A hydrometer measures the weight of a liquid in relation to water. The weight of water on a Specific Gravity scale is expressed as 1.000. As you add sugar or other soluble solids, the numbers after the decimal point will increase, i.e. 1.010.

The Balling (or Brix) scale expresses percentage of sugar by weight.

The Alcohol scale is actually measuring potential alcohol. In order to determine the alcohol content of a brew, you will need to make two readings. one before fermentation commences and another after fermentation stops. Subtrace the final figure from the first reading and you will have the alcohol content percentage by volume.

For example:

1st Reading:      16%
2nd Reading:       4%
Alcohol Content:  12%
To use the hydrometer, put a sample of the brew in a hydrometer testing jar or similar clear glass container. Spin the hydrometer to dislodge air bubbles. At eye level read the figures on the stem of the hydrometer where the surface of the liquid cuts across the stem. This figure will tell you how much sugar is in your brew and the potential alcohol. You can then adjust the amount of sugar according to the type of brew you wish to produce.

Temperature Corrections

The hydrometer gives an accurate reading when the temperature of the liquid is 60% F. The higher the temperature the less dense the brew is. The following tables show how to correct for temperature difference.

Temperature (F)  Specific Gravity Correction

       50               Subtract 0.5
       60                     0
       70                   Add 1
       77                   Add 2
       84                   Add 3
       95                   Add 5
      105                   Add 7


Temperature of brew is 84 F.

Specific Gravity is:    1.100
Correction is:              3
Corrected Sp. Grav is:  1.103

Specific gravity is best measured using a hydrometer when the brew is between 78 F. Measurements should be made before pitching the yeast. Yeast should be pitched if the temperature of the brew is between 68 and 78 F.

The combination of 4.5 to 5 gallons of water with 5 lbs. of malt extract and / or corn sugar will result in a specific gravity of about 1.035-1.042. As the yeast ferments the dissolved sugars into alcohol and carbon dioxide the density of the liquid drops due to the lack of sugar in the solution and because alcohol is less dense than water.

Regardless of this, the final specific gravity reading taken before bottling should yield a reading of 1.005 to 1.017 (even higher for very heavy, all-malt beers). The reading will not reach 1.000 as there will be some residual, unfermented sugars that will give the beer body and a roundness to the flavor.

The brew is ready to bottle if the hydrometer readings remain unchanged for 2-3 consecutive days.

The alcohol content of the brew can be calculated by using the specific gravity or balling scale. For the balling scale, multiplying the difference betwen initial and final balling readings by 0.42 will yield approximate alcohol content.

Likewise, with specific gravity, multiply the difference of the readings by 105 to get percent alcohol by weight. (E.g. 1.040 - 1.010 = 0.030. 0.030 * 105 = 3.15%)

To convert between percent alcohol by weight and percent alcohol by volume, multiply percent by weight value by 1.25.

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Created: 08/09/2001
Last Modified: 08/10/2001