How to Use a Hydrometer

Hydrometer was one of the main things I missed while brewing my first homebrew, if not the only one. You can brew without it, especially if you’re using a beerkit and are just mixing the necessary ingredients together for fermentation. When you follow the instructions, ferment it for the number of days recommended and add nothing, you will get (most of the times) whats promised and no harm done, unless you’d like to be absolutely sure that fermentation is done and to know exactly how much ABV is in it. Those were the two things I needed the hydrometer for with my first homebrew and because of that it was my first purchase after bottling.

What is a Hydrometer

A hydrometer is used to measure the specific gravity of liquids. It is a simple tool, usually made out of glass, that shows the difference of a liquid’s density to the density of water. The history of hydrometer goes back to late 4th, early 5th century and the basics of the instrument have remained the same since. It looks like a thermometer – a tube of glass, bulb below, weighted to keep it upright. Hydrometer has a scale on it to show the specific gravity, when it is floating freely in the liquid. Scales can be diffrent, depending on the purpose of a particular tool and every hydrometer is calibrated according to its intended use. The simple hydrometer I use is scaled in a way that “0” marks the density of distilled water and it goes up to 120. Although meant for wine, it is perfectly ok for measuring beer.

My hydrometer, meant for wine


How to use it

Using a hydrometer is even simpler then the tool itself – you just put it into the liquid and look at the scale, done! There are a few things to keep in mind though:

  1. Use a testing cylinder. While you can put the hydrometer directly to the fermenter, it is not recommended, specially at the later stages, when it would require you to open the fermenter. Using a testing cylinder the beer remains calmly in the fermenter, no unnecessary air gets near it and you don’t have to worry about any additional bacteria from the hydrometer getting into the precious liquid.
  2. Know your tool. Every hydrometer is calibrated for use at specific temperature, usually 20 degrees Celsius (68F). If your wort is at different temperature, you have to calculate the correct readings. Couple of degrees more or less will not make a big difference for a homebrewer and because I need to measure before adding the yeast, the temperature has been always at the correct level for me.
  3. Wait for it. Dont’t be hasty, let the hydrometer float and wait for the airbubbles to escape. Then center it if needed and take the reading from the line where beer meets the air.
  4. Test your hydrometer. If you are unsure about what the numbers on scale are meant for or have doubts if your hydrometer shows the correct readings just test it. Measure the density of distilled water at 20 degrees Celsius (or whatever is marked on your hydrometer) and where the air meets water, there is specific gravity at 1000.
Hydrometer in water

Hydrometer in water

For what and when

I use my hydrometer for two things – to see how the fermentation is going (is it fermenting at all, is the beer ready) and for calculating the alcohol level. To do that I take a reading just before the yeast is added (called the Original Gravity, OG) and write it down. At that point you can also check if the wort is as it should be, taking into account you know what gravity it should have (beer kits and also most recepies include this information).

Now I wait until the airlock implies that fermentation is over and take another reading. If it happends too soon, there may be a problem with it and if the readings show high gravity, the fermentation has stopped prematurely and you either have to restart it or give up and start all over again. It has never happened to me, but it is one of the best things about the hydrometer – you can be sure that everything is ok, without it you must only hope.

Usually I take 1-2 readings before the end and mark all the numbers down, eventhough I really need the lastone – the Final Gravity (FG). This is used to calculate the alcohol level of your beer. For this calcuation there are several slightly different formulas available, but most of them are variations. I use thisone:

ABV = (OG – FG)*132.715

If you want to be more accurate, you can get formulas from different sources, like Wikipedia – Gravity


Having a hydrometer is the simplest and cheapest way of finding out how your wort is doing and how strong your beer is. Its price compared to the value of the information is not worth mentioning, so I believe it is a must for a brewer at any level.