Primary Fermenter Size

updated 07/30/2024

A new member on WMT asked about container size, e.g., how big should the primary fermenter be?

Overflow

This question is a great one — if a primary is too full, a vigorous fermentation can overflow it.

In 2013 I juiced a bushel of apples to make wine. When done, I had a huge amount of glistening apple pulp, so I decided to try making a second run wine from it. I treated it like a grape second run, adding water, sugar, tannin, and acid in a 1-to-2 ratio, e.g., for each 2 gallons of juice I added 1 gallon of water.

I used a 7.9 gallon (30 liter) fermenter, and it overflowed. Twice. It taught me a valuable and somewhat messy lesson.

My general rule is to never fill the primary more than 3/4 full, and if the must has a lot of fine solids in it, going 1/2 full may be safer.

For juice, this isn’t typically a problem as most don’t foam up a lot. Musts with a lot of solids are more likely to overflow, as the emitted CO2 floats the solids up. The apple pulp was the worst I’ve encountered, as the solids-to-liquids ratio was higher than typical.

How to Determine Primary Size?

To determine primary container size, first determine must volume.

For juice? This is easy — if you have 23 liters of juice (typical kit size), the volume is 23 liters. A 30 liter (7.9 US gallon) primary fermenter is a good choice, as is any larger container.

For grapes? Several sources state that containers of  12 to 15 US gallons capacity is required for 100 lbs of grapes, and this is a reasonable estimate.

This is scalable, so take the weight of the grapes and multiply by 0.12 and 0.15 to determine the minimum and maximum container size, e.g., for 150 lbs of grapes, container volume of 18.0 to 22.5 gallons is required.

Note that for primary fermentation, a larger sized container is not a problem. Larger helps ensure the fermentation will not cause an overflow.

For other fruit? This is trickier, since grape wines typically need to account only for the grapes. Unless making a 100% fruit wine without added water, it’s necessary to add in the water volume.

Use the same scale as grapes, but add in the volume of added water, e.g., for 30 lbs of fruit where 5 gallons of water will be added, the initial container size is 3.6 to 4.5 gallons, then add in the water volume + 10%, so assume 9 to 10 gallons capacity.

This will vary by fruit, and always think about how full the container is. If it’s more than 3/4 full, you need a bigger container, regardless of what the calculations say.