How To Use Your Hydrometer

HBO_StaffHBO_Staff Administrator Posts: 2,115

You can use your hydrometer for a couple of useful reasons. One of its purposes is to see what stage of fermentation your brew is at, to see if its finished its first stage of fermentation (ready for bottling/barrelling) or stopped prematurely. The other purpose for your hydrometer is to calculate the alcoholic ABV% of your brew. To use your hydrometer to see if your brew has finished its first stage of fermentation and is ready for bottling or barrelling is simple. Firstly sterilise your hydrometer to make sure of no contamination. Next, gently place it into your fermenting vessel to take a gravity reading (alternatively you can take a sample of your brew into a trial jar and place your hydrometer in the trial jar so it floats). If using a kit there is sometimes a guideline gravity reading of what your brew should be in your beer, cider or wine kit instructions. After you've taken your first reading you'll have to leave it for 24 or 48 hours before you take your second. If your first and second readings are the same after 48 hours and all signs of small bubbles rising through the liquid to the surface have completely stopped, this usually means fermentation has stopped. If in doubt leave it another day or so, or if the reading is too high and your fermentation has stopped prematurely have a look at the post below: To calculate the alcohol content of your brew: When brewers talk about Specific Gravity they will usually specify either the Original Gravity (OG), or Final Gravity (FG). The OG is a measurement of the beer or wine before it ferments, and the FG is the measurement that is taken after fermentation is complete. The difference between the two tells you how much alcohol is in the brew. As an example, a typical Pale Ale will start off with an OG of around 1.045 and finish off with an FG of around 1.008.

Enter your readings into the calculator with the correct decimal points on this link (eg 1.045 and 1.010);

Alternatively calculating the percentage of alcohol is as simple as plugging some numbers into the following equation.

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

 So, given a few numbers suggested above:

OG = 1045
FG = 1008

The equation would look like this: ((1.05 x (1045 – 1008))/1008) / 0.79 x 100

So, this beer would be about 4.9% alcohol.  There are various tables with the alcohol conversion numbers to work out the alcohol content, any of the proven ways can be used, but if you're not sure then post your questions here and we'll be happy to help. These hydrometers come in their own handy trial jar and are easy to read;


  • markpaynemarkpayne Member Posts: 3
    Making my 1st batch now, Started primary fermentation last Tuesday the 1st. Gravity reading is now 1.014. Woodfords Reserve, Do you think its save to keg up now.? Thanks
  • HBO_StaffHBO_Staff Administrator Posts: 2,115
    Yeh thats perfect, that'll be ready for barreling.
  • markpaynemarkpayne Member Posts: 3
    OK. Thankyou, but after a quick an gentel stir, Its off again blowing bubbles past the air lock. Think it was just a drop in temp which slowed it down. will check the hydrometer reading in the morning an barrel her up after work moro. Thankyou again. PS this kit i bought from you is fantastic. Should be a good xmas!! me misses dog the wood burner bad films an good beer.
  • BrenBren Member Posts: 2

    Ive been doing my primary fermentation for 6 days now. However, in retrospect the temp whilst ive been doing this has been slightly lower than it should have been (16-20c).

    How do I know the fermentation has worked? I bought the kit from yourselves so I have a hyrdometer but im pretty sure I dont have the guideline gravity readings.

    The brew is Nelsons revenge.

    Any advice would be appreciated.

  • markomarko Administrator Posts: 76
    Hi Bren. The guidline hydrometer reading for the Nelsons revenge is 1014. So if your brew has been some where around that reading for a couple of days then its ready for its next stage. If its not there yet or any problems let us know.
  • BrenBren Member Posts: 2
    Hi Marko

    Thanks for the reply. It was slightly higher at 1016-18. Ive started the secondary fermentation now so hopefully it will turn out okay!!!!
  • markomarko Administrator Posts: 76

    Here's a helpful link to a ABV calculator. The site it links to is an interesting blog.



  • MattPMattP Member Posts: 14

    Hi, not sure I follow your calculations for %ABV ?

    I see that the equation when calculated using your suggested values does give an answer of 0.0487, but don't understand how you come up with 4.9% alcohol from this as it's out by a factor of 100 ?

    Am I missing something, or do you just multiply the answer given by your equation by 100 ? In which case, why does the equation not end in x100 ?

  • markomarko Administrator Posts: 76
    edited March 2012
    @MattP thanks for pointing that out. I'll edit the OP.
  • angus5954angus5954 Member Posts: 4


    In reply to your How To Read Your Hydrometer. I have found an  easier way to find the A BV% .Using your example readings of OG1.045 and FG1.008 drop the 1.on both readings.For example 045 minus 008 =37 devide that by 7.5 and you get a reading of 4.9 % alcohol the same as you get quicker.

  • davetechdavetech Member Posts: 2
    If you have an android phone there are plenty of free brewing related apps that you can download which calculate abv based on start & finish gravity. Slightly easier than getting the calculator out for the equation posted above.
  • paultait1985paultait1985 Member Posts: 4

    Hi guys

    im new to the whole home brew thing. My parents bought me one of your Cider Brew buddy kits for xmas. I started the kit off on the 30th Dec and its been sat in the front room since then at about 21C. I tested the cider yesterday and again today with the hydrometer and im getting a reading of 1.000 is this a good reading? and am i ready to bottle?

    The cider still looks a little cloudy in the brew bucket.... any ideas on how to clear it up a bit?


  • HBO_StaffHBO_Staff Administrator Posts: 2,115
    edited May 2013

    The BrewBuddy kits are very popular, the reading does sound nice and low and the brewing temperature is good, as long as it stays the same for 2 days in a row you can now go ahead and bottle it. It will still be cloudy at this stage but if you transfer over into bottles trying not to disturb the sediment at the bottom of the vessel, prime each bottle with sugar, and then leave somewhere warm for a week, then move somewhere a bit cooler, this will help it to clear, it will also carbonate and the flavours develop fully.

    We also have a good deal on the BrewBuddy refills at the moment for a limited time only which may be of interest, FREE brewing sugar with each refill on the cider, beer and lager versions;

  • paultait1985paultait1985 Member Posts: 4
    fantastic thanks for your help! i'll keep you up dated on the progress
  • Shaky9000Shaky9000 Member Posts: 2
    May be a silly question but my fermenter does not have a tap, to get a reading i need to take the lid off and put a glass in. Is this ok aor will the exposure to air do it no good?
  • HBO_StaffHBO_Staff Administrator Posts: 2,115
    Whilst the brew is fermenting it does not need to be airtight so you can take the lid off if needed, people usually do one of two things, either clean a trial jar (plastic tube included with our hydrometers) or a glass and scoop some brew out, then take a reading. Alternatively you can clean the hydrometer and pop it in the vessel and take a reading in situ, either way, just make sure that the jar/glass/hydrometer are well cleaned to minimise the risk of contamination
  • geraldogeraldo Member Posts: 1
    hi folks, just signed up. i was hoping someone could put me straight on the use of liquorice in the making of irish stoute /london porter. 
  • HBO_StaffHBO_Staff Administrator Posts: 2,115
    edited October 2013
    Liquorice can be used to flavour beers including stouts, to make it impart it's flavour fully it needs to ideally be dissolved, you may try soaking it in warm water to see if this helps, it can then be mixed in with your brew whilst it ferments. The amount of liquorice you would add is down to experimentation and how much flavour you want to add
    There is a faster (fastest) was to determine the ABV - the delta from hydrometer readings divided by 7.5 = the ABV. If adding any sugar to bottling containers, add 0.5% more for overall ABV.
  • timdegtimdeg Member Posts: 1
    Is it possible for a hydrometer which is unbroken to produce unusual readings? I am doing an American IPA and OG was 1.020, and FG after 11 days is 1.006. It is in clear water right now and reading well under zero. I have been using this one for over a year with no problems. Since I am aiming for 1.011 FG I am a long way off.
  • HBO_StaffHBO_Staff Administrator Posts: 2,115

    Hi Timdeg, a hydrometer wont read 1.000 in water, so it cannot be tested that way, unusual readings may have an explanation though. The OG sounds too low - it may be that all the sugars in the malt had not fully dissolved so the reading was lower than expected.

    If after 11 days the current gravity reading is 1.006 then this is good and well below the 1.011 you are aiming for, so as long as it is constant now the brew can be bottled or barrelled.

    If the reading is 1.060 then this would indicate it still has along way to go, just check it is warm enough, at least 20 degrees C for most brews, maybe give it a good stir to re-suspend the yeast, and if the reading wont lower then it will need a new yeast to get it going.

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