​Wonderful Wine Geekery: Using Math to Fortify your Wine

​Wonderful Wine Geekery: Using Math to Fortify your Wine

Posted by Matteo Lahm and Matt Pruszynski on 2nd May 2024

Ever wanted to fortify your wine but got intimidated by how to do it? If so, today is your lucky day. We are going to geek out with some math! Sounds thrilling right? Well, not to be sarcastic but, it is exciting. In this article you will learn the actual equations for calculating how much spirits you need to add to reach your desired ABV. Fortifying wine is not a matter left to chance. You want to be precise because adding too much or too little comes with different sets of problems. Let’s embark on a spirited investigation and learn how to do the job right.

Let’s start with why you might want to fortify your wine. The practice is either a matter of preference or to save a batch that would be otherwise undrinkable. With the former, it is pretty straight forward. You made wine and you want it to be stronger. In the case of the latter, there are several scenarios that call for fortification.

If you have a low-ABV wine that tastes flabby and might not have enough alcohol to prevent long term spoilage, fortification is a great option. If your wine falls into this scenario, it is important to err on the side of caution. If you do not plan to back sweeten, a little more alcohol can really impact the flavor profile and, you need to make sure the ABV increase can be supported by other factors like tannins and/or oak. Lighter wines, especially whites cannot really support anything over 14% without the alcohol dominating the other flavors. However, an oaky and tannic red can be pushed to 16 or even 17%. If you are unsure, it is best to be prudent. You can always add more alcohol but, you cannot get it out. So, once you have calculated your desired ABV, add the spirits of choice incrementally.

But sometimes, fortifying wine is more of a solution than just an option. It happens to be an ideal way to salvage a batch that has undergone a wild yeast, AKA pellicle infection. Pellicle infections are identified by that white film that grows over the top of the wine. Sometimes it even grows bubbles on the surface and has visible striations. They are most common with wines that have been stored in carboys with too much headspace. They like O2 and surface area. When both are present, they can and will grow.

Pellicle infections are common when you ferment fruit that has not been purged of wild yeasts. They cause your wine to become sour. To read more about pellicle infections, click here. Pellicle infections look scary but, they are not a death sentence for your wine and do not render it undrinkable. After all, it is a wild yeast infection but, an infection can add flavors to your wine that are not desirable if you do not take steps to correct it. In this case, fortifying your wine is a perfect solution because it will bring the ABV high enough that it kills off the infection. Also in this scenario, you can push your ABV up to around 18-20% and you will definitely want some sweetness to your wine to balance out the sour. 

So now that we have clarified why you would want to fortify your wine, let’s dig into how you do it.

Calculating the ABV of a Fortified Wine

Equation Defined 

(A x B) + (C x D) = E(D + B)

Variables Defined

A = ABV of Wine in percent

If you took a hydrometer reading and your wine fermented dry, you should know your final ABV.

B = number of Bottles of wine yielded from Batch

If you made a wine kit, your yield should be 28-30 bottles however if you have a fresh fruit batch and you are bottling in 750 ml bottles, you will get about 5 bottles per gallon.

C = ABV of Distilled Spirits

Just look at the proof and half it. That will give you the ABV of whatever spirits you are using.

D = number of bottles of Distilled Spirits added to batch of wine

This is one of the variables we will solve for in the example 2 below. If you know your desired ABV, this will be the unknown in your equation.

E = Target ABV of Total Batch

This is the other variable that will be addressed in example 1. If you want to calculate starting from a known number of bottles of spirits, this will be the unknown in your equation.

Example 1: In this example, we will solve for E (final ABV) and figure out the result of adding 4 ounces of distilled spirits per bottle to a Merlot.

A=0.135 – Your wine’s ABV is 13.5%

B=26 – You have enough wine to fill 26 bottles. This is calculated without the addition of the spirits.

C=0.50 – You are using spirits that are 100 proof, so your spirit ABV is 50%.

D=4.7 – For a 4 oz. per bottle addition, you will have 4.7 bottles of spirits for 26 bottles of wine.

E= Unknown 

Calculate E (ABV of Fortified Wine)

Step 1: (0.135 x 26) + (0.50 x 4.7) = E(4.7+26) Multiply the numbers in parentheses on the left. Add the numbers in the parentheses on the right.

Step 2: 3.51 + 2.35 = 30.7E

This is your initial result. Now add the numbers on the left together.

Step 3: 5.86 = 30.7E

Now divide both sides of the equation by 30.7. 30.7 is cancelled out on the right leaving you with just the variable of E.

Step 4: E = 5.86/30.7

Divide 5.86 by 30.7 and you have your answer.

Final result: E=0.191 which means your final ABV will be 19.1%.

Example 2: Now we will solve for D. You know your desired ABV but do not know how many bottles of spirits you need to add to a batch of Moscato.

A=0.085 – Your wine has an 8.5% ABV fermented dry.

B=26 – The number of bottles of wine you will have before adding the spirits.

C=.50 – You are adding spirits that are 100 proof or 50% ABV


E=.191- You want your wine to have a 19.1% final ABV.

Calculate D (# of bottles of Distilled Spirits needed)

Step 1: (0.085 x 26) + (0.5 x D) = .191(D+26)

Do your multiplication and parentheses first. On the right, you will have to multiply both figures in the parentheses by .191.

Step 2: 2.21 + .5D = 0.191D + 4.966

Your initial result. Now you need D in only one side of the equation to solve it. Subtract 0.191D and 2.21 from both sides of the equation. This will place both D variables on the left and both known numbers on the right.

Step 3: .5D - .191D = 4.966 - 2.21

Now perform subtraction. This will leave you with one number on the right and the combination of the two numbers on the left times D.

Step 4: 0.309D = 2.756

Your last step is to divide both sides of the equation by 0.309.

Your Result: D = 8.92 Bottles of Distilled Spirits needed to bring the Target ABV to 19.1%.

So, there you have it! Now you know how to calculate wine fortification. Like we always say, winemaking is both an art and a science. Today we really got into the science part and, if nothing else, now you see that your junior year algebra class had real, and important practical application. Here’s to fortified wine, and math! Cheers.