One of the common questions asked when I mention to friends that I make mead is how do you determine the alcohol by volume. And until I researched this post, I thought this was a relatively straightforward process. It only required a simple math equation, right? If you start with a known quantity of sugar dissolved in a must, measure the starting specific gravity, and compare to the finished gravity, the entire change can be attributed solely to the production of alcohol, right? Well, it turns out this logic is flawed, and the solution will vary depending on the source.
Some Definitions
Before we get started, here are a few definitions that will help with the explanation to follow.
ABV (Alcohol by Volume): the percent of a fluid that is alcohol as measured by the solution’s volume
ABW (Alchohol by Weight): the percent of a fluid that is alcohol as measured by the solution’s weight
Alcohol Tolerance: a basic property of the yeast used to ferment a beverage. When a yeast reaches its alcohol tolerance, it usually goes dormant and stops fermenting. However, depending on other factors in the must, the yeast may over- or under-shoot the stated alcohol tolerance. As such, use this as a guide more than a definite measure.
Brix: See Degrees Brix.
Degrees Brix: measurement of the dissolved sugar to water mass ratio of a liquid. 25 deg Brix means 25 grams of sucrose dissolved in 75 grams of water (25% of final solution is sucrose [25/(25+75)]). Commonly used in commercial winemaking.
Degrees Plato: measurement of the dissolved solids to water mass ratio of a liquid. 25 deg Plato means 25 grams of solids dissolved in 75 grams of water (25% of final solution is dissolved solids [25/(25+75)]). Most often used measurement most for commercial and home brewing practices.
Gravity Point: one thousandth of a specific gravity measurement. If the SG is 1.050, there are 50 gravity points.
Original Gravity (OG): the specific gravity of a must or wort when yeast is first pitched to start fermentation
Final Gravity (FG): the specific gravity of a finished mead, wine, or beer
Plato: See Degrees Plato.
Specific Gravity: a measure of density, in the form kg/L, where the specific gravity of water at 60 deg F is 1.000
Starting Gravity: See original gravity.
Yeast Tolerance: See alcohol tolerance.
The Spirit Indication Test
The most accurate method available to most home meadmakers to calculate alcohol by volume is the Spirit Indication Test. This method involves boiling a known sample of mead (wine or beer) to remove all its alcohol, reconstituting the solution to the original sample volume, and then comparing the SG of the two samples to determine the alcohol by volume.
These steps are defined below:
1. Measure the SG of the finished mead.
2. Measure 250 mL of the finishedmead (wine or beer) with a graduated cyclinder and pour into a flask, beaker, or pot that can be boiled.
3. Boil the mead to half its volume. As alcohol’s boiling point is less than that of water, all of the alcohol in the mead will have evaporated.
4. Reconstitute the sample to its original volume, 250 mL, using distilled water. Distilled water has an SG of 1.000, so we can ensure that the only changes to SG from the original sample to the final sample are the reduction of alcohol and replacement with water.
5. Calculate ABV using either the table available here, or with an equation presented here.
One great feature of this method is that it only requires measurements of the final mead, not a measure of the initial must. However, that in itself presents issues, as the meadmaker does not know the status of the mead during its fermentation, and may schedule nutrient additions at inopportune times.
This method also involves the sacrifice of a portion of your finished mead. So, for those looking to calculate ABV without sacrificing their mead, a method using only measurements is ideal. 250 mL is only 1.25% of a 5 gallon batch (6.25% of a 1 gallon batch), but that is still a cup you can’t drink!
A similar method involves using an ebulliometer, but for home hobbyist use, this is an unnecessary investment. Instead, keep reading below to find ways that don’t require a significant sacrifice of the finished mead to determine alcohol by volume.
The Basic Calculation
One of the most referenced means to determine alcohol content is to subtract the final gravity from the initial gravity and multiply by a factor that has been determined experimentally. This is actually the method used in The Complete Guide to Making Mead, published just last year.
The general equation looks like this:
ABV = (OG – FG) * factor
*** Factor varies from 125 to 136, depending on the source
Generally in the brewing realm, you will see the factor most quoted around 131 (source, source, source). By comparison, winemaking sources often cite a larger value, with a popular factor working out to 135.87 (source). The difference in the multiplication factor is due to a number of factors, but most directly is a result of the relationship between alcohol content and the difference of OG and FG not being linear. Over large alcohol content ranges, using a single factor is insufficient for an accurate measurement.
The UK Government has produced guidance for breweries to calculate ABV, adjusting the factor depending on the quantity of OG-FG. The factors provided in the link are a factor of 1000 off the 125-136 range provided above. Their measurement is using gravity points, not specific gravity. Interestingly, I could not find a similar method searching the TTB website. If you have a link to one, let us know in the comments section or on twitter, and we’ll be sure to post it.
Potential Alcohol as a Function of SG
Another method uses the SG of the must and finished mead to estimate final alcohol by volume by comparing the potential alcohol of both solutions. This is very similar to the basic calculation, but with the use of the lookup table, one only has to subtract the potential alcohol of the initial must from the potential alcohol of the final mead.
The general steps to calculate SG using a potential alcohol measure are as follows:
1. Measure the intitial must SG.
2. Ferment your mead.
3. Measure the final initial gravity.
4. Determine the initial potential gravity using one of the following tables: American Homebrewers’ Association PABV from SG or Brewhaus’ PABV from SG.
5. Determine the potential gravity of the final mead using the same table as in step 4.
6. Subtract the step 5 value from the step 4 value. This is the mead’s alcohol by volume.
Note, the tables for this method (well, for all methods in this post), are not in agreement with eachother.
More Advanced Methods
The Hall Equation
In the Summer 1995 edition of Zymurgy Magazine, Michael Hall postulated a more advanced formula to calculate alcohol by volume. The article expands upon the flaws identified with the simple (OG-FG)*factor logic, and fits curves to experimental data to come up with an improved method to calculate ABV that does not require look up tables.
Building off the work of Carl Joseph Napoleon Balling, Fritz Plato, and Jean De Clerck, Hall presents the following equation based solely off OG and FG:
ABW = 76.08 * (OG – FG) / (1.775 – OG)
ABV = ABW / 0.794, where 0.794 is the SG of ethanol
This is the equation you will find in many of the more advanced calculators available online today.
Convert SG to Brix and Back
Another common method calculates ABV from the degrees Brix measurement. Degrees Brix is a measurement of the dissolved sugar to water mass ratio of a liquid. This is similar to SG, as degrees Brix can be converted to SG. To get to ABV, one calculates the potential alcohol content assuming the apparent extraction of alcohol from sugar. Ben Rotter at BRSquared has a great overview of this method here.
This method is actually very similar to the Hall equation, and uses the same Bates et al study from 1942 to correlate SG to degrees Brix as the basis for the calculations.
A simplified equation to calculate Brix from SG is as follows, where we have re-entered the Bates et al data to calculate a MeadMakr Brix equation as a function of SG:
Brix = 182.9622*(SG)^3 – 777.3009*(SG)^2 + 1264.5170*(SG) – 670.1831
By weight, the sugar to alcohol conversion results in 51.1% of the sugar mass converting to alcohol and 48.9% converting to CO2 (the 51.1 and 48.9 come from the chemical reaction to create alcohol from sugars, C6H12O6 –> 2(CH3CH2OH) + 2CO2, where the amu of sugar is 180, 2 alcohols is 92, and 2 CO2s is 88; for alcohol –> 92/180 = 0.511, and for CO2 –> 88/180 = 0.489). This equates to an alcohol conversion efficiency of 64.37% when converting back to volume (0.511 / 0.794). Thus, one would multiply the Brix measurement by .6437 to get the potential alcohol available.
However, like in the basic calculations, not all of the sugar is converted into alcohol. Some is lost due to the yeast population expanding and the yeast cells repairing themselves; some is lost with as other chemical byproducts are created; and some is lost to the atmosphere. Many calculations assume about an 90% efficiency (10% of the sugar is lost to these activities), so you would multiply by 57.93% instead of 64.37% to get the alcohol by volume approximation.
Potential Alcohol = Brix * 0.5793
Alcohol by Volume = Initial Pabv – Final Pabv
Additional reference tables for methods to convert Brix, SG, and potential alcohol may be found here, here, and here. Those looking for additional information may also consult this article over at morebeer.com.
Limitations to Alcohol Calculations
Although the general equation to create alcohol only has two by-products, ethanol and CO2, real world experience proves there are many other byproducts to this reaction. Thus the calculation of ABV is not just a measure of how much sugar was dissolved, but also how much other stuff may have been created.
Miscibility further complicates the problem. When alcohol is mixed with water, the total fluid volume is not simply the sum of the initial alcohol and the initial water (Does it add up?). Instead, the water molecules move into the interstitial spaces between the larger alcohol chains, resulting in a lower volume than may be expected. And this reduction in volume isn’t even predictable! Since volume is not conserved, by its nature a calculation for the amount of alcohol by volume is just an approximation, and not an absolute measurement.
It should also be noted that much of these calculations are based on what would occur in beer wort or wine must, and not in a mead must. Balling’s studies noted that the SG increases almost solely as a function of sucrose additions, not other dissolved solids. But that almost provides room for additional study with honey as its focus. And knowing that honey provides little in the way of yeast nutrients, much of the yeast assimilable nitrogen must come from a nutrient source. How this impacts these measurements is unknown.
The Best Method
So with all that said, what is the best method?
There truly isn’t one best method among all of these. If you prefer simpler calculations, stick to the basic calculation, (OG-FG)*factor, where factor varies from 125 to 136. Those seeking more advanced methods might consult the Hall equation or become familiar with Brix. Interestingly, no matter what method you use, you will likely be within 1% of the actual percent alcohol, and it is almost certain you will not predict the alcohol content exactly.
Even the TTB’s requirements allow for error in the measurement, as they only require that a wine’s alcohol content be within 1.5% of the actual content if the alcohol content is below 14%, or within 1% if above 14%.
There are many factors at play in calculating alcohol content, and even with thousands of years of fermentation experience (the oldest artifact with known alcohol content dates to 7000 BCE), humanity has a lot left to learn.
Let us know in the comments section what method you prefer, or if you know of a better one than we’ve mentioned.
If you like this article, or want to know more about how to make mead, check out the MeadMakr Guide and the MeadMakr Podcast. If you’d like to get in touch, hit us up through our Contact Page or follow us on twitter. We’d love to hear from you.