What is tamping pressure for espresso and does it matter?

Tamping compresses the coffee puck to create resistance for the 9-bar pressure. The commonly cited 30lbs (14kg) of pressure is a baseline — consistency matters more than the exact force. An uneven tamp (tilted) causes channeling far more than under- or over-tamping. Use a calibrated tamper (one that clicks at the right pressure, like the Normcore) to build consistency. The tamper diameter should match your basket — 58mm for most standard machines.