This might be a STUPID question but.....

[BostonJ]

When you are doing a 5 gallon boil some of the liquids boil off, right? Do you replace that water you lost when transferring the wort to the carboy?

 

[Reelale]

You can, but the standard method is to start with a volume of water that accounts for the boil off. This way, your gravities come out right.

 

[busted_knuckle]

Yes, after you transfer your wort to your carboy top it up to 5 gallons with water.

 

[Yooper]

Yes.

Many of us start with a 6.5 gallon boil, just so we can allow 1 gallon per hour boil off, and then some trub losses in the boil kettle and still have 5 gallons or so going into the fermenter. You can also top up with water at the end, if you need to.

 

[mikeroscope]

So topping it off to 5 gallons with water at the end messes up your gravity reading, or not doing that does?

 

[KCBrewer]

Adding water will always lower your gravity reading. Extract kits will come out to the gravity the kits state at 5 gallons no matter what, so when your done with your boil, top it up to 5 gallons if you're short. If you're over 5 gallons, your OG will be lower than expected, in which case you can continue to boil until you reach 5 gallons. I usually go about 5.25 gallons on extract kits to allow for trub.

Honestly I don't even bother with OG readings on extract kits anymore. I'll take a reading after transferring it to the keg for sh!ts and grins, and that's about it. Unless I question whether it had a good fermentation.

 

[Reelale]

If your expected gravity is based on 5 gallons, and you get 4 into the fermenter, your gravity is most probably higher due to concentrating the wort. Adding the correct amount of water to obtain the original volume should bring the gravity back to the estimated value.

Conversely, if you start with say, 6.5 gallons, your pre-boil gravity will be low. But by boiling down to 5 gallons, it should be really close to estimated. It's just two different ways to skin the same cat.

Taking gravity readings during the process allows you to adjust for high or low values.

 

[munklunk]

If you use and online brewing calculator to get your strike and sparge temps, it will calculate volume also taking into account all of that stuff (boil-off, trub loss, and such)

Here's the one I use.
http://www.brew365.com/mash_sparge_water_calculator.php