The Best Grind Size for a Phin

What medium-coarse looks like

Rub a little between your fingers. It should feel gritty and distinct, like coarse sand or the raw sugar you get in a café packet. You should be able to see individual particles without looking hard.

If it feels like flour or smooth powder, that is espresso grind and it will choke the phin. If it looks like coarse breadcrumbs, that is French press grind and the water will fall straight through.

Why a phin needs this particular range

A phin has no pressure. Water sits on top of a bed of coffee roughly two centimetres deep and works down through it under its own weight.

The bed must seal. The press plate does not compress the coffee; the coffee swells during the bloom and pushes up against the plate. A grind that is too coarse leaves too much space between particles for that seal to form, and water finds a channel round the edge.

Water must still pass. A grind that is too fine packs into something close to a solid, and the drip stops entirely.

Medium-coarse is the band where both hold. It is narrower than it is for most brew methods, which is why grind is such an effective lever here.

Why a grind that looks right can still behave wrong

A grinder does not produce one size of particle. It produces a distinctly bimodal distribution: a population of large particles and a separate population of very fine ones. The fines carry most of the accessible surface area, because surface area to volume climbs steeply as particles get smaller.

For a brewer that runs on gravity alone, that matters a great deal. Two grinds can look identical in the chamber and behave completely differently, because what governs the drip is not the average particle size but how many fines are sitting in the gaps between the big particles, plugging them.

It also explains a few things that otherwise look like bad luck.

Blade grinders are unpredictable. A blade smashes rather than cuts, so it produces fines and boulders simultaneously. The boulders stop the bed sealing and the fines choke it, in the same chamber, at the same time. This is why a blade grind often drips fast and tastes over-extracted.

The same setting drifts as the grinder warms up. Warm burrs change the shape of the distribution as well as the size, so grinding finer on warm burrs does not reproduce what cooler burrs gave you. If your second phin of the morning behaves unlike the first, this is why.

Stale pre-ground coffee can drip faster than fresh. Less trapped gas means less resistance, which is a brew-time question more than a grind one. See phin brew time.

Reading the drip

The drip tells you the grind is wrong before you taste anything.

What you see	What it means	What to do
Done in under 2 min	Too coarse	One step finer
A steady drip, done in 4–6 min	Correct	Nothing
Barely dripping at 8 min	Too fine	One step coarser
Nothing at all	Much too fine, or press too tight	Loosen the press first, then grind coarser
Fast, and water visible round the edge	Channelling	Level the bed and reseat the press; this may not be grind at all

That last row matters. Channelling looks like a too-coarse grind, but grinding finer will not fix it. Check that the bed is flat and the press is sitting square first.

When the drip runs too fast

Grind is the third thing to check, not the first. Work in this order, because the first two cost nothing and an unlevel bed presents exactly like a coarse grind:

1. Is the bed level? Tap the phin on the counter until the surface is flat. Water takes the shallow side of a sloped bed and runs straight down it.
2. Is the chamber at least a third full? Too little coffee cannot seal against the press plate however fine you grind it. Add 2–3 g. See how much coffee goes in a phin.
3. Only then, grind one step finer. Change one thing per brew. Change two at once and you cannot tell which one did it.

Grinding finer to compensate for a shallow bed is the specific mistake to avoid. It does slow the drip, but by compacting the bed rather than sealing it, and compacted beds channel.

When the drip stalls or barely moves

Reverse the order. Here the cheap fix is the press:

1. Back the press plate off until it is barely touching the coffee. A press screwed down hard is the most common cause of a phin that will not drip, and it is instant to undo.
2. Check you have not overfilled the chamber. Past about half full there is no headroom, and the bed has nowhere to swell into except against the plate.
3. Then grind one step coarser.

A phin that stalls with the press loose and the chamber correctly filled is ground too fine, and that is usually an espresso grind bought or set by mistake. The full symptom-by-symptom version of this is on how to use a phin.

Grinder settings

These are the settings commonly reported by people brewing in a phin. We have not tested these grinders, and burr wear, burr alignment and burr temperature all move the answer, so treat them as somewhere to start rather than a specification.

• Hand grinders (1Zpresso, Comandante, Timemore): roughly 26–32 clicks from zero on most models, a touch coarser than you would use for a V60.
• Baratza Encore: around 20–24.
• Blade grinder: possible but difficult. Pulse in short bursts, shake between them, and accept that you will get fines and boulders together. A slower drip than expected is normal.

Buying pre-ground, and when not to

Almost all Vietnamese coffee sold domestically is pre-ground, in the right grind, and it works. For most people starting out, this is the sensible option.

The cost is aromatics. Grinding increases both the porosity and the surface-to-volume ratio of the coffee, and that accelerates degassing and staling together. That is the mechanism behind the advice to grind fresh.

How long you have is less settled than the advice implies. The Specialty Coffee Association’s review of the staling literature describes the mechanism and cites measurements of individual aroma compounds falling away within days, but the reviewed studies do not converge on a shelf life for ground coffee, so this page does not give one. An opened bag on a warm shelf is worse than a sealed one in a cupboard; beyond that, trust the cup.

Dark-roast robusta is somewhat more forgiving here than a light arabica, because much of what you are tasting comes from roast development rather than from delicate origin character.

If you drink Vietnamese coffee more than a few times a week, a hand burr grinder is the single change that will improve it most. See grinders.

The other levers, and how they interact, are collected under brewing.