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Using a hydrometer to do gravity readings


In order to calculate the alcohol content of a brew, you need to take two samples:

  1. The Specific Gravity (SG) of the initial wort/must on Day 0 when it is This measurement is also known as the Original Gravity (OG).
  2. The Specific Gravity (SG) of the final beverage after fermentation and when cold (preferably after clarification). This measurement is also known as the Final Gravity (FG).

From these two measurements we can calculate the alcohol %. The higher the initial SG, the higher the alcohol % will be and the lower the final SG, the higher the alcohol % will be.



When the wort/must is full in the vessel and before the yeast is added, give it a good stir so that all the contents are mixed

well. Take a 100ml sample out of the top surface using a clean glass and add this to the 100ml measuring cylinder. Then add the hydrometer to the cylinder and give the hydrometer a spin as you let go. The spinning helps remove air bubbles from

the hydrometer. Let the hydrometer come to a stop and then measure the SG. An example is shown in Figure 62.

You may need to blow off any bubbles so you can read the hydrometer better. Then read off the scale at the top point of the liquid where it rises and touches the hydrometer.

Note: some hydrometers require a slightly different reading point, so read the instructions of the hydrometer you have.

For a WilliamsWarn Standard Kit consisting of a 1.7kg (3.75 lbs.) can of liquid malt extract and a 1.36kg (3lb) pack of dry malt extract made to 23 Litres (5 UK Gallons/6 US Gallons), the SG will be 1.0454.

Specific gravity is a measure of density. Water has a density of

1.000 SG. So 1.040 means the wort/must is denser than water due to the dissolved matter from the barley grain or other raw materials that are now in your fermenter along with the water. This includes various sugars, proteins, amino acids, minerals and vitamins and other compounds.

To get the most accurate reading, the temperature of the sample should be 20°C (68°F). A few degrees on either side doesn’t matter too much, but the closer you are to this target the more accurate the reading. Note: Some hydrometers have different calibration temperatures than this, so read the instructions of the hydrometer you have.



Once the pressure has built up in the unit, you can take a sample out of the draft tap at any time. If you’d like to measure the SG each day you may do that. However unlike the wort/must sample, there are now carbon dioxide bubbles in the fermenting beverage so you need to de-gas it first. Take 50ml of the beverage out of the draft tap and discard this down the drain, this will clear the line so that you may sample the fresh beverage. Then take a 150ml sample from the tap and pour it between two glasses for a minute to create turbulence and remove the CO2. The more turbulent the pouring between the glasses, the more the removal of CO2. Then let it settle and ensure it's close to 20°C (68°F) in temperature and then add it to the measuring cylinder. Add the hydrometer with a little spin and read the SG.

Different yeast will ferment at different speeds and different fermentation temperatures also affect the rate. Some graphs


of typical fermentation rates, as measured by the drop in SG, are shown is Appendix 7.

The yeast is consuming the sugars, amino acids, vitamins and minerals and excreting alcohol and CO2 from its cells into the wort/must. The SG reduces each day because these compounds, which are denser than pure water, are being reduced in quantity. Most of the resulting CO2 is emitted out the VPRV (we keep about 10%) and the alcohol stays in the beer. The alcohol is also less dense than water so the more that is made the lower the SG becomes as well.

Beers will eventually stop at a certain SG depending on residual starch in the ingredients and the yeast type. Yeast cannot eat sugar molecules above 3 sugar units long and these remain in the beer to give body and some taste. The amount of these starches (or “dextrins”) depends on how the malted barley was mashed, the stage in a brewery brewhouse when crushed grains are mixed with water and starches are converted into sugars.

For extracts like WilliamsWarn extracts, this is controlled when the extract is made. All grain brewers can control this themselves

depending on their temperature and time regimes during mashing.

Ingredients made up of mainly sugar, like the musts of cider, wine and mead, will ferment right down when using a non- flocculent yeast as they have no residual dextrins because the raw material is fruit or honey, not starchy grains. We use a very flocculent yeast for our cider so it stops at about 1.005 – 1.007 if given sufficient time during fermentation.



Once fermentation is complete, the SG will not reduce any lower and you can take a final SG sample. This could be before cooling if the yeast has finished fermenting, but the most accurate reading is at the very end when the beer is cleared. This is because if fermentation hasn’t stopped during cooling, the SG will continue to lower, and in addition, the clarification process mixes that entire tank contents very well to make sure you get an accurate reading of the total tank SG.

You will need to take a sample, ensure it is close to 20°C (68°F) and de-gas the beverage by pouring 150ml between two glasses before putting it in the measuring cylinder with the hydrometer.

To increase a cold beverage's temperature, you can hold the glass of sample under a tap of ambient or slightly warm water while swirling the glass to get a good heat transfer. Then measure the SG as described above.



The alcohol % cannot be determined by floating the hydrometer in wort or beer and reading the alcohol scale on the hydrometer. A calculation is required.

There are various calculations to calculate alcohol by volume (ABV) from an initial and final SG reading.

Here are some options (you may find others on the internet).

  1. A basic calculation is:

(Initial SG x 1000) – (Final SG x 1000)/7.46

So for a beer starting at 1.0454 and finishing at 1.010, the alcohol content by this formula will be (1045.4-1010)/7.46 = 4.75% alcohol by volume.

  1. Another simple formula is that is commonly used is: ABV = (Initial SG - Final SG) x 131

So in this example:

(1.0454 – 1.010) x 131 = 4.64% abv.

You will note there is a discrepancy between the formulas. These formulas provide a guideline only as true alcohol is measured in a lab from the final beverage using equipment a homebrewer does not possess, so different authors prefer slightly different formulas.

  1. A more complex formula which attempts to provide greater accuracy at higher specific gravities is:

ABV = (76.08 x (Initial SG-Final SG)/(1.775-Initial SG)) x (Final SG/0.794)

You may want to use this for beverages that have an initial SG above 1.060.

Using formula B, you can see the effect of a lower Final

S.G. on the ABV.



























The yeast we use for our extracts all ferment to this range, so for the same starting point with a Standard WilliamsWarn Kit you will end up with slightly different alcohol %’s.

Nottingham Ale yeast will ferment to about 1.010 in 3 days to make 4.75% alcohol ales and we use this with our English beer styles.

S-23 and W34/70 are lager yeasts and will ferment slower than the ales (they are also fermented at lower temperatures which slows the process down) but they reduce the SG further to about

1.008 in 5 days. They therefore produce 5.01% alcohol lagers for 1.0454 initial SG worts.

For our Belgian Beer we use T-58 which will end at about 1.012.

Our cider when added as two cans with yeast nutrient will produce a 1.038 must and will ferment to 1.007 or just below and produce a 4.06% alcohol cider. To leave some residual sweetness in that cider kit you can put the cooling on before the fermentation is finished to end at 1.010 which will result in a 3.70% cider.

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