Bitterness to Gravity Units Ratio
This page is about the "Bitterness to Gravity Unit Ratio", which you need to know about if you have the details of a beer which you like but you want to make a stronger or weaker version.
If you look at my recipes you'll notice that there are some where I've modified the strength of a beer which I like but which may be too strong for me. If you, too have a recipe for beer which you want to make and would like to change the strength of it, then you need to change the bitterness as well, to match. There is a simple way of doing this which works well, which involves maintaining the "Bitterness to Gravity Units Ratio".
Bitterness Units are also known as IBUs or 'International Bitterness Units'. Gravity Units are just the last two digits of the Original Gravity of a beer, so if a beer has an Original Gravity of 1055 then it has 55 Gravity Units. If you want a mathematical formula it's simply GU=(OG-1000).
The Bitterness to Gravity Units Ratio is simply the first number (IBU) divided by the second (GU) and if you change the strength of a beer then you need to change the bitterness as well in order to maintain the same ratio that it originally had. If, for example, the original beer was 1068 with an IBU of 44 then it has a Bitterness to Gravity Units Ratio of 44/68. If you want to reduce the gravity to 1058, then you simply need to reduce the bitterness to keep the same ratio - i.e. to 58x44/68 = 37.5