How to Read a Hydrometer
Submitted by: Bob Zormeir

Hydrometers are calibrated for readings at 60^o but the adjustment between 60^o and room temperature is minimal. To make your readings (and your ABV calculations) consistent the main thing is to always measure at the same temperature. To adjust the specific gravity reading for temperature differences, add +0.001 if you're measuring at 70^o instead of 60^o. Adjust by +0.002 at 77^o. Adjust by +0.003 at 84^o. Adjust by -0.005 at 50^o. If want to just measure the alcohol, but you don’t care about the precise alcohol level, adjustments for temperature fluctuation is an area that easily lends itself to the "Relax, don’t worry, have a homebrew" rule.

Before measuring, make sure both the hydrometer and hydrometer jar are clean. Hydrometers are made of rather thin glass, so exercise due caution. Strangely enough, even fingerprints on the bulb can theoretically affect the reading you will get. Make sure you wort is also clean and free of hop particles, etc. Pour the wort carefully into the hydrometer jar to avoid bubbles, then stir gently to release any existing bubbles.

Carefully lower the hydrometer into the wort, holding the hydrometer at the top of the stem. Note the approximate reading, and then push the hydrometer into the liquid a fraction of an inch beyond its equilibrium position. Don’t overdo this because excess liquid on surface of the hydrometer can also affect the reading.

When you release the hydrometer it should rise and fall steadily, and after a few oscillations settle down to a position of equilibrium. Make sure the hydrometer is not touching the side of the hydrometer jar, and you're ready to take a reading.

Read the scales at the bottom of the meniscus rather than the top of the meniscus where the surface of the liquid curves up to touch the hydrometer stem. To do this, take the reading by viewing the scale across the surface of the wort, adjusting your line of sight until it is at the same level as the liquid surface of the wort.

And don’t forget to get both the starting AND final gravity, as I sometimes do. Not all fermentations convert all the sugar to alcohol so when the fermentation has finished, you need to take a second reading to see how much sugar is left over, if any. Compare the two readings to determine the current alcohol of your beer. The most reliable readings will be before fermentation starts, and after it stops.

A hydrometer has three scales.

• Specific Gravity This scale is based on the weight of water. A hydrometer floating in water it will read 1.000 Specific Gravity. A beer at 1.100 weighs ten percent more than the same volume of water. A beer at 1.090 weighs nine percent more. A beer at 0.995 weighs half a percent less. Alcohol weighs less than water, so when all the sugar is turned into alcohol you can have a Specific Gravity reading that is slightly less than water.
• Potential Alcohol This scale tells the brewer how much alcohol can potentially be made from the sugars that are currently in the wort, if all of the sugars are fermentable. A single reading on the Potential Alcohol scale can NOT tell how much alcohol is already in your beer, or exactly how much can be made.
• Plato/Brix/Balling This scale is used mostly by commercial breweries. The Plato scale is based on percentage of sugar that is in the liquid by weight. The scale goes from 0 to 30 on most hydrometers. If your wort reads 10 on the Plato scale, it contains 10% sugar by weight.

How accurate is you hydrometer, and how good a job are you doing at reading it? If you float you hydrometer in distilled water at 60^o and Specific Gravity reads "1.000" you both both passed the test.