The share of the fermenter that must be left empty, as a fraction.
25 % is the homebrew rule of thumb, and it's about krausen, not spillage. A
vigorous ale at 20 °C throws a head that fills most of the free space in the
first 48 hours, and a batch filled past this line reaches the lid. It's a
fraction of the vessel because the foam scales with the batch, not the
bucket.
Some brewers fill tighter than this on purpose and use a blow-off tube, so
the finding is a note until the beer won't physically fit, and only then a
warning.
The share of the fermenter that must be left empty, as a fraction.
25 % is the homebrew rule of thumb, and it's about krausen, not spillage. A vigorous ale at 20 °C throws a head that fills most of the free space in the first 48 hours, and a batch filled past this line reaches the lid. It's a fraction of the vessel because the foam scales with the batch, not the bucket.
Some brewers fill tighter than this on purpose and use a blow-off tube, so the finding is a
noteuntil the beer won't physically fit, and only then awarning.