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ABV vs extract - how beer strength is really calculated

A beer label shows the alcohol content, but how is it actually known how much there is? The strength of beer is not magic or guesswork, but results from simple physics and mathematics, based on measuring the extract, that is the sugar content, before and after fermentation. The difference between these two measurements reveals how much sugar the yeast turned into alcohol. In this post we will explain how the strength of beer is really calculated: what original and final extract are, how degrees Plato work, what the ABV formula is, what attenuation is and why a distinction is made between apparent and real extract. It is a more technical topic, but explained step by step it will become clear. After reading, you will understand what really lies behind the percentage figure on the bottle.

Beer strength is not magic

The alcohol content of beer, given as ABV, that is percent by volume, is not a made-up value or one measured directly with a simple instrument. It results from a calculation based on measuring the density of the beer before and after fermentation. The crux is a simple principle: yeast eats the sugars dissolved in the wort and turns them into alcohol and carbon dioxide. The more sugar that has disappeared, the more alcohol that has formed. By measuring how much sugar there was at the start and how much remained at the end, you can calculate how much was lost, and from that how much alcohol was produced. That is why the strength of beer is not magic, but mathematics based on the physics of fermentation. Brewers and home brewers routinely perform these measurements and calculations. Understanding this principle demystifies the percentage figure on the label and shows that behind it stands a concrete, measurable process. It is the foundation on which the rest of this topic rests.

Original and final extract

Two measurements are key: original and final extract. The original extract, marked as OG from original gravity, is the density of the wort measured before fermentation, telling how much sugar was dissolved in the water. The final extract, marked as FG from final gravity, is the density of the beer measured after fermentation is complete. The drop from OG to FG shows how much sugar the yeast turned into alcohol. It is the difference between these two values that is the heart of the strength calculation. The bigger the difference, the more sugar fermented and the stronger the beer. The measurement is made with a hydrometer or refractometer. OG and FG are the basic data a brewer records for every batch. Without them you cannot calculate the alcohol. Understanding that the strength of beer is simply the difference between the sugar at the start and at the finish is the key to the whole calculation. The rest is just converting this difference into percent alcohol.

Degrees Plato versus gravity

Extract can be expressed in two ways, which is sometimes a source of confusion. The first is specific gravity, where pure water has a value of 1.000, and wort with sugar a slightly higher one, for example 1.050. The second is the Plato scale, often also called Balling degrees, expressing the percent by weight of sugar in the solution. For example, wort of an extract of 12 degrees Plato contains roughly twelve percent sugar by weight. Both scales describe the same thing, just differently, and can be converted. Roughly, one degree Plato corresponds to about four points of gravity, that is a value of 1.004. Importantly, there is a practical rule of thumb: every one degree Plato of original extract gives roughly about 0.4 percent ABV. So wort of an extract of around 10 degrees Plato will give after fermentation roughly four percent alcohol. It is a handy approximation that lets you quickly estimate the strength, knowing only the original extract.

How alcohol is calculated

With the original and final extract, you can calculate the alcohol with a specific formula. The simplest and most popular is: ABV is approximately equal to the difference between OG and FG, multiplied by 131.25, where OG and FG are given as specific gravity. For example, a beer of OG 1.050 and FG 1.010 gives a difference of 0.040, which multiplied by 131.25 gives about 5.25 percent ABV. This formula is easy to remember and accurate enough for most beers. For very strong beers more complex formulas are used, which better account for the non-linearity of the process, but the principle stays the same. This shows that calculating alcohol is simple arithmetic, once you know two density measurements. The multiplier 131.25 is a constant resulting from the physics of the conversion of sugar into alcohol. Thanks to this formula every home brewer can calculate the strength of their beer, having only a hydrometer and two numbers. It is a tool that demystifies the whole process.

Attenuation - the key to strength

A concept closely related to strength is attenuation, that is the degree of fermentation. Attenuation is the percentage of extract the yeast managed to process: it tells what part of the available sugar was fermented. The higher the attenuation, the more sugar was turned into alcohol, the more dry and strong the beer, and the lower the final extract. Low attenuation means more sugar remained, so the beer is sweeter, fuller and weaker. That is why two beers of the same original extract can have different strength: the more fermented one will be stronger and drier, the less fermented one weaker and sweeter. Attenuation depends on the yeast strain, the fermentability of the wort and the conditions of fermentation. It is a key parameter that links extract with the strength and character of the beer. Understanding attenuation explains why the original extract alone is not enough to predict strength, you also need to know how much of it was fermented. We write more about this in our post on attenuation.

Apparent versus real extract

Here we come to a subtlety that confuses many beginners: the difference between apparent and real extract. When you measure the density of finished beer with a hydrometer, you get the so-called apparent extract. The problem is that this measurement is distorted by the presence of alcohol. The real extract is the actual amount of sugar that really remained in the beer after fermentation. These two values differ, because alcohol affects the density reading. The apparent extract is always lower than the real one, that is the hydrometer shows less sugar than actually remained. There are formulas converting apparent extract into real, accounting for the influence of alcohol. For the ordinary beer drinker this distinction is a curiosity, but for the brewer and in precise calculations of calories or real attenuation it has real significance. Understanding that the hydrometer reading from finished beer is the apparent extract, rather than the real sugar content, is an important step in grasping beer strength measurements.

Why alcohol lowers the reading

Why does alcohol distort the density measurement? The reason lies in physics: alcohol is lighter than water, it has a lower density. When alcohol appears in beer, it lowers the overall density of the solution below what would result from the amount of dissolved sugar alone. A hydrometer measures the total density of the liquid, so the presence of light alcohol pulls the reading down. As a result the instrument shows a density lower than would result from the actual amount of remaining sugar, giving precisely the apparent extract. That is why the apparent extract is lower than the real one: alcohol masks part of the sugar, lowering the measurement. The more alcohol, the greater this distortion. Understanding this mechanism explains why the distinction between apparent and real extract exists at all and why correction formulas are needed. It is one of those nuances that at first glance complicate the topic, but once explained become logical. The lightness of alcohol is the key to this whole measurement puzzle.

A step-by-step example

Let us look at a strength calculation with a concrete example, step by step:

The table shows the full course of the calculation. We start with two density measurements: before and after fermentation. Their difference, multiplied by the constant 131.25, gives the alcohol content, here about five percent. We calculate attenuation as the ratio of the fermented extract to the original, here about seventy-six percent. It is a simple, repeatable scheme: measure OG, measure FG, subtract, multiply. With these data, anyone can calculate the strength of their beer.

What it means in practice

What does this knowledge give in practice, especially to home brewers and curious consumers? First, it lets you calculate the strength of your own beer, having only a hydrometer and two measurements, without relying on guesswork. Second, it explains why two beers of the same original extract can have different strength and character, depending on attenuation. Third, it helps understand labels and compare beers consciously. Fourth, it explains why a strong beer does not necessarily have a high final extract, because what counts is the difference, not the final value alone. Fifth, it makes you realise that the fullness and sweetness of beer relate to the remaining extract, and the strength to how much of it was lost. This knowledge turns the numbers on the label from an abstraction into concrete, understandable information about the beer. For the brewer it is a tool of control over the batch, for the consumer a key to a conscious choice. We write more about the numbers on the label in our post on the numbers on beer.

Common misunderstandings

A few misunderstandings have grown up around beer strength, worth setting straight. The first: that a high original extract always means a strong beer. Not necessarily, because what counts is the difference between OG and FG and the attenuation, not the original extract alone. The second: that a hydrometer reading from finished beer shows the actual sugar content. No, it is the apparent extract, lowered by alcohol. The third: that a strong beer must be sweet or dry. It depends on attenuation, because a strong beer can also be dry, when heavily fermented. The fourth: that calculating alcohol is complicated. The basic formula is simple arithmetic. The fifth: that extract and alcohol are the same thing. They are different: extract is the sugar content, and alcohol is the product of its fermentation. Dealing with these myths helps you better understand beer and its labels. The strength of beer is a logical, measurable process, not a set of mysterious numbers. It is enough to understand a few basic concepts to see clearly.

The key points in a nutshell

The strength of beer results from measuring the extract, that is the sugar content, before fermentation (original extract OG) and after it (final extract FG). The difference between them shows how much sugar the yeast turned into alcohol. The simplest formula is ABV approximately equal to the difference of OG and FG multiplied by 131.25; roughly one degree Plato of extract gives about 0.4 percent alcohol. Attenuation, that is the degree of fermentation, is key, because two beers of the same OG can have different strength. A hydrometer from finished beer shows the apparent extract, lowered by the light alcohol, not the real one. Strength is logical mathematics, not magic. Want to record the measurements and strength of your beers? Keep notes in the GustoNote app. See also our posts on the numbers on a beer label and attenuation.