How much headspace should ideally be left in a growler when filling?

Leaving around 5% headspace in a growler when filling is advisable for optimal beer storage. This small amount of headspace allows for the natural expansion of the liquid as temperatures change and helps to mitigate the risk of over-pressurization, which can lead to spillage or even breakage of the container. Additionally, it creates a buffer zone for carbonation. If a growler is filled to the brim (0% headspace), any movement of the growler can cause the beer to foam and overflow, leading to a loss of product. Conversely, excessive headspace (10% or more) can result in increased oxidation over time, which negatively affects the beer's flavor and can lead to off-flavors. Thus, aiming for approximately 5% headspace strikes a balance between minimizing foam during transport and maintaining beer quality.