I’ve brewed two batches and both times my ABV has been below what the kit has estimated. Am I doing something wrong?
maybe. we need to know a little more. are you doing extract or all grain? are you adjusting your hydrometer for temperature changes? or do you use a refractometer?
There are only two variables, extract weight and wort volume. Extract is consistent both in terms of sugar content and the poundage supplied by commercial vendors, that leaves your estimate of the final volume as the most likely source of error. Theres also the possibility that your OG measurement is off, hydrometers can be off. Check yours with water to see if it reads 1.000, and be sure you are measuring at the right temp. It says the temp on the hydrometer, usualy its 60F or 70F.
there is only one way to really tell: drink a crap-load of the beer in question. if you’re not as drunk as you should be, there’s a problem somewhere. try to trace down the problem & fix it.
if you’re doing an extract kit, final volume is most likely the culprit. what are your O.G. & F.G. readings?
What you mean to ask is why is your finishing gravity high. You may be stopping fermentation before it completes. Be sure to give the yeast 2-3 weeks in the primary fermentation (unless you know what you’re doing 
).
For example, my last batch was NB Bavarian Hefeweizen, which is an extract kit. The kit said the OG should be 1.049. Mine was 1.050. The estimated FG (according to iBrewMaster) is 1.013. Mine came out at 1.019, which comes out to 4.06% ABV. The recipe calls for 5.11% ABV. I think I’m waiting long enough to let fermentation finish because no bubbling is happening in my airlock and the my FG readings are steady. The only thing I can think of is my boil is only 2 1/4 gallons versus the 2 1/2 gallons recommended.
That’s a reasonable FG for an extract beer. To get attenuation on par with an all-grain recipe, you’ll need to use only light/extra light/pilsner extract. If you want higher attenuation that what that gives you, substitute sugar for some of the extract.
Ignore any program that claims to provide an FG. It’s just multiplying the OG by an assumed attenuation, without accounting for the recipe.
Make sure you are oxygenating your wort.
Pitch enough healthy yeast (1 pack dry/2 packs liquid or make a starter for a 1.050 beer)
Make sure your temps are within range of the yeast strain.
This has nothing to do with your boil concentration.
