Alcohol content

So I brewed my first batch of beer
and took the original gravity and
finishing gravity, subtracted the two
then multiplied by .129 as I read to do
in a book to get my alcohol content.
The prob is after this equation I'm
Getting that my content is 2.5%
this seems really low. Does this
sound possible or have I just probably
goofed up reading the hydrometer.

Oh yeah, it's a nut brown ale made
from muntons malt and hops extract cans
( the two can dump) and it fermented
for a week before I bottled and all
signs pointed that the fermentation was
done.

What were your OG and FG readings? If the OG is lower than it should have been, odds are the wort and water didn't totally mix when you topped up. Assuming you did a partial boil and added top up water in the fermenter before pitching your yeast. This happens all the time because you can stir the crap out of it, and by the time you pick up your wine thief/turkey baster to get the sample, the heavier wort sinks to the bottom and the lighter stuff floats to the top. If you take your sample nearer the top, you get a lower OG reading, if you take it from nearer the bottom, you get a higher one.

My OG was 1.040 (40) and my FG was 1.020 (20), so 40-20 x .129=2.58%

First I'd like to point out this is the first time that I have brewed beer so there is a good chance I mis-read the hydrometer. I took both readings from the fermenting bucket; I did not turkey base a sample into another container I didn't know that was needed. I took the first reading pretty quickly after I mixed the boiled wort with the 4 gallons of water, probably just a matter of minutes afterwards. So I suppose any of those could be factors, I just don't want to have made an almost non alcoholic beer.

What size batch did you do? 5 or 6 gallons?

I always multiply by 131 (not 129) but that's still very close. For a 6 gallon batch, two cans of LME would give you an OG of 1.039, for a 5 gallon batch you'd get 1.047.

If you did a 6 gallon batch, with two cans of extract, a 2.5% ABV seems reasonable. For a 5 gallon batch, two cans of extract would get you 3.54% or so.

If you use a better yeast, the beer should finish lower, giving you a higher ABV and less sweetness in the finished beer. 1.020 isn't an uncommon place for extract beers to stop, especially with a crap yeast like Munton's, but if you use a better yeast you would probably finish at 1.010-1.014.

First, I hope you sanitized your hydrometer when you took your readings.

Do a fast google for "ABV calculator", and you'll find http://www.rooftopbrew.net/abv_calculator.php, which confirms your calculation. That is, if your readings were correct.

Extract brewing is pretty fool proof. As long as your measurements were correct, you should have hit your target OG. I agree with da cat, it could have been a slightly unmixed OG reading to be that low.

It was a 5 gallon batch. I did sanitize the hydrometer both times before using. I didn't use the yeast that came with the cans, i used an 11g pack of brewers yeast that the brew store gave me (don't know the brand).

Like is said there is a good chance my readings were wrong. It just seems like 2.5 is low, I thought most beers came in the 4.5 range.

I have always used the factor of .131. It doesn't change the out come very much though.

we just brewed a hefe and the OG was 1.034, which seems really low - any ideas why this may be?

Was the Hefe AG or Extract? If AG maybe the grains werent cracked enoough. If Extract it may not have been stirred evenly when your water was added to make 5 gallons. Just a couple ideas.