How do you use a hydrometer to measure alcohol content?!
You take the difference and multiply by 131.
ex.: SG (starting gravity) is 1.052, FG (final gravity) is 1.005
1.052 - 1.005 = 0.047 * 131 = 6.157% ABV
Answers: The way to find ABV with a hydrometer is to measure your specific gravity before fermentation has started (before you pitch your yeast), and then again after fermentation.
You take the difference and multiply by 131.
ex.: SG (starting gravity) is 1.052, FG (final gravity) is 1.005
1.052 - 1.005 = 0.047 * 131 = 6.157% ABV
You are actually measuring the Specific Gravity (SG) of wine or beer before fermentation.
You need to draw of a quantity of your brew into a Hydrometer Jar or similar item with sufficient depth. The brew should not be hot to prevent damage to your hydrometer.
Spin the hydrometer between thumb and fore-finger to eliminate air bubbles. (a false reading could be obtained if you have any bubbles!)
Take care not to read the hydrometer at the top of the meniscus (curved surface of the liquid) as a lower SG could be recorded.
The SG will allow you to estimate the potential alcohol content when fermentation is complete. You can also work out how much sugar you need to add to give the alcohol content you require.
If you have a hydrometer you should have instructions that will give you a chart showing SG and potential alcohol content.