Homebrew How To's - Hambleton Bard
Homebrew How To's - Hambleton Bard

Wine and beer making hydrometer - useful formulas

When using your hydrometer in wine making, beer making or for making spirit (moonshine), you will need some basic formulas for determining things like alcohol, sugar content etc. This page contains the most used formulas (slightly simplified for ease of use).

wine making hydrometer
  • 1 kg of sugar dissolved will fill 0.63 litres
  • 2.7 g of sugar in one litre of liquid will increase s.g. one unit
  • 20 g of sugar in one litre will make 1% (vol) of alcohol when fermented.
  • Fermenting 7.4 s.g. units on the scale will produce 1% (vol) alcohol.
  • The specific gravity of alcohol is 0.80 (the weight of a litre is 0.8 kgs)
  • Specific gravity = grammes per litre

Example no.1

You have a wine kit making 23 litres in total and the instruction says to add 4 kgs of sugar. The volume of the concentrated juice is 1.5 litres. How much water do you need to add?

Count down from 23 litres, first you have the sugar (dissolved); 4 kgs takes up 4 x 0.63 = 2.52 litres space. Remove that and you have 23 - 2.52 = 20.48 litres left. That is your concentrated juice plus the water you add. Remove the juice: 20.48 - 1.5 = 18.98 litres. That is the amount of water to add (let's not go silly on accuracy, so make it 19 litres).

Example no.2

You have a beer kit where you just followed all instructions and mixed things together, but it hasn't started to ferment yet. Then you decide you would like to make a higher alcohol beer.

First: Let's check HOW much alcohol you can expect if you don't do anything. Take a hydrometer reading (make sure all ingredients have mixed well). Let's assume that is 1040 (pretty normal for beer kits). What you don't know is the final gravity of course, but let us assume that is normal. Normal for a lager might be around 1004. So if all goes fine, you will ferment from 1040 down to 1004, that is 36 fermented units. We know that for every 7.4 units you will get 1% alcohol (by volume), so this would give you 36/7.4 = 4.9% alcohol. Not bad, but you would like 6%.

So you need to add enough sugar to produce another 1.1% alcohol. Well 20g per litre will give you 1%, so 1.1 x 20 = 22 grammes per litre should do the trick. Your kit is a 23 litre kit so you need to add 23 x 22 = 506 grammes, or just make it half a kilo that will do fine.

Watch it so you don't aim too high, the beer yeast isn't designed to cope with high alcohol but 6% should work fine. It may add a day to your fermentation though.

Next page: alcohol calculations with the hydrometer.

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