Flow rate is the live rate at which espresso is arriving in the cup, in grams per second. Better scales calculate and display it as the shot runs.
Why it is useful
A total weight and a total time tell you what happened. Flow rate tells you what is happening, while there is still information in it.
Reading it
A slow, gradually rising flow that settles around 1.5 to 2 g/s is a healthy shot.
A sudden jump early — flow leaping to 3 g/s in the first few seconds — is channelling. Water has found a path around the puck. You will taste it as sour and bitter at once.
Flow that never gets going and stays under 0.5 g/s is choked.
Why it beats time alone
A channelling shot can hit a perfectly normal 28 seconds and 36 g, because the numbers are averages. Flow rate shows the shape of the extraction, which is where the fault is visible.
What it does not replace
A bottomless portafilter, which shows channelling directly and unmistakably. Flow rate infers it; the naked basket shows it.
Between the two, the portafilter teaches faster and the scale is useful every day.
Which scales have it
The Timemore Black Mirror Basic 2 at around 55 euros is the cheapest good one. Acaia and similar cost several times that.
Last reviewed 8 August 2026