This is what SoapMaker 3 says about SG:
"
The specific gravity (SG) of a substance is the ratio of its density to that of water. Water has a specific gravity of 1. Oils are slightly lighter than water, and generally have a specific gravity around 0.9 to 0.95.
SoapMaker uses Specific Gravity (SG) to implement several features:
· You can record your supply purchases (and track your stock) by volume if that's how you buy them. e.g. If you buy a gallon of oil, you can record the price you paid per gallon, and SoapMaker will tell you how many gallons are left in stock each time you make a batch.
If you record a purchase in volume units (e.g. ml), the SG is important because internally SoapMaker always saves unit cost as cost per gram, even if you choose to display it in volume units. If the SG is wrong, the conversion from the volume units to grams will be wrong, and so will the unit cost.
· You have the option to enter recipe quantities in volume units (cups, Tbsp, etc.) and SoapMaker will convert to weight units (e.g. oz or lbs).
Note: Using volume measures for dry or granular ingredients is inherently inaccurate because of entrapped air - use weight measures instead.
· You can specify your product size in volume units. (e.g. if you make liquid soap and sell it in 10 fl oz bottles, you can tell SoapMaker your "portion size" is 10 fl oz and it will calculate the number of portions in a recipe and the cost per portion.)
· You can have SoapMaker resize a recipe to fit a mold.
All of these features involve conversion between weight and volume, and this can only be done accurately if we know the SG of each ingredient."