How to use a Hydrometer in Home Brewing

The hydrometer is an essential bit of kit for the home brewer providing gravity readings which allow you to calculate ABV and measure the progress of fermentation. This is a guide to some of the terminology used and how to use the hydrometer.

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Specific Gravity

Refers to the measure of your beers density as compared to water. There are a number of dissolved substances within the unfermented wort which contribute to it’s density, sugar is only one of them, other proteins are present and when using a hydrometer you are measuring all of them with your initial reading.

Original Gravity/Start Gravity

This reading is taken before fermentation it measures the density of the liquid giving the home brewer an idea of what their final alcohol level will be. However as mentioned above there are other unfermentable substances within the wort so it is only a rough guide to ABV. Generally a high original gravity reading around 1.060 means a denser liquid and therefore a higher content of fermentable sugars. The original gravity reading is taken before the yeast is pitched. Make sure all the sugars are dissolved in the liquid before taking this reading otherwise it won’t be accurate.

Original gravity readings for beer vary but as a rough guide they start at about 1.040 some stronger beers start higher, for example a stout I recently made started at 1.070 and will have a higher alcohol content than a beer that starts at 1.040 because there are more fermentable sugars in it than the 1.040 beer.

Original gravity readings for wine also vary depending on the fermentable and unfermentable sugars however again as a rough guide for a 12% wine your start gravity will need to be about 1.090.

This reading can be taken by using a trial jar as in the picture above or by placing your hydrometer directly into the fermenter, but before doing this you must thoroughly sterilise your hydrometer to prevent the introduction of any infections which may result in off flavours in your final brew. Once you have gently lowered the hydrometer into the liquid give it a twist to stop any bubbles from sticking to it, this will make it easier to read the scale.

Final Gravity

During fermentation all the fermentable sugars are eaten up by the yeast which produces CO2 and alcohol as by products. The CO2 escapes out of the liquid into the fermenter but the alcohol remains. Something to bear in mind while fermenting is that the CO2 forms a blanket over the top of the brew as it is heavier than O2. This protects your brew from any bacterial infection that would ruin your brew. However this only applies in the initial fermenter where the CO2 is trapped and an airlock is present preventing any introduction of O2, if transferred to a secondary vessel this introduces more O2 therefore reducing the effectiveness of the CO2 blanket.

Take the final gravity reading once fermentation is complete. Most malt extract kits contain instructions as to when to take a final gravity reading. You can then work out the ABV using the original gravity and final gravity readings. See equation and ABV calculator at the bottom of the page.

As with the Original Gravity reading this can be taken by placing your sterilised hydrometer directly in the fermenter or if you prefer you can use a trial jar.

How do I know when fermentation is complete?

The only way to be sure that fermentation has finished is to take two hydrometer readings over a 48 hour period, if the readings remain stable this means the brew has fermented, however if the readings change this means it is still fermenting and you will need to wait until they are stable before kegging or bottling. If a beer is bottled whilst it is still fermenting this can cause over carbonation resulting in a gushing beer or exploding bottles. In wine  it can also cause glass bottles to break as the CO2 from fermentation cannot escape.  However don’t let this worry you as these are worse case scenarios and can be easily avoided by following the procedure above.

Fermentables (Beer):

Refers to the amount of fermentable sugars present within the wort. These are gathered from starches, like grain, through the mashing process which converts starch to sugar or through non mashed sugars like fruit and honey.

Unfermentables (Beer):

Sugars with a complex build that cannot be broken down by yeast. These include roasted and dark caramel malts. These add sweetness, body and mouthfeel to your beer.

Fermentables (Wine):

Glucose and Fructose are some of the fermentable sugars in the must.  At the time of harvest grapes contain between 15% and 25% of simple sugars which can be broken down during fermentation.

Unfermentables (Wine):

These, like the sugars in beer, are more complex and can be found in the wine after fermentation, adding flavour. A few examples of some of these sugars are arabinose and rhamnose which are five carbon sugars and are not broken down during fermentation.

ABV Calculation:

Below is a link to an ABV calculator:

For those of you who would like to do the maths here is the equation:

% Alcohol = ((1.05 x (OG – FG)) / FG) / 0.79 x 100

So if your OG = 1.045 and FG = 1.008 the equation would look like this:

((1.05 x (1045 x 1008)) / 1008) / 0.79 x 100

So this beer would be 4.9% alcohol. Here’s a link to the Stevenson hydrometer just in case you would like to take a look:


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