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Alcohol and Attenuation Calculator

Enter Your Specific Gravity Readings into the calculator below to estimate your beer's alcohol content and your fermentations attenuation. Please note that SG readings must be entered in full. eg. 1.059. (1059 or 59 will not work).

 OG (eg. 1.059) FG (eg. 1.010) Alc/Vol % Alc/Weight % Apparent Attenuation (%) Actual Attenuation (%)

Explanation of the Calculator.
If you like you can read what equations are used in the above calculator below.

Calculating Alcohol by Volume (Alc/Vol)

There are many calculators out there that estimate the alcohol content of beer using Specific Gravity (SG) readings. They all pretty much follow the formula:

Alc/Vol % = (OG – FG) x f

Where:
OG = Original Gravity = Specific Gravity of the beer before fermentation
FG = Final Gravity = Specific Gravity of the beer after fermentation
f = a correction factor connecting change in gravity with alcoholic strength

Unfortunately f is not constant and ranges from 125 for weak beers to 135 for strong beers (Briggs, Boulton et al. 2004). Many alcohol calculators will simply use an f value of around 130 and not take into account variations. To make our calculator more accurate than most we have used f values produced by the Laboratory of the Government Chemist in the UK. For taxation purposes the UK government have released a table of f values to be used in alcohol calculations for small breweries. This is shown in Table 1 below:

Table 1. f values for various beer strengths (HM Revenue and Customs (UK) 2008).

 (OG – FG) % ABV Factor (f) Up to 0.0069 Up to 0.8 125 0.0070 – 0.0104 0.8 – 1.3 126 0.0105 – 0.0172 1.3 – 2.1 127 0.0173 – 0.0261 2.2 – 3.3 128 0.0262 – 0.0360 3.3 – 4.6 129 0.0361 – 0.0465 4.6 – 6.0 130 0.0466 – 0.0571 6.0 – 7.5 131 0.0572 – 0.0679 7.5 – 9.0 132 0.0680 – 0.0788 9.0 – 10.5 133 0.0789 – 0.0897 10.5 – 12.0 134 0.0898 – 0.1007 12.0 – 13.6 135

Note: SG and f values have been changed from the original HM Revenue table by a factor of 1000 to adjust for SG values written in true form in our calculations (eg. SG as 1.051 instead of 51 and f as 125 instead of 0.125).

From Table 1 we created a graph, using an excel spread sheet, and inserted a linear trend line (see figure 1 below). The equation for this trend line (f = 100.3 x (OG – FG) + 125.65) is used in our calculator to calculate the f value (please note that the correlation between beer strength and f value is not exactly linear, however it is very  close and is more than good enough for the purposes of this calculator).

Figure 1. Chart of f values vs change in SG showing linear trend line and equation used to calculate f. By inserting the equation for f into our original alcohol calculation formula it can be rewritten as follows.

Alc/Vol % = (OG – FG) x f

Alc/Vol % = (OG – FG) x (100.3 x (OG – FG) + 125.65)

This is the equation used in our calculator. It is probably the most accurate calculator you will find on any homebrew web site.

Calculating Alcohol by Weight (Alc/Weight)

The following formula from New Brewing Lager Beer by Greg Noonan (Noonan 1996) is used to calculate Alcohol by Weight:

Alc/Weight % = Alc/Vol % x 0.814

Calculating Apparent Attenuation

Attenuation is a measure (usually expressed as a %) of the amount of wort sugar that is fermented into alcohol and carbon dioxide. The apparent attenuation is calculated by comparing the drop in SG during fermentation to the OG of the beer before fermentation using a hydrometer. This is done using the following equation that is used in our calculator:

Apparent Attenuation % = ((OG-1)-(FG-1)) / (OG-1) x 100%

(note: the - 1’s in the equation are used to exclude the contribution of water ( SG = 1.000) in the measurements)

If you are doing this equation by hand it is easier to use Gravity Units (also known as Excess Gravity units) where Specific Gravity is expressed as a whole number using the significant figures after the decimal point. eg. 1.048 becomes 48 and 1.009 becomes 9. If you use Gravity Units the equation can be rewritten as:

Apparent Attenuation % = (OG-FG)/ OG x 100%

Apparent Attenuation is so named as it measures the amount of sugars that appear to be fermented using measurements taken on a hydrometer. Apparent Attenuation does not take into account the influence of the lighter density of the alcohol that is in solution after fermentation and therefore the actual amount of attenuation is lower than the apparent attenuation. As Apparent Attenuation is easy to measure it is the most common form of attenuation referred to in texts, recipes and yeast specs sheets.

Calculating Actual Attenuation

As already mentioned Apparent Attenuation doesn’t take into account the influence of the lighter density of alcohol compared to water (At 20 degrees C, SG of water = 1.000 and SG of alcohol = 0.798) (Noonan 1996). To estimate the actual amounts of sugars fermented we need to take this into account.  This can be done by converting Apparent Attenuation into Actual Attenuation using the following formula, from Brewing Lager Beer by Greg Noonan (Noonan 1996), that is used in our calculator:

Actual Attenuation % = Apparent Attenuation % x 0.814

Bibliography

Briggs DE, Boulton CA, Brookes PA, Stevens R (2004) 'Brewing - Science and Practice.' (Woodhead Publishing Limited: Cambridge)

HM Revenue and Customs (UK) (2008) BEERPOL12030 - Appendix D – Alcoholic strength: Calculation of Alcoholic Strength. In 'Beer Policy Guidance'. (HM Revenue and Customs (UK),)

Noonan GJ (1996) 'New Brewing Lager Beer.' (Brewers Publications: Boulder, Colorado, USA) pint.com.au Web

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