Calculations
Calculations are a key part of beer brewing, some of which are essential for the home-brewer, and some of which are not essential but still interesting. I have included a Design Form, that can be used to record your beer design, and in which all of the following calculations are done automatically. After completing the Design Form, a brew day Log Sheet is automatically loaded with your design, which may then be printed. You may also record your own design, or upload one of mine, on the Save/Load page.
Mash thickness is the ratio of mash liquor (in litres) to the total grain bill (grist weight) (in kilograms).
The recommended range is between 2.5 and 3: the lower the ratio, the thicker the mash.
Total mash volume is a useful calculation, to ensure that your mash tun will be large enough:
Mash volume = (0.67 * grain bill in kilograms) + mash liquor volume (in litres)
Estimated Absorption, that is, the amount of mash liquor that the mash absorbs, may be calculated as approximately 1.04 litres for every kilogram of grist.
Mash efficiency, that is, a measure of how thorough your mashing and sparging is, may be calculated as (Actual OG – 1000) / (Target OG – 1000)
So if your target original gravity is 1048, but your gravity after sparging is measured as 1040, then your mash efficiency equates to 83%.
Brew House Efficiency
A real world brewing system has additional losses after the mash process is complete. These includes boil off, deadspace during the boil and transfer and trub losses (the gunk left after the boil). Each of these will further reduce your original gravity into the fermenter. The number that captures all of these losses plus the mash efficiency is called the brewhouse efficiency. Brewhouse efficiency is typically about 72% for most home brewing size systems.
Water Amount
The total amount of water needed for your brew is calculated as follows:
(Target final volume / 0.96) + Loss due to evaporation during boil + Grain absorption + Equipment loss
where:
0.96 represents the generally assumed 4% loss during cooling.
Loss during the boil – during a 60 minute boil I usually lose 3 litres due to evaporation.
Grain absorption usually uses a figure of 1.04 litres per kilogram of grist weight, as noted above.
Equipment loss is an estimate of how much wort is left in the boiler, plus how much beer is left in the fermenter.
This total is the sum of mash water + sparge water. The amount of water needed for the mash is based on the mash thickness ratio (see explanation above). The amount of water need for the sparge is then the total minus the mash requirement.
Colour – EBC
The calculation of the colour of your beer in terms of its EBC value is quite involved – refer to the Beer Colour page.
Original Gravity is a function of the grain bill, efficiency and target batch size. The calculation of OG is quite involved – refer to the page on Original Gravity.
If you wish to predict the FG, you can estimate this by taking into account the attenuation rate of the yeast. For example if you have a yeast with a 75% attenuation rate and your original gravity is 1050, the estimated final gravity would be about 1012. This is only an estimate, and can be lower or higher, because there are many factors that affect the final gravity. These factors include fermentation temperature, amount of yeast pitched, the health of the yeast, the amount of oxygen present, the mash temperature, the amount of adjuncts used, the amount of nutrients available in the wort, and the flocculation rate of the yeast strain.
Bitterness – IBU
The calculation of the bitterness of your beer, in terms of its IBU value, is a function of the type of hops added to the boil, at what stage in the boil they are added, and the target batch size and OG. The calculation of IBU is quite involved – refer to the Bitterness page.
Alcohol content may be calculated from the Original Gravity (OG) and Final Gravity (FG), measured with a hydrometer (see photo):
Alcohol by Volume (ABV) percentage = (Original Gravity – Final Gravity) x 0.13
For example, for a beer whose measured original gravity (OG) is 1054, and whose final gravity (FG) is 1017, the resulting ABV would be 4.8%.
Volumes CO2
Force carbonating beer in a keg requires the correct regulator setting. The Brewers Friend Keg Carbonation Calculator calculates keg pressure (in PSI) to set the regulator to, at a given temperature and given volumes of CO2.