Does Demijohn Need to Be Full?

A Demijohn, carboy, or any container that is fermenting or aging wine MUST ABSOLUTELY be filled to near the top to absolutely minimize the amount of oxygen that is present.

How full should I fill a demijohn?

Half sized demijohns are perfect when you're making wine from fruit and haven't got quite enough fruit to make a full gallon. ... Or simply when you don't have enough of your homemade cider must left to fill a whole demijohn, but a half size would do the job nicely.

Does a carboy have to be full?

Once the wine is mostly degassed, it is extremely important to limit the exposure to oxygen to a minimal and topping up is one of the best strategies to do so. You would think that by not ever opening a 3/4 full carboy, the heavy CO2 gas would protect the wine and never escape.