Gravity

Ok I have been reading the book written by John Palmer at www.howtobrew.com and it talks about the starting gravity of the wort and how you can work this out I.E that one pound of malt extract per gallon would give you a gravity of 1.036 - 1.038 but the part I don't understand is how does this convert in to alcohol content ? I know it also depends on your yeast strain an the amount of the sugars that they are able to process but I am just after a general idea thans for any help

Justin
New brewer

The general idea is that for each 2.065 grams of extract (sugar) consumed yeast produce, on average, 1 gram of ethanol, 0.11 grams of biomass (more yeast) and 1.054 grams of CO2 (relying on memory here so these numbers may not be exact - they are only averages anyway). Thus if a brewer knows how much extract the yeast have consumed he can estimate the amount of ethanol. The extract consumed is estimated via the specific gravities of the wort pre and post fermentation. As a general rule ABW = 0.421*(OG - FG) where OG is the original gravity and FG the final gravity expressed in degrees Plato. Multiply degrees Plato by 4, divide by 1000 and add 1 to convert to specific gravity e.g. 10 °P = 1.040 SG. FG is the apparent final gravity and ABW is the alcohol by weight. Multiply by 1.25 to get ABV (alcohol by volume). The factor 0.421 is an average - the actual number to use is a function of OG.

This is the famous 'Balling Formula' first promulgated by Karl Balling. There are equivalents in wider use by the home brewing community.

Thank you for your help it sounds very complicated lol I will go through it all later and try and make a little more sense of it.

scorpien222 - while it may sound rather complicated, that is because you asked the question in the "Brew Science" forum . Generally, lets say you make up your wort to say 1.055 Original Gravity (OG), and the yeast finishes at around 1.012 Final Gravity (FG) which are numbers pretty typical for a basic ale recipe. If you look at the side of most hydrometers, across from where it gives you your Specific Gravity (SG), it will also give an Alcohol percentage. So a 1.055 OG = 7% (potential alchohol content if all sugar is used up), and a 1.015 FG = 1% (unused potential), so the actual beer would be roughly 7%-1% or 6% ABV (Alcohol By Volume).

These are just rough numbers, but they are close enough for most homebrewers. Remember, you can measure what your starting wort's OG is, but you cannot accurately predict how much sugar the yeast will consume until the fermentation is finished. Some manufacturers provide a "Attenuation Range" which is a estimate of how much sugar a yeast strain will use based on experience, but each different recipe will have slightly different attentuation.

Excellent thank you for your help that is more at the level I am at the moment I am still at the researching the hobby stage I haven't even bought any kit at the moment my journey of mastering home brewing has barely begun thanks again for the help and any more tips and pointers would be greatly received.

It's pretty easy to calculate ABV if you have both the OG and FG:

ABV = 131*(OG - FG)

So in the example given above with and OG of 1.055 and an FG of 1.015, your ABV would be:

131*(1.055 - 1.015) = 5.24%

The reason it is lower than the 6% estimated above is that the FG is taken from a sample that contains alcohol, so the potential alcohol scale on the hydrometer is inaccurate at that point.