Start with some real basics just so we are on the same page.

Extract is the term given to everything in solution in the wort.

The amount of extract is measured in two ways. SG is times the mass of an equivalent Volume of water so if we had 1 Litre of wort and it weighed 1.050kg we would say it has an SG of 1.050.

The other way to measure wort is Plato (oP) or Brix (oB). Plato is the equivalence to a given amount of Sucrose (sugar) in water, but it is %WW (weight in weight) if we made a 12.5% Sugar in water solution, it turns out that 1L would weigh 1.050kg.

It isnt exact but the conversion between the two scales (SG an oP) across the brewing range can by described by

SG = (4*oP)/1000+1 There is some error but it’s smaller than the ability of most brewers to measure, so close enough.

One more equation

Mass of extract = Volume * SG * oP (NB its %ww so your equation will need oP/100)

Let’s say we took that can of Coopers and diluted it to 12L.

You will find that most LME, kits, honey, golden syrup... are going to be very close to 80% Solids (extract) and 20% water. Given that the kit weighs 1.7kg, we can say there is 1.7*0.8=1.36kg of extract

If we plugged that information into the equation above

1.36=12*SG*oP

I would shove that into a spreadsheet and use Goal Seek (in Excel, Data Tab, What if Analysis, Goal Seek) to solve, doing that gives an SG of 1.0434669428456 and a oP of 10.8667357114. Let’s call it an SG of 1.043 and 10.9oP.

Unlike Liquid Malt Extract (LME) Dry Malt Extract (DME) is pretty much water free, as is Sugar but its handy to know that Dextrose is 91% solids and 9% water.

From there you should be able to calculate exactly what SG you will get from any addition.

Just remember that it’s always mass in volume or mass in mass, the amount of liquor you end up with is just as important as the amount of mass in it.

Also mass has volume. Sugar is used as a model because it is very similar to wort constituents and it’s cheap, pure and highly available

If we took 100g of sugar and added 900g of water we would have a 10oP wort, it would have an SG of 1.040 but there would be only be 0.963L.

Mark

If you play with Excel this might help