This article is going to tell you about a simple calculation that will ensure that you hit your target Original Gravity (OG) every single time.
Whether you’re a home brewer or a pro brewer, this trick is for you.
By using this calculation several times during brew day, you will ensure that you have consistent wort and therefore consistent beer.
Consistency is key to product quality and ultimately, brand loyalty.
Your consumers depend on you to brew consistent, quality product.
Before You Brew – Set Your Product Specification
If you’re a professional brewer or you run your own brewing business, it’s imperative that you write out a specification of your product before you even think of mashing in.
That means writing your beer recipe in a format that’s irrespective of batch size.
So whether you’re pilot brewing a batch or you’ve been brewing the same product for years, you can ensure that your product stays the same without “tweak creep” setting in.
“Tweak Creep” is where a brewer constantly tweaks a recipe from batch to batch – which is perfectly OK – but only if you’re documenting your recipe changes.
If you don’t document your recipe changes, you’ll never be able to recreate that awesome batch you brewed last month.
While there are other numbers you need to specify for your product, for today’s purposes, the wort Original Gravity (aka OG) is the number we are interested in.
Mash Efficiency Is A Number You Cannot Fully Control
I speak to many home brewers and professional brewers and often they get themselves wrapped up in the intricacies of mash efficiency.
They often try and target as higher mash efficiency as they can.
But the thing is, while there are ways that you can skew mash efficiency in one direction or another (e.g. mill gap, liquor to grist ratio etc), it’s really a number you have little control over.
Once you have your equipment dialled in, you’ll find that mash efficiency varies with something as silly as the weather!
So what’s the point of making that number a target?
The same goes for the volume of wort you should collect.
Many brewing recipe apps say something like “collect X litres/gallons of wort.”
But what’s the point if those X litres/gallons of wort aren’t at the gravity you targeted?
The only number you have full control over on brew day is your OG.
You’re going to get whatever mash efficiency and volume you’re going to get on brew day.
So it’s your job as a brewer to hit your product specification’s OG.
Enter The Dilution Calculation
I’ll let you in on a secret – wort gravity is merely a ratio of water to sugar in your wort.
If you think about wort in this way, then you can extrapolate a calculation to determine what volume of wort at your target gravity if you know the volume and gravity of a sample of wort in your kettle.
The beauty of a ratio calculation is that units of measure become irrelevant.
So for gravity measurements, you can use SG, Plato, Brix, Baume or kg/m3 – it really doesn’t matter.
Same goes for volumetric measurements. Litres, BBLs, hL, Firkins, Hogsheads – same goes here.
As long as you are consistent with your units of measure, this calculation will work for you.
To make this calculation, you will need the following data and you’ll need to be able to accurately measure where required:
- Current Gravity (CG) – Use a refractometer or density meter to measure wort gravity quickly hot side. A hydrometer probably won’t cut it here because your wort may be too hot for an accurate hydrometer reading.
- Current Volume (CV) – Maybe your kettle has a measuring stick, sight glass or flow meter. Make sure this is calibrated and accurate.
- Target Gravity (TG) – This comes from your product specification.
The formula is as follows:
Nail That OG Every Time
So let’s run through an example of this calculation on a hypothetical brew day.
Our data is as follows (note, with SG we’re only going to use the gravity “points” i.e. SG – 1):
- CV = 1500L
- CG = 1.060 (that’s 60 points)
- TG = 1.056 (that’s 56 points)
Target Volume @ TG = 1500 x 60 / 56 = 1607L
Now, if we’re at the end of the boil, we need to adjust our wort gravity to hit our target spec of 1.056.
By subtracting the current kettle volume, we can determine the amount of liquor required to hit our target OG.
1607L – 1500 = 107L of liquor should be added to the kettle to make the wort 1.056.
Let’s try it again with US Units of Measure such as Plato and Barrels (bbl):
- CV = 12.82 bbl
- CG = 14.7P
- TG = 13.7P
Target Volume @ TG = 12.82 * 14.7 / 13.7 = 13.75bbl
And therefore our dilution volume would be 13.75 bbl – 12.82 bbl = 0.93bbl liquor dilution required to hit target gravity.
As you can see, the numbers for both calculations are identical.
There’s no reason that you shouldn’t be hitting your target OG for your product each and every time.
Personally, during a brew day, I use a refractometer to make this calculation during lautering so I can determine how much run off there is to go.
Making this calculation at the start of the boil is a great way to dial in more accurate hop charges in the kettle to hit target IBU.
Making the calculation at the end of boil helps you to work out how much liquor to dilute with so you hit your target OG.
And remember, it’s more difficult to take water out of wort than it is to put it in.
So it’s a good idea to boil your wort at about 10-20% over target gravity and dilute on every brew so you know you will hit your specified OG every time.
If you’d like your products specified in a way that ensures product quality and consistency each and every time, contact me to discuss your requirements.