thanks for that sound ok might have a go last part was alittle dificult to get my head around but we will have a good beer at the end hopefully cheersHangover has a point, you cant just consider the kit but have to include everything that goes into the fermenter.
The upper limit is really what the yeast can cope with (kit yeast really is less than ideal - investing in a bigger better yeast is a good idea). Most kit yeasts are able to go to 8% ABV, some might go a bit higher (up to perhaps 10%) but with the very small pitch (amount of yeast) it will be struggling. Yeast under stress will make some off flavours and lots of higher alcohols (fusels) which take some of the fun out of drinking and tend to make the next morning pretty painful.
If you used a 1.7kg kit and 1kg of dextrose, at 23L your OG would be 1.038.
Reduce that to 18L and the OG would be 1.048. Both are well within the capacity of the yeast to ferment, as above you will get a bit higher alcohol content, a bit higher FG and a bit more flavour.
Those numbers only work for 1kg of Dextrose, if you are using more than 1kg or something else like sugar or malt extract the numbers will be slightly different but for 1kg not much, if you used 1.5kg you should still be fine.
For anyone interested
The equation we need is: -
Mass of extract = Volume of wort * SG * oP
Volume in the first case 23L
SG and oP we want to calculate, given SG=(4*oP)/1000+1
We know the mass of extract, the kit is about 80% solids so the mass from the kit is 1.7*0.8=1.36kg and the dextrose is 91% solids so 0.91kg added together gives us 2.27kg of solids (things that aren't water).
Shove the whole thing in excel and use goal seek to solve for SG.
Just note if you are putting the equation together for example a 12 Plato (12oP) beer has an SG of 1.048, but oP is percent weight of extract in total weight as a percent so in the equation that would be 0.12 (12/100) not 12.
You should be fine, but temperature control and more of a better yeast will help you get a better beer.
Mark
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