Fermenter Headspace (How Much in the Primary and Secondary?)

New homebrewers often wonder how much headspace they should have in both the primary and the secondary fermenter to achieve the best results. Since mistakes are frequently made in this area I thought it would be a good topic of discussion.

The amount of headspace available during primary fermentation should be about 20 percent of the total volume of the container to avoid foam and liquid escaping during active fermentation. During secondary fermentation, there should be little or no headspace available to avoid oxidation of the beer.

Although those rules are short and easy to remember it is important to understand the reason the rules are there in the first place. Let’s take a closer look at what fermenter headspace is and why you should have different amounts in the primary and secondary fermenting vessels.

What is fermenter headspace?

Before we get into too many details, let’s first define what exactly we are talking about here.

Fermenter headspace is simply the amount of space between the top of the beer and the top of the fermenter. Depending on the type of fermenting vessel you are using (carboy, bucket, or conical) you will need to measure this space in different ways.

I remember when I was making my first batch of homebrew I had no idea that headspace inside the fermenter was something important. I even remember thinking that I should fill up the fermenter as high as possible because that meant more beer to enjoy!

Unfortunately, this mistake led to a pretty messy primary fermentation and I got lectures from my wife for many months about what happened. Luckily, I think she has mostly forgotten as this point!

Now that we know what it is, let’s talk about why the headspace inside the primary fermenter should be different than the secondary.

How much headspace do I need in the primary fermenter?

Primary fermentation is a period of intense activity that produces foam and an incredible amount of carbon dioxide.

The ideal amount of headspace inside the primary fermenter is about 20-25% of the total volume inside. This will leave enough space for foam and yeast to accumulate without blocking the airlock and causing a blowout.

This rule of thumb will be sufficient in the vast majority of cases and you can’t really have too much headspace during the primary. With that being said, be careful with having too much headspace in the primary if you plan to leave your beer in there for a long period of time to condition.

At that point, you will want to have the same amount of headspace that you would in a secondary.

What if there is not enough headspace in the fermenter?

I mentioned earlier that I made a pretty big mess during my first batch of homebrew years ago.

If there is not enough headspace in the primary fermenter then you run the risk of foam and yeast blocking the top of your fermenter, unable to escape the airlock quickly enough. Eventually, the opening could be blocked completely which will often result in a blowout from the pressure of the carbon dioxide building up inside the vessel.

There are ways to mitigate this problem, such as using a blow-off tube, but the easiest solution is to simply provide more space for the yeast and foam to accumulate during active fermentation. This activity will die down after a few days and there will be much less risk of any blowouts occurring.

What size fermenter do I need for primary?

The answer to this question will come down to the size of the batch you are making because it’s a question of volume and the specific fermenter that you are using.

Generally speaking, a 5-gallon batch of beer will need a 6.5-gallon fermenter during the primary fermentation to ensure that there is enough headspace available for the extra yeast and foam during the active phase of fermentation.

With that in mind, you can simply adjust the size of your fermenter depending on your batch size. Just remember that most supplies out there are designed with either a 1-gallon, 5-gallon, or 10-gallon batch size in mind. That means it would be difficult to purchase a ready-to-use fermenter for sizes in between.

How much headspace do I need in the secondary fermenter?

Secondary fermentation is a bit misleading because there really isn’t much fermentation happening at all.

Ideally, there will be little or no headspace inside the secondary fermenter to reduce the amount of oxygen coming into contact with the beer. While oxygen is important during the time yeast is pitched, it should not be introduced after fermentation is complete because it will eventually cause oxidation to happen. This oxidation produces undesirable off-flavors to develop in the final product.

This is a good rule of thumb to follow during the second phase, but your situation could be a little different.

Generally speaking, the risk of oxidation grows over time so if you are just planning on keeping your beer in the fermenter for a couple of weeks to clear or dry hop, for instance, then you might not have to worry about headspace as much. Conversely, if you plan on conditioning a big beer for many months then minimizing the headspace will be a top priority.

By the way, you might not even need to do a secondary fermentation. I recently wrote an entire resource about whether or not secondary fermentation is necessary if you want to check it out.

What size fermenter do I need for secondary?

The answer to this question will depend on the size of your batch of beer. Since it relies on the volume, you’ll have to do a little math to see what size you will need.

Generally, you can use a smaller fermenter for the secondary than you needed for the primary fermentation. For a 5-gallon batch of beer, you can use a 5-gallon fermenter for the secondary to ensure that the amount of oxygen coming into contact with your beer is minimized.

With that in mind, you should probably have a couple of different sizes of fermenter available for your homebrew. It’s also common advice to use a glass carboy for your secondary fermenter because it will not let oxygen pass through it and into your beer. Over time, oxygen can pass through a plastic fermenter and lead to foul off-flavor-producing oxidation.

What if there is too much headspace in the fermenter?

Remember that secondary fermentation is not the same as the primary fermentation and, as such, they will have different needs.

Generally speaking, the secondary fermenter needs very little or no headspace in order to minimize the potential for oxygen entering the beer because oxidation will cause bad off-flavors to develop in the final product.

The primary includes a lot of activity during the active fermentation phase while the secondary includes very little if any, yeast activity.

If you end up with too much headspace in your secondary fermenter then you have a couple of options:

  • Add glass marbles to the fermenter – this will take up space at the bottom and reduce the headspace at the top. Just be sure that you properly sanitize the marbles and minimize the amount of splashing when adding them to the fermenter so you don’t mix extra oxygen in.
  • Add a couple of cups of distilled water – This is an option if you are just a bit short. Since carboys have a big slant at the top, a small amount of volume missing can lead to a huge surface area.