Fresh coffee holds CO₂ from roasting. If you pour the full brew at once, that gas pushes water away and the extraction goes patchy. The bloom is a small first pour that lets the gas vent before the real work starts.
What you're watching for
The bed swells. Foam pops at the surface. The smell turns sweeter and rounder. That's CO₂ leaving the grounds and the particles opening up. 30 to 45 seconds is enough for most fresh coffees.
How much water
Twice the weight of the coffee. 15g of coffee gets a 30g bloom pour. Enough to wet every particle, not so much that water starts dripping through before the gas escapes.
When you can skip it
Coffee more than three weeks past roast date has already vented most of its CO₂. The bloom does less. You can shorten it to 15–20 seconds, or skip it entirely on stale beans.
What it sets up
A clean main pour. Without a bloom, every pour after fights gas pockets. With a bloom, the bed is uniform and water goes where you put it. Everything downstream — clarity, sweetness, even flow — depends on this 30 seconds.