Attenuation is a term often thrown around by home brewers at parties to impress non-brewers, but understanding the different forms of real and apparent extract and attenuation can help beginning and advanced home brewers alike. So in this two part series on the BeerSmith blog we take a look at beer attenuation in all of its various forms, and how you can use it in recipe design.

### What is Attenuation?

So you are at a party looking to impress the non-brewing muggles, but instead a brewing geek comes up and starts talking about original extracts, apparent extracts, and ABVs. Here’s how to tell if he really knows what he’s talking about:

Attenuation is nothing more than the percentage of the original extract that has been converted via fermentation to CO2 and alcohol (and a few lesser compounds like esters in small quantities). Recall that the basic brewing process for all grain starts with the mashing process, which converts your barley grain into sugary wort. If you are an extract brewer, then you just start with sugary wort syrup.

You boil the sugary wort, cool it, add some yeast, and fermentation starts. During fermentation a portion of the sugary wort is converted to alcohol (primarily ethanol). That portion of the sugar, expressed as a percentage, is the attenuation of the beer. Apparent attenuation is very easy to calculate as follows:

**Apparent_Attenuation_in_% = 100 * (OG – FG)/(OG – 1.0)**

where **OG **is your original gravity and **FG **is your final gravity. So if you have a beer with an original gravity of 1.050 and it finishes with a gravity of 1.010, the math works out to be 100*(1.050-1.010)/(1.050 – 1.000) which is exactly 80%. So for this example, 80% of the available extract in the wort fermented to become alcohol and CO2.

### What is Apparent Extract and Real Extract?

The gravity of beer is most often measured using hydrometers. However, hydrometers are calibrated to measure the sugar content of a solution of water. Finished beer, however, contains alcohol (ethanol) which skews the hydrometer reading because alcohol is less dense than water. Therefore, a hydrometer reading taken on finished beer will show lower (less extract content) than the beer actually contains.

Apparent extract (often written as AE) is the measured hydrometer reading for the finished beer, usually expressed in degrees plato by professional brewers. For a homebrewer, this is the same as your final gravity (FG), but convert it from a specific gravity to degrees plato if you want to sound like the pros. To do a rough Plato calculation in your head, one degree plato is approximately 4 points of specific gravity, so a finished beer with a specific gravity of 1.012 (1.012 is “12” points) is approximately 3 degrees plato. If you want an exact calculation you can use a tool like BeerSmith or an online converter.

Real extract (often written as RE) is the real extract content of the finished beer, accounting for the actual alcohol content and imperfect nature of hydrometers. Real extract can be calculated from the starting gravity and apparent extract (final gravity) as follows:

**Real_extract = 0.188 * Original_extract + 0.8192 * Apparent_extract**

where **Real_extract, Original_extract **(which is just your OG) and **Apparent_extract** (your FG) are all in degrees plato.

Now you know enough to be dangerous at dinner parties. Please join us next week for part two of this article, where we explore real attenuation, and also how to make proper use of attenuation when designing your own beer recipes. Thank you for joining me on the BeerSmith Home Brewing Blog. Don’t forget to subscribe to our weekly brewing newsletter, and have a great brewing week!

MPBCBrewerThanks Brad, I enjoyed this article. Is there any chance that part 2 will explore the use of a refractometer reading to calculate the RE. I think Colin Kaminski described this in a BYO article, but I got lost somewhere in the calculations 🙁

Brad SmithHi – I will likely have to put the refractometer in a separate article, as the calculations get complex. However I will try to write an article soon on this, as the use of refractomers is not well understood.

Brad

PeteBeerSmith software is awesome and makes the beer brewing process even more fun. I love spending time designing beers and seeing calculated stats. Then on brew day, I just print and brew. Although after reading this article, I have one slight confusion.

The article has a formula of Apparent_Attenuation_in_% = 100 * (OG – FG)/(OG – 1.0) When I plug in 1.047 OG and a 1.014 FG, I can calculate it is: 100 * (1.047 – 1.014) / (1.047 – 1.0) = 70.2% attenuation.

But in Brewsmith I get 69.2% using the Alcohol % tool. I only double checked it because it is the first time I got attenuation numbers below 70. I like dry beer.

Brad SmithHi – internally the program uses a slightly more accurate formula which is (oe-re)/(2.0665 – 0.010665 * oe) where OE and RE are the plato versions of the OG and FG.

MALACHY UDEZEWHAT IS THE DIFFERENCE BETWEEN OG AND OE. WHAT IS RDF. MALACHY

Nicolas SpogisHi Brad!

I have a simple question here. I’m using in my recipe an S-04 Yest, that have an min attenuation of 71% and a max attenuation of 75%.

When I go in my recipe on the Tab Fermentation, I found:

Est Original Gravity – 1,060

Est Final Gravity – 1,011

Using your formula, I’ll found an attenuation of 81,67% (1,060-1,011)/(1,060-1,000). This will be so hight an more than the max yeast attenuation.

There is a bug here?

Bets regards,

Nicolas

Brad SmithDo you have any sugar in your recipe? Sugar will result in higher attenuation and BeerSmith does factor this in when calculating.

Nicolas SpogisYes Brad! There is 6% of Demerara Sugar in my recipe.

Thanks for you reply! Regards, Nicolas

AndayImpressive stuff. That Dog’s Head of circa 1900 is almost 100% atetnuated which must largely explain the very high ABV for this bottling. Dog’s Head was I believe a trade mark used in the Far East. I wonder if long shipment of bottle-conditioned beer to warm climes would explain the unusual dryness of the beer.Gary