brewed Midwest Brass Ale, gravity came out right on, but must have not used enough sparge water as I ended up with four plus gallons in primary. Fermentation with dry yeast, all going well, looks great. Ten years ago when I was brewing I added boiled water to primary to take up air gap.

I have searched the forum for my answer, but cannot find any opinions.

Essentially, when you come up short, do you just continue on and not top off?

Is the empty space in primary fermenter(carboy) an issue? I am ok with ending up with fewer bottles, and it seems logical that adding water would reduce the gravity and I guess just make a weaker watered down beer. I guess I would prefer to just finish this batch and end up with less total beer, but better quality.

My next batch I made sure i used the calculator and ended up with over five gallons(slightly) going into the primary fermentor.

I plan on racking to secondary in a few days, and with less fermentation going on I wondered about the air space. Of course I will rack "quietly" and try not to add any oxygen for that phase.

Any thoughts about topping off? Most photos I have viewed on forums show empty space in primary and secondary carboys. TY

The answer is, it depends. You have correctly answered your question.
If you DO add more water, then you are watering down the final product and so you end up with more beer of less potency. That may be a good thing or a bad thing, depending on what you are aiming for. If you are trying to brew a barley wine, then leave it alone and just go with less of the stronger beer.
If you are brewing a lower gravity session beer, then by all means, boil up some water and top it off. The idea there is to have a flavorful beer that will not fill you up or get you drunk, so if there's a bit less alcohol, no problem with that either.

IMO, the biggest thing though, is that you MUST boil it or treat it to remove chlorine. I would bet that the vast majority of the off flavor phenolic beers that I judge in competitions are made by newbies who simply top up with tap water, or if they are boiling, their water has chloramine in it which boiling will not remove.
In those cases use a bit of campden tablet to remove the chlorine. Campden tablets are about the size of an aspirin and they cost only about two or 3 dollars for a package. 1 tablet is enough to treat 20 gallons of water, so you can even use a pill splitter to divide the tablet down even further. it is well worth the minor cost and your beer will much less prone to phenolic off flavors if you do so.

HTH-

The place to top up is in the kettle. You should have a supply of water in the HLT left over from sparging. Use this to top up the kettle at the completion of the boil. You should know how much liquid is in your kettle as a funtion of the distance of the surface of the liquid from the rim. For example, in my kettle, each inch below the rim represents 1.8 gal. This is corrected for temperature so that if the liquid at heat off is 1.8 inches below the rim I know that were it cooled, the kettle would contain 1.8 gal less than the full capacity of the kettle. And so on. Anpther advantage to doing it in the kettle is that you can add as much or little as is necessary to get the OG you want.

If you wait until after the boil to top up you should boil the makeup water before adding it to be sure it is sterile.

If you add cold water to the fermenter it is important that it be dechorinated/dechloraminated but much more important is that it be deoxygenated. Campden tablets do scavenge oxygen to some extent but unless you have a DO meter you will be hard pressed to know how much bisulfite to dispense to get both the chloramine and the oxygen and not leave an SO2 residue. Better to dechloraminate and then boil. This will drive of O2 and SO2. It should be clear from this that it is better to add makeup water at knockout.

I don't see why you need to add deoxygenated water when topping up.
You are just going to aerate when you pitch your yeast anyways.
And also, 99% of the time the newbies are topping up is because they don't have a big enough boil kettle anyways.

As far as missing gravity and topping up, adding to the boil kettle is better but a lot of new ag'ers miss volume for a variety of reasons including hops soaking up too much wort, too much trub, underestimating tubing losses, etc. By then, also, the boil is over and all they really want to do is hit the 5 gallon mark and be done with it.

I actually said that this is important if you are topping up in the fermenter and while it wouldn't make any difference if done before aeration and pitching it certainly would after fermentation is partially or totally complete and I guess that's what I envisioned from the OP i.e. he was about ready to bottle and decided to top up to the original designvolume. There is nothing so important as keeping air out of the packaged beer in terms of prolonging shelf life. I suppose if one is dumping into a bottling bucket, filling into bottles and adding yeast for conditioning it again doesn't matter as the yeast will consume the oxygen but if one is coming from a sealed cylindroconical and doing counter pressure filling of bottles or kegs deoxygenated water is critical. Commercial operations spend big bucks on deaerating plant.

so in a nutshell, make sure you are boiling enough that with trub loss and hop loss you end up with your five gallons. topping off should be avoided, as contamination risks are there.

so, in the primary and secondary fermenter(carboy) having a void shouldn't be an issue as we are not agitating and there is carbon dioxide to keep that air sterile. Correct? I am going to be more careful to start the boil with the proper amount of wort after sparging, but there might be some other newbies that end up short on a batch or two like i did. TY

something to keep in mind is that it's going to throw off your ibu's. unless you're really unhappy with the amount of beer you got, i'd be tempted to just let it be. i'd rather have 4 gallons of good beer than 5 gallons of watery weird beer.

Also, if you are bottle conditioning, either calculate how much priming sugar you need for your ~4 gal volume or use carbonation tabs to dose per bottle. If you throw the full amount for a 5 gal batch in your 4 gal of finished beer, you will end up with more sugar in each bottle, which can lead to over carbonation.

As far as your headspace question, a larger headspace isn't as much of an issue when ferementing rapidly as the CO2 being produced will have purged any O2 from the vessel. My concern would be transferring to a secondary without enough activity to push the O2 out of your carboy in a reasonable timeframe. If it were me, I'd just leave it in the primary until I was ready to package.