Inconsistency always has a cause, and there are only a handful of candidates.
Things you are probably not measuring
The dose. If you grind by time rather than weight, the dose changes as beans age and as the hopper empties. A gram either way on 18 g is over five per cent.
Retention. Single-dosing with a grinder that holds a gram means part of every shot is yesterday’s coffee.
Group temperature. On a machine without a PID, whether you flushed and how long you waited changes brew temperature by several degrees.
Things that vary on their own
Bean age. Fresh coffee degasses and runs fast; the same bag at three weeks runs slow. Re-dialling weekly is normal.
Puck prep. If you sometimes stir the bed and sometimes do not, you have introduced the largest variable there is.
The diagnostic routine
Fix everything measurable for three days: dose by weight, yield by weight, same timing, same prep, same flush.
If shots are still varying, the remaining variable is one you cannot measure — usually temperature, or a dirty machine.
The most likely single answer
Weigh the dose. It removes the commonest cause in one step and costs twenty euros.
Last reviewed 8 August 2026