Hydrometer – what is it and why do I need one?

A hydrometer is a tool used to measure the weight of a liquid in relation to to water. This measurement is called the Specific Gravity (SG) of a liquid.

In home brewing we use a hydrometer to measure the Original Gravity (OG) and Final Gravity (FG) of our beer. These two measurements, and a little math, help us to determine when fermentation is complete and how to calculate the alcohol content of our brew. Pretty cool, eh?


  • Always measure the Original Gravity (OG) on brew day after cooling your wort and before adding the yeast. (Most hydrometers are calibrated at 60F, but we can adjust for temp later.)
  • Take the Final Gravity (FG) reading after fermentation is complete (typically 2-3 weeks later).
  • Give the hydrometer a spin to dislodge any air bubbles.
  • Measure at eye level where the surface of the liquid cuts across the stem of the hydrometer.


Is fermentation complete? OG / 4 = FG

A simple guideline is that your FG should be about 1/4 of your OG.

Pure water has a Specific Gravity (SG) of 1.000 at 60F. As you add sugar the number goes up. Your wort might be 1.055 on brew day because you’ve added fermentable sugars to the liquid. Then you pitch your yeast and store the fermenter at a consistent temp (60-70F) for two weeks. As the yeast eats the fermentables the SG of the liquid goes back down. So two weeks later lets say you get a reading of 1.014.

Do a little math…
OG / 4 = FG

So in this example, take the OG of 1.055 and grab the last two digits, 55. Divide by 4, and see what you get.

55 / 4 = 13.75

Rounding up makes 14. Since your FG was 1.014, it’s safe to assume your yeast is done, fermentation is complete and you are ready to bottle.

What’s my alcohol content? (OG – FG) x 131.25 = ___% ABV

Another cool thing about using a hydrometer is calculating your alcohol content.

Do a little math…
(OG – FG) x 131.25 = ___% ABV

Let’s determine the Alcohol By Volume (ABV) of the brew in the hypothetical example above.

1.055 – 1.014 = 0.041
0.041 x 131.25 =  5.38125
rounding up gives us 5.4% ABV

temperature adjustments

Most hydrometers are calibrated at 60 degrees F. If your liquid is not at 60F you can still take your measurement and adjust for temp. Here’s a table to correct for temperature.

Temp in degrees F SG correction
50 Subtract .005
60 no correction needed
70 Add .001
77 Add .002
84 Add .003
95 Add .005
105 Add .007
110 Add .008
113 Add .009
118 Add .010

So, let’s say it’s brew day and you are chilling your wort. Rather than waiting for it to get down to pitching temp, you decide to take a hydrometer sample early and correct for temp. If the temp of your wort is 84F and your SG is measured at 1.052, what’s the corrected OG?

1.052 + .003 = 1.055 OG

It’s just that simple.

My head just exploded

If numbers and equations make your head explode, here’s a detailed video which explains Hydrometer usage in simple English. Sometimes seeing is believing.

Hydrometer – detailed explanation

If you have any questions ask below and I’ll do my best to answer.

Cheers, and happy home brewing!

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