What is a knock box?
A knock box — also called a grounds bin or marc bin — is a container into which the barista taps the portafilter to eject the compressed coffee puck (spent grounds) after extraction. It is typically built around a metal or rubber-covered bar on which the portafilter strikes, protecting both the portafilter and the bar from damage. It eliminates the need for an open bin nearby and centralises used grounds for sorting, composting or disposal.
The knock box is one of the simplest accessories in the espresso bar, but its choice deserves thought since it is used dozens to hundreds of times per day in a professional setting, and several times a day at home.
Materials vary. Stainless steel knock boxes are the most durable and easiest to clean — a dishwasher cycle is enough. Robust plastic models (ABS or similar) are lighter and less expensive but can crack over time under repeated impact. Hard rubber or silicone models are quiet — a significant advantage in an apartment or office where the knock box clang can disturb people — but less durable.
The knock bar is the critical component. A bar that is too rigid without padding can gradually damage the spout or rim of the portafilter. A well-cushioned rubber or silicone bar protects the portafilter and reduces noise. On quality knock boxes, the bar is replaceable separately, extending the product's lifespan.
Capacity is a practical criterion: a 1-litre knock box holds 10–15 double shots before emptying, which corresponds to a day's domestic use. Active bars use 3 to 5-litre bins to avoid too-frequent emptying during service.
Coffee grounds from the knock box are 100 % compostable. They are rich in nitrogen, magnesium and potassium, and make an excellent garden amendment. Many specialty bars offer their used grounds free of charge to local residents — a waste valorisation practice well established in the coffee community. At home, grounds can also be used as a natural body scrub, an odour absorber in the refrigerator, or a natural repellent against certain insects.