Hitting your target original gravity (OG) is important to brewers for many reasons – when you plan a recipe your OG will influence the balance of your beer, affecting the total alcohol, the balance of malt character and the perceived bitterness (missing your target OG can throw off your BU:GU ration and completely change the finish of your beer).
GU x WORT VOLUME (Gallons)
So what good does knowing the total gravity do us? Well, we know it doesn’t change so we can say that the total gravity at the beginning of the boil will be the same as the total gravity at the end of the boil. And then if we break total gravity into the components that make it up we get the equation;
And if you divide total gravity at the beginning by the volume at the end you should end up with the gravity units at the end which can be expressed as;
So to run through an example, if you have 28 L of preboil wort which has a specific gravity of 1.043 and you know that post boil you will have 23 L you can work out your final gravity like this;
If you need to dilute your volume for any reason simply use the volume after dilution as the ‘volume at end’ value.
Knowing what your gravity is at the end enables you to make the necessary adjustments in order to accurately achieve your target gravity. So if the result comes out lower than you expected for your target you can either boil for longer, lowering the ‘Volume at end’ figure until you achieve the gravity you want or you can add malt extract in order to raise the gravity.
Similarly, if the gravity is higher than you expected you have multiple options. You can produce a larger end volume, either by liquoring back or boiling for a shorter period or you can reduce your start volume by removing a portion of the wort prior to the boil.
This is where that total gravity figure comes in useful again. If you know you want a final volume of 23 L (6 US Gal) at 1.052 you multiply those figures (52 x 6) to get a total gravity of 312 GU. Now you know your total gravity units if you fall short or overshoot it’s easy to see what adjustments you make. If you calculate your final gravity at 1.048 you know that your total gravity is (48 x 6) = 288 GU which is 24 GU short of what you need.
So in the above example you might want to add extract to raise your gravity units. You can work it out using this equation;
Which is taking the difference between your target gravity and the actual total gravity and dividing it by the extract/pound value for the type of extract that you use.
If you are using liquid malt extract, divide the amount of gravity units you are short (24 GU) by 38 GU per pound to get 0.6 lb.
If, on the other hand you decide to try and hit your gravity by boiling, you can calculate the volume of beer needed for a gravity of 1.052 by dividing the total gravity (preboil) by the desired gravity units (52). Using the example above, this is;
and taking the example where you missed your GU by 24 you get;
So you can see how you can quickly adjust to ensure you don’t miss your targets.
If you find you are going to overshoot your target gravity, simply divide your total gravity units by your desired gravity units to see how much wort you need to hit your target gravity, for example;
A page of equations can always look pretty scary but essentially you are concerned with specific gravity (expressed as units) and volume. The equations just use these to help you determine and hit your gravity.”
Once you get into the habit of measuring your gravity throughout the process and using these equations, adjusting your gravity will become easier and you’ll get much better results from brewing the beer you planned rather than just accepting what you get. A handy tool for measuring your gravity before fermentation is a refractometer.