There are many brewing programmes and apps available these days that allow you to calculate beer recipes very effectively.

However, it's a lot easier to devise recipes if you have an understanding of how the quantity and type of ingredients you use will affect the strength of the finished beer. This page explains just what you need to know to work out what's going on with your grist.

Alcohol in beer is formed by yeast acting on fermentable sugars in the wort. The sugar can come from several sources: -

- Malt Grains
- Malt extract
- Adjuncts such as rice and maize
- Directly from household sugar (sucrose)

Malt is, of course, malted barley and in our British ales, the vast majority of fermentable sugar comes from Pale Malt. Malt contains starch and also enzymes that, under suitable conditions, can break the starch down into two types of sugar: -

- Maltose; a readily fermentable sugar
- Dextrose; a somewhat less fermentable sugar, which therefore contributes to residual sweetness in the finished beer

The 'suitable conditions' are, of course, exactly those that we strive to create during the mash.

We can also use darker malts, which have been heated further after initial malting, caramelising some of the sugars to produce different flavours. Generally speaking, dark roasted malts (with the exception of Crystal Malt) provide little fermentable sugar for the brew. Dark malts are added when we wish to produce a darker beer, such as brown ales or stout.

Malt extract is simply a concentrated solution of wort which has already been mashed, and so contains maltose and dextrose.

Usually in the form of flaked rice or flaked maize. These adjuncts contain starch, and rely on the fact that the enzymes in the malt can convert more starch than is present in the malt itself.

In order to work out the strength of beer our recipe will produce, we need to know how much sugar each of the ingredients will contribute to the wort. This depends on three factors: -

- The potential sugar in the ingredients
- The efficiency with which that sugar is extracted
- The final volume (length) of beer

Each ingredient provides a certain amount of sugar; the following table shows how much extract can be obtained from each type. The table shows the approximate potential extract (expressed in degrees of gravity) available for 1lb in a gallon (UK) and 1kg in a litre of water.

Ingredient | Extract for 1lb in 1 gal |
Extract for 1kg in 1 litre |

Pale Malt | 30 | 300 |

Lager Malt | 30 | 300 |

Crystal Malt | 24 | 240 |

Malt extract | 30 | 300 |

Flaked Rice | 30 | 300 |

Flaked Maize | 30 | 300 |

Flaked Barley | 30 | 300 |

Household Sugar (sucrose) | 37 | 370 |

Glucose | 30 | 300 |

To work out the total potential extract, we work out the extract from each ingredient and add them together. The formula for calculating extract is below: -

Example; take the following recipe for 5 gal (23 litres) of beer: -

- 3.8kg of Pale Malt
- 250g of Crystal Malt
- 500g Flaked Barley

First, calculate the potential extract from each ingredient:

Pale malt: | extract = 3.8 x 300/23 = 49.57 | |

Crystal malt: | extract = 0.25 x 240/23 = 2.61 | |

Flaked barley: | extract = 0.5 x 300/23 = 6.52 |

The above calculation gives the theoretical maximum figure for the original gravity of the brew. In practice, losses will occur throughout the brewing process due to:-

- Unconverted starch after mashing
- Extract left in the grains after sparging
- Extract left in the ullage (the liquid remaining in the vessel) when sparging and boiling

The above factors are dependent on your particular brewing setup and the only way to find them out is to measure your actual original gravity with a hydrometer and compare it to the theoretical figure. In practice, most people achieve an efficiency of between 80 and 90%. Thus for the above example, assuming an 85% efficiency, the actual extract would be:

58.7 x 0.85 = 49.9 degrees of gravity

Thus your hydrometer should show an original gravity reading of around 50 (or 1.050).

The next thing to consider is how original gravity is related to the alcohol content of the finished beer.

The sugars in the wort do not ferment out completely; if they did, the beer produced would be thin and dry tasting. Instead, due to the presence of relatively unfermentable dextrin, fermentation abates at what is known as the 'quarter gravity' stage. Thus, for our example above, we would expect fermentation to stop at around a specific gravity of 12. The actual figure depends on the mash conditions and the recipe, but will always be in the same ballpark, assuming the brewing procedure has been carried out correctly.

So armed with this knowledge, we are in a position to calculate the alcohol content of our brew!

Alcohol content depends on the amount of sugar converted by the yeast, and this is indicated by the difference in specific gravity before and after fermentation (I will refer to this as the 'gravity drop'). Once the gravity drop is known, a very simple formula can be used to calculate alcohol content: -

Thus for our recipe above, the ABV = (50 - 12)/8.06 = 4.7%

This is the usual way alcohol content is stated on beer in the shops; if you prefer to know alcohol content by mass (weight) use the following formula: -

*Note: there are varying opinions on the exact value of the constants to use in the above formulae - mine err on the conservative
side for alcohol content.*

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