Winemakers routinely need their fruit juice (must) to have a certain specific gravity before allowing it to ferment. The specific gravity (SG) of the must is a direct measure of its sugar content. Winemakers typically want the must to contain more sugar than is normally in fruit juice, so they will need to add table sugar to the must.
Things You'll Need
 Hydrometer or saccharometer
 Calculator
 1 L must
 156 g sugar

Measure the specific gravity of the must with a hydrometer. Place the hydrometer in a sample of the must and read the value on the scale at the surface of the must. Assume for this example that the SG of the must is 1.05.

Calculate the percentage of sugar in the must, also known as the Brix value. This is given by the equation Brix = 261.3 x (1  1/SG). In this example, Brix = 261.3 x (1  1/1.05) = 12.4. A must with an SG of 1.05 is therefore 12.4 percent sugar. A special type of hydrometer known as a saccharometer will automatically perform this conversion for you.

Determine the Brix value for the desired SG. Winemakers commonly want the must to have an SG of about 1.1, so the desired Brix value in this example would be 261.3 x (1  1/1.1) = 23.75. The desired Brix value is therefore 23.75.

Measure the volume of the must. Some carboys will have markers that will allow you to measure the volume directly. You can also calculate the volume as the sum of the volume of the ingredients in your must. In other cases, you may need to pour the must into a measured container. For this example, the must will have a volume of one liter.

Calculate the amount of sugar to add to the must. This is given by the equation M = V x (SG) x (Bf  Bi) / (100  Bf) where M is the amount of sugar to add in kilograms, V is the volume of the must in liters, SG is the current specific gravity of the must, Bf is the desired Brix value from step 3 and Bi is the current Brix value from step 2. In this example, M = 1 x 1.05 x (23.75  12.4) / (100  23.75) = 0.157 kilograms. You would therefore need to add 157 grams of sugar to one liter of must to raise the SG from 1.05 to 1.1.