Measuring Specific Gravity and Alcohol Content

Specific gravity is a great indicator of when meads and wines are done fermenting. 

For very sweet (dessert-like) meads a final specific gravity of 1.020 is suggested. For very dry meads a specific gravity around 1.000 is suggested (or even less). The closer to 1.020, the sweeter the mead. The closer to 1.000, the drier the mead.  Based on your preference you can continue fermenting your mead to reach the desired sweetness.

You can measure the specific gravity by floating a hydrometer in your mead.  It's easiest if you use a graduated cylinder that is taller than your hydrometer (or any sort of container taller than your hydrometer).  Make sure you sanitize your tools before they come in contact with your mead!

The specific gravity can also be used to determine the alcohol content if you take it at the start of fermentation and when fermentation is complete.

All you do is subtract the final SG from the initial SG and multiply by 136.

Example:
Sweet mead: (1.100-1.020)*136=11.56
Dry mead: (1.100-1.000)*136=13.6

If you are interested in a mead with a specific alcohol content you can start by calculating the theoretical initial specific gravity.  You would do this by first determining the volume of honey you plan on adding to your mead. A good estimation is a cup of honey usually weighs about 12 ounces, so a pound of honey occupies about 1 1/3 cup. Specific​ gravity for home brewing purposes is the ratio of the density of a substance over the density of water. A good estimation for the specific gravity of honey is 1.41. To calculate the specific gravity of your mixture you would multiply the specific gravity of water by the volume of water and add it to the specific gravity of honey multiplied by the the volume of honey.  You would then divide that number by the total volume (weighted average).

Example:  I usually use about 3 pounds of honey in my 1 gallon batch.  3 pounds of honey is about 4 cups.  There are about 16 cups in a gallon, so the remaining volume is water.

[(SGw*Vw)+(SGh*Vh)]/Vt
[(1*12)+(1.41*4)]/16=1.1025

For the last 1 gallon batch of mead that I made with 3 pounds of honey, the hydrometer reading had an initial specific gravity of 1.1 - so although this is a rough estimation, we came pretty darn close.
DISCLAIMER: Many assumptions were made and this is for estimation purposes.  If you need a more accurate estimation, let me know your specific recipe and I may be able to help.

Now that you have your theoretical initial specific gravity, you can calculate your theoretical final alcohol content if you know what you want your final specific gravity to be.

If you do not enjoy doing these mathematical calculations - gotmead.com has a very great tool for determining the alcohol content. The link is: http://gotmead.com/blog/the-mead-calculator/