The disc of spent grounds left in the brewer at the end of a brew. The word comes from espresso — where the compacted, wet hockey-puck-shaped cake of grounds is the literal physical object you knock out of the portafilter — but it's now used for filter coffee too.
Reading your puck is one of the cleanest free diagnostic tools you have. The shape, surface, and texture of the spent bed tell you what happened during the brew with no equipment required.
Pour-over puck shapes and what they mean
Flat, slightly concave (centre very slightly lower than rim). The ideal. Even pours, even bed, even extraction. This is the shape Scott Rao's "spin" was designed to produce.
Crater, also called a doughnut (low centre, high rim — a ring of grounds around a hollow). Water bored down through the middle and left the outer ring behind. Usually a pour that was too forceful or too centred. The rim under-extracted, so the cup reads sour and thin. Pour gentler, and spiral wider instead of drilling the centre.
Mounded (high centre, eroded rim). The opposite failure: water ran down the walls instead of through the bed, washing the edges away and leaving the middle untouched. That's bypass. Check that the paper is rinsed and seated flat, and stay 5 mm clear of the rim when you pour.
Sloped (one side higher than the other). The brewer wasn't level, or you poured asymmetrically. Boring fix: brew on a level surface.
Espresso puck reading
For espresso the puck should come out as a single intact disc, slightly damp on top but firm. Crumbly, soupy, or holey pucks indicate distribution or extraction problems. The colour should be uniform brown — light spots mean dry channels, dark spots mean wet ones.
Take ten seconds at the end of every brew to look at the puck. It costs nothing and over weeks of brews it teaches you more than any TDS reading will.
