How fast water leaves your kettle, measured in grams per second. Pour 150 g of water over 30 seconds and you poured at 5 g/s. It's the variable most home brewers never measure and most competition brewers rehearse.
Flow rate is not the same as brew time. Brew time is how long the whole thing took, and the bed controls most of it. Flow rate is the part you control — the rate at which you hand water to the bed.
Why it changes the cup
Pouring faster does two things at once, and they pull in the same direction. It raises the water level above the bed, which pushes harder and speeds drainage. And it hits the slurry with more energy, which is agitation — more contact between water and grounds, faster extraction.
So a fast pour extracts more per second but gives the bed less time overall. A slow pour is gentler and longer. Two brews with identical dose, grind, ratio and temperature can land a long way apart on nothing but this.
The number that matters is your own
There's no correct flow rate. Most filter recipes land somewhere between 3 and 7 g/s, and the recipes that specify one are usually competition routines where the barista rehearsed the pour until it was repeatable.
What matters at home is holding whatever rate you chose. If the same recipe gives you a different cup on different days and the grind, dose and water are identical, pour speed is the usual culprit — it's the one variable that drifts without anyone noticing, because nobody is looking at it.
How to get a feel for it
Put the kettle on the scale and pour into a cup for ten seconds. Divide the grams by ten. Most people are surprised — a "slow, careful" pour is often 8 g/s, and a genuinely slow one feels uncomfortably patient at first.
A gooseneck kettle helps enormously here, not because it's precise but because a narrow spout makes a slow stream possible. A wide kitchen kettle can barely pour under 8 g/s without dribbling.