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?

### TIPS FOR USING A HYDROMETER

- 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.

### MATHEMATICS AND BEER

## 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 1.0**00 **at 60F. As you add sugar the number goes up. Your wort might be 1.0**55** 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 1.0**14**.

Do a little math…

**OG / 4 = FG**

So in this example, take the OG of 1.0**55** 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!