Need help ASAP
I've noticed in my 1st 2 5 gallon AG brews that I've actually only gotten around 23 gallons of wort into the fermenter.
In the extract brews, you fill the fermenter with water to the 5 gallon mark. Am I supposed to still do that with AG or am I just doing something really wrong? I really need to know ASAP Thanks 
You need to make enough wort (like 6.5 gallons) to boil down to 5 gallons. You don't top up with water.

Most of the brewing software will help you figure out how much water you'll need initially to account for boil off....You can also do a test boil...Mark a target amount like two gallons on the outside of your kettle... add 3 gallons of water, and measure the amount of time it takes to boil off the additional gallon of water...It will help you figure out a rough idea.
Or mark 5 gallons, add 6 1/2 and time that boil that down to see how long it will take... 
Take a gravity reading, if it is way too high add some water back. It the gravity is correct leave it and correct on the next batch like the previous poster stated.

Normally after your strike water(initial water to mix the grain with) you'll sparge (rinse the grain) once or twice with additional water. Generally you'll sparge until you have about 6.5 or so gallons in your pot (as Yooper said) and then boil down to 5.5 or so from there and transfer that into the fermenter.
Here's a great water temperature/volume calculator that's free (and did I mention great?!) http://njnear76.bravehost.com/ 
Also, when calculating the amount water needed, you need to account for the loss of water that is absorbed in the grain. You will lose about 1/8 of a gallon of water for every pound of grain.
For example, I have a recipe with 12 pounds of grain that yields 5.5 gallons of wort into the fermenter and I start with 9 gallons of water. 
check out EdWort's Haus Pale Ale recipe for a link to another cool excel based calculator. It will help you figure out how much sparge water you need to attain a certain volume preboil(like 6.5 galls as Yoop mentioned).

Oh well, I figured something had to be wrong. I followed both recipes exactly but I guess I should have known this.

I guess to be more clear, the easiest way to figure out how much water you need (short of using the calculators that are posted here and there) is to start at the end and work backwards.
Example with 12.5 pounds of grain: End: 5 gallons of wort If you know you lose 1.5 gallons to boil evaporation and hop absorption, you need 6.5 gallons at the beginning of the boil (if you don't know for sure, 1.5 gallons of evaporation is a decent estimate for a 90 minute boil, but you should start measuring this for your brew system) Your 12.5 pounds of grain will absorb 1.5 gallons of water, so you need 8 gallons of water combined for mashing and sparging Next, you figure out how much of that is for mashing and how much for sparging. If you want to do a 1.2 qt/pound mash thickness (that would be fairly medium thickness) you will mash with 15 quarts or 3.75 gallons and the remaining 4.25 gallons is your sparge water. As Yooper said earlier, all of the water goes through the grain  no top off water. Hope this helps. 
I have been fighting the same issue, having started AG brewing last year after 25 years of extract+steep brewing. One other way we lose volume of wort in the fermenter is thermal expansioncontraction, believe it or not. Water contracts nearly 4% in volume as it cools from boiling to room temperature, or about a quart in five gallons. So what is 5 gallons at the boil in the kettle will be 4.75 gallons when cooled and in the fermenter.
When you add all these losses up: loss to grain in MT, loss to evaporation, loss to hops, particularly to leaf hops, loss to thermal contraction, you may need as much as 8.5 gallons of water to start with, to get five gallons into your fermenter. You may also want to increase what you want out of the boil kettle by another half gallon so you can fill your secondary and leave the trub and yeast undisturbed in you primary fermenter when you rack it. 
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