.
Despite our best efforts, all grain brewers will often need to adjust our post-mash/pre-boil gravity and/or volume to hit our targets. Mashing is not a totally smooth and predictable process so even with focus, experience, and care, we will often be off more than we’d like. If you happen to knock back a few beers during brew time, your focus on process becomes even more questionable. I know, I’ve been there. With that in mind, I thought I’d share my notes for the ways you can make corrections to realign your pre-boil gravity and volume with your targets.
Do You Know Your Pre-Boil Targets?
Before we talk about making adjustments to your pre-boil gravity and/or pre-boil volume, we need to make sure you know what your pre-boil volume and pre-boil gravity targets are! You have two ways of getting your targets. The first is to use software like BeerSmith or Brewer’s Friend to enter your grain bill, equipment details, and other information so the software provides these values as part of its normal calculations. Since software also provides a number of other important details (e.g. mash water volume, IBU levels), this is my recommended way of getting your target pre-boil volume and gravity. Find a homebrewing software platform to use. It’s worth the money.
The second way is to calculate it yourself using information provided in the recipe (batch size and boil time) and your kettle’s hourly boil-off volume (e.g. 1.5 gallons/hour). In the Process section that follows, I provide the steps and formulas for calculating a recipe’s target pre-boil volume and target pre-boil gravity manually.
If you are creating your own recipe from scratch, you’ll need to use the first option – software. While you can manually calculate all of the software data points manually, including final gravity and volume, that goes WELL beyond the scope of this Homebrew Note.
Our Example
Calculations are always easier to understand with an example. With that in mind, I’ll use the following example data for the calculations that follow.
- Recipe’s Target Final Volume = 5 gallons (going into fermenter)
- Recipe’s Target OG = 1.045 (going into fermenter)
- Boiling time = 90 minutes (1.5 hours)
- Your brewing system’s boil-off rate = 1.5 gallons per hour
Items Needed
NOTE: This post contains affiliate links. In the event of a sale, I will be awarded a small commission (at no additional cost to you). Don’t worry, I’ll just use the commission to buy more brewing stuff anyway.
- Calculator
- Food Scale
- Water
- Dry Malt Extract (DME)
Process: Calculating Your Target Pre-Boil Volume and Pre-Boil Gravity
REMINDER: This section isn’t necessary if you’re using brewing software. That software should provide your target pre-boil volume and target pre-boil gravity.
Step 1: Calculate the target pre-boil volume
FORMULA: Pre-Boil Volume = Recipe Batch Size + (Hourly Boil-Off Volume * Boil Time in Hours)
Example Calculations:
Pre-Boil Volume = 5 + (1.5 * 1.5)
Pre-Boil Volume = 7.25 gallons
Step 2: Calculate Your Target Pre-Boil Gravity
DEFINITION: Specific Gravity Points are the portion of the specific gravity reading to the right of the decimal point multiplied by 1000. In the example’s target OG above, a specific gravity of 1.045 is 45 specific gravity points (AKA gravity points). You can think of SGPs as the numbers left after dropping the 1.X (1.045 = 45, 1.105 = 105, 1.008 = 8, etc.)
FORMULA: Pre-Boil Gravity Points = (Post-Boil Volume * Post-Boil Gravity Points) / Pre-Boil Volume
Example Calculations:
Pre-Boil Gravity Points = (5 * 45) / 7.25
Pre-Boil Gravity Points = 31.03
Pre-Boil Gravity = 1 + (Pre-Boil Gravity Points / 1000)
Pre-Boil Gravity = 1.031
Following the above calculations, you know:
Target Pre-Boil Volume = 7.25 gallons
Target Pre-Boil Gravity = 1.031
Brewing software’s estimated values might not be identical to what the formulas above produce but they should be very, very close. For the gravity and volume adjustment calculations that follow, we’ll use these example target values as calculated.
OK, now that you have your target pre-boil volume and target pre-boil gravity, either from your brewing software or from your manual calculations, you’re ready to compare data once your mash is finished.
When your wort is post-mash and post-sparge (if applicable), you’re ready to confirm and/or adjust your volume and gravity. When you compare your targets to actual, one of these scenarios will occur:
- Scenario 1: Gravity Correct & Volume Correct
- Scenario 2: Gravity Correct & Volume High
- Scenario 3: Gravity Correct & Volume Low
- Scenario 4: Volume Correct & Gravity High
- Scenario 5: Volume Correct & Gravity Low
- Scenario 6: Gravity Low & Volume High
- Scenario 7: Gravity Low & Volume Low
- Scenario 8: Gravity High & Volume High
- Scenario 9: Gravity High & Volume Low
Below, we’ll see how to correct for each scenario.
Process: Correcting Your Pre-Boil Gravity and/or Pre-Boil Volume
SCENARIO 1: Gravity Correct / Volume Correct
You’ve hit your target pre-boil gravity and target pre-boil volume? Get to boiling and pour yourself a cold homebrew. You’ve earned that Brewmaster title today!
SCENARIO 2: Gravity Correct / Volume High
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 | Pre-Boil Measurements Pre-Boil Volume = 7.75 gallons Pre-Boil Gravity = 1.031 |
Only Step: Make no additions, just pour off additional volume to hit target pre-boil volume
FORMULA: Amount to Pour Off = Current
Pre-Boil Volume – Target Pre-Boil Volume
Amount to Pour Off = 7.75 – 7.25
Amount to Pour Off = 0.5 gallons
NOTE: Both gravity and volume are now at target!
SCENARIO 3: Gravity Correct / Volume Low
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Pre-Boil Measurements Pre-Boil Volume = 6.8 gallons Pre-Boil Gravity = 1.031 |
Step 1: Add water to reach target volume
FORMULA: Amount of Water to Add =
Target Pre-Boil Volume – Current Pre-Boil Volume
Amount of Water to Add = 7.25 – 6.8
Amount of Water to Add = 0.45 gallons
NOTE: Volume is now at target but the additional dilution has made gravity below target
| NEW Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 | NEW Pre-Boil Measurements Pre-Boil Volume = 7.25 gallons Pre-Boil Gravity = To Be Measured |
Step 2: Re-measure your gravity including the water addition
Step 3: With new volume and gravity values, jump to the SCENARIO 5: Volume Correct / Gravity Low section below
SCENARIO 4: Volume Correct / Gravity High
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 | Pre-Boil Measurements Pre-Boil Volume = 7.25 gallons Pre-Boil Gravity = 1.035 |
Step 1: Calculate and add water needed to adjust gravity to target
FORMULA: Volume of Addition = (Volume
before Dilution * (Actual Gravity Points – Target Gravity Points)) / Target
Gravity Points
Volume of Addition = (7.25 * (35-31)) / 31)
Volume of Addition = 0.935 (just under 1 gallon)
NOTE: Gravity is now at target but the additional water has brought volume above target
| NEW Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | NEW Pre-Boil Measurements Pre-Boil Volume = 8.2 gallons Pre-Boil Gravity = 1.031 gallons |
Step 2: Make no additions, just pour off additional volume to hit target pre-boil volume
IMPORTANT: Before you pour off the additional volume, make sure the wort is well mixed. You might even consider stirring the wort numerous times over several minutes (up to point of boil) before you drain the excess.
FORMULA: Amount to Pour Off = Current
Pre-Boil Volume – Target Pre-Boil Volume
Amount to Pour Off = 8.2 – 7.25
Amount to Pour Off – 0.95 gallons
NOTE: Both gravity and volume are now at target!
IN CASE YOU’RE WONDERING ABOUT THAT PREVIOUS SCENARIO: Yes, the amount of water you added (step 1) and the amount of wort you’re draining (step 2) in the above example are near identical. This is coincidental based on the example. However, it’s still helpful for understanding to think through the logic. Initially, you’re reducing the gravity of the full volume of wort by diluting the wort with plain water (no sugar). Subsequently, you drain off the wort (both water and sugar) to get it back to your target volume. Since the water and extract/sugar are being drained
in the right proportion, the gravity remains unchanged.
SCENARIO 5: Volume Correct / Gravity Low
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Pre-Boil Measurements Pre-Boil Volume = 7.25 gallons Pre-Boil Gravity = 1.027 gallons |
Only Step: Calculate the amount of dry malt extract (DME) to add in order to reach target gravity
FORMULA: Weight of Extract to Add = (Volume of Wort * (Target Gravity Points – Actual Gravity Points)) / Extract Potential Points
HELPFUL INFORMATION: DME Potential Points = 45 & LME Potential Points = 36
Using DME, the calculation would be:
Weight of DME to Add = (7.25 * (31 – 27)) / 45
Weight of DME to Add = 0.644 lbs or 10.3 oz
NOTE: Both gravity and volume are now at target!
SCENARIO 6: Gravity Low / Volume High
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Pre-Boil Measurements Pre-Boil Volume = 7.75 gallons Pre-Boil Gravity = 1.027 gallons |
Step 1: Pour off additional volume to hit target pre-boil volume
FORMULA: Amount to Pour Off = Current
Pre-Boil Volume – Target Pre-Boil Volume
Amount to Pour Off = 7.75 – 7.25
Amount to Pour Off = 0.5 gallons
| Post-Step 1 Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Post-Step 1 Pre-Boil Measurements Pre-Boil Volume = 7.25 gallons Pre-Boil Gravity = 1.027 gallons |
NOTE: Volume is now at target but gravity is still below target
Step 2: Calculate the amount of dry malt extract (DME) to add in order to reach target gravity
FORMULA: Weight of Extract to Add = (Volume of Wort * (Target Gravity Points – Actual Gravity Points)) / Extract Potential Points
HELPFUL INFORMATION: DME Potential Points = 45 & LME Potential Points = 36
Using DME, the calculation would be:
Weight of DME to Add = (7.25 * (31 – 27)) / 45
Weight of DME to Add = 0.644 lbs or 10.3 oz
NOTE: Both gravity and volume are now at target!
SCENARIO 7: Gravity Low / Volume Low
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Pre-Boil Measurements Pre-Boil Volume = 6.8 gallons Pre-Boil Gravity = 1.027 gallons |
Step 1: Add water to reach pre-boil target volume
FORMULA: Amount of Water to Add =
Target Pre-Boil Volume – Current Pre-Boil Volume
Amount of Water to Add = 7.25 – 6.8
Amount of Water to Add = 0.45 gallons
| Post-Step 1 Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Post-Step 1 Pre-Boil Measurements Pre-Boil Volume = 7.25 gallons Pre-Boil Gravity = To Be Measured |
NOTE: Volume is now at target but due to further dilution, gravity is even lower than before
Step 2: Measure new gravity
Step 3: With new volume and gravity values, jump to the SCENARIO 5: Volume Correct / Gravity Low section above
SCENARIO 8: Gravity High / Volume High
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Pre-Boil Measurements Pre-Boil Volume = 7.75 gallons Pre-Boil Gravity = 1.035 gallons |
Step 1: Add water to reach pre-boil target gravity
FORMULA: Volume of Addition = (Volume
before Dilution * (Actual Gravity Points – Target Gravity Points)) / Target
Gravity Points
Volume of Addition = (7.75 * (35-31)) / 31)
Volume of Addition = 1.0 gallon
| Post-Step 1 Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Post-Step 1 Pre-Boil Measurements Pre-Boil Volume = 8.75 gallons Pre-Boil Gravity = 1.031 gallons |
NOTE: Gravity is now at target but volume is even further above target
Step 2: Drain kettle to target pre-boil volume
FORMULA: Amount to Pour Off = Current
Pre-Boil Volume – Target Pre-Boil Volume
Amount to Pour Off = 8.75 – 7.25
Amount to Pour Off = 1.5 gallons
NOTE: Both gravity and volume are now at target!
SCENARIO 9: Gravity High / Volume Low
This scenario is the trickiest to fix because it really depends on which measurement is furthest from its target. To simplify things, I suggest adjusting the volume to target first by adding some water. After that adjustment, you can then see if your new gravity is above or below its target and take appropriate action. If you add the water to reach target volume and the new gravity is close to the target gravity, you can stop there. If not, follow the steps as detailed below.
| Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Pre-Boil Measurements Pre-Boil Volume = 6.8 gallons Pre-Boil Gravity = 1.035 gallons |
Step 1: Add water to adjust Pre-Boil Volume to Target Pre-Boil Volume
FORMULA: Amount of Water to Add =
Target Pre-Boil Volume – Current Pre-Boil Volume
Amount of Water to Add = 7.25 – 6.8
Amount of Water to Add = 0.45 gallons
NOTE: Volume is now at target but gravity is unknown
Step 2: Re-measure gravity
If re-measured gravity is now below target pre-boil gravity (example = 1.028), take the following Step 3. If it is above target pre-boil gravity, skip down to the next Step 3.
| Post-Step 2 Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Post-Step 2 Pre-Boil Measurements Pre-Boil Volume = 7.25 gallons Pre-Boil Gravity = 1.028 gallons |
Step 3: Calculate the amount of extract to add in order to reach target gravity
FORMULA: Weight of Extract to Add = (Volume of Wort * (Target Gravity Points – Actual Gravity Points)) / Extract Potential Points
HELPFUL INFORMATION: DME Potential Points = 45 & LME Potential Points = 36
Using DME, the calculation would be:
Weight of DME to Add = (7.25 * (31 – 28)) / 45
Weight of DME to Add = .0483 lbs or 7.7 oz
NOTE: Both gravity and volume are now at target!
If re-measured gravity is now above target pre-boil gravity (example = 1.035):
| Post-Step 2 Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Post-Step 2 Pre-Boil Measurements Pre-Boil Volume = 7.25 gallons Pre-Boil Gravity = 1.035 gallons |
Step 3: Calculate the amount of water to add in order to reach target gravity
FORMULA: Volume of Addition = (Volume
before Dilution * (Actual Gravity Points – Target Gravity Points)) / Target
Gravity Points
Volume of Addition = (7.25 * (35-31)) / 31)
Volume of Addition = 1.0 gallon
NOTE: Gravity is back at target but volume is now above target
| Post-Step 3 Example Data | |
| Recipe Targets Target Pre-Boil Volume = 7.25 gallons Target Pre-Boil Gravity = 1.031 gallons | Post-Step 3 Pre-Boil Measurements Pre-Boil Volume = 8.25 gallons Pre-Boil Gravity = 1.031 gallons |
Step 4: Drain kettle to target pre-boil volume
FORMULA: Amount to Pour Off = Current
Pre-Boil Volume – Target Pre-Boil Volume
Amount to Pour Off = 8.25 – 7.25
Amount to Pour Off = 1.0 gallon
NOTE: Both gravity and volume are now at target!
ALTERNATIVE: So you are aware, an alternative to the water/DME adjustment approach outlined above does exist. You could reduce your boil time in an attempt to boil off less – lowering final gravity, increasing final volume. Or, you could increase your boil time in an effort to boil off more – increasing final gravity, decreasing final volume. However, since both values (gravity and volume) would be moving in opposite directions, it’s highly unlikely that you would end up with the perfect amount of boiling time to hit both targets. As a result, you’d probably end up making one of the above adjustments anyway. That’s why I prefer to keep it simple and get my pre-boil gravity and pre-boil volume right with the above calculations and adjustments. I just wanted to make you aware that the boil time adjustment is an option in case you’d like to research it further.
You’ve successfully corrected your pre-boil gravity and volume! Since your wort is now aligned with your targets, you’re more likely to hit the final gravity and volume you’re after. Save and/or print the summarized Homebrew Notebook Page for use in the future.
Now that you’ve read this Homebrew Note, let me know if you have a question, recommended improvement, or other thoughts in the comments below. As I mention in About Homebrew Notes, these are living documents and your feedback is appreciated!