This calculator's main function is to predict the original gravity of a batch of beer based on the ingredients in the recipe. The original gravity is a measure of the sugars present at the start of brewing. Later, most of these sugars will be converted to alcohol by the fermentation process. The original gravity of the batch is a function of three things: the volume of water, the amount and type of ingredients that contribute sugars, and the efficiency with which sugars are extracted from the ingredients.

The first box allows you to enter the Batch Volume in gallons. This is the total volume of wort at the start of fermentation. Increasing this volume while keeping the total amount of sugars constant has the effect of diluting the wort and lowering the gravity.

The second box allows you to enter the Brewhouse Efficiency. This value represents how efficienctly you are able to extract sugars from grains at your brewery. It is the percentage of potential sugars in the grains that make it into the wort. This value only applies to grains. By contrast, ingredients like refined sugars and extracts (DME and LME) will disolve completely in the wort and always contribute 100 percent of their sugars. To estimate the efficiency of your brewing equipement you must work backwards from measured original gravities.

In the table below, each row represents the addition of one fermentable ingredient. You can use any combination of pounds and ounces (in whole or decimal numbers) to enter the weight of an ingredient. Next in the field labeled "Fermentable" you can type the name of a fermentable or select from a list of commonly used ones. After a fermentable is selected the remaining fields will fill with default values for that fermentable.

The rest of the table row provides a link to a page where you can learn more about the fermentable. The "Potential" field indicates a typical potential extract for the fermentable given as the specific gravity that would result from one pound of the fermentable in one gallon of water. The value of the "Potential" field can be changed if you believe the ingredients you are using differ from this typical value. The "°L" field gives the color for the fermentable in degrees Lovibond. The same as for potential, you can change the default value. The "%" field gives the percent of the total grains this fermentable comprises. Extracts and sugars are exempt from this calculation. "Part S.G." gives the partial specific gravity that would be produced by this fermentable ignoring the others. Due to rounding errors, these partial values will sometimes appear to have minor inconsistencies with the total original gravity value.

Below the fermentables table are the summary results for this batch. This lists the total weight of grains used, the original gravity of the batch of beer, and a prediction of the beer color in SRM.