How Does Grind Size Change Coffee Brewing and Extraction?

Grind size sets the contact area between water and coffee and how long extraction lasts, and that shows up directly in the cup. Before dialing in grind, the beans must be freshly roasted. Check for a one-way exhaust valve and a clear roast date, since carbon dioxide released during roasting needs three to seven days of resting.

Brewing beans right after roasting lets escaping carbon dioxide disrupt extraction, making results unstable. One roaster suggests resting light-to-medium roasts three to four days. After resting, grind coarser or finer by taste, and use a No. 20 sieve to calibrate for different coffee machines and brewing methods.

The coarseness of coffee grounds directly determines the contact area between water and coffee and the extraction time, which in turn affects the final taste. But grind size is also the most headache-inducing step for many enthusiasts. This article, drawing on Front Street's practical experience, systematically explains how to judge resting time based on the roast date, the actual impact of grind coarseness on flavor, and how to calibrate grind size for different brewing methods using a No. 20 sieve. The article also compiles the suitable equipment for five particle sizes ranging from extra-fine to coarse, helping you roughly determine the grinding direction even without a professional sieve, and find the grind parameters that suit your taste.

The size of grind particles plays a key role in coffee brewing—it determines the contact area between water and coffee grounds and how long the entire extraction process lasts, ultimately reflected directly in the taste in the cup. However, choosing the grind size is precisely the step that confuses the most people. This time, Front Street will talk about how to judge whether the grind should be coarser or finer for different types of coffee beans.

Before that, there is a prerequisite that must be met: the single-origin beans or espresso blends in your hands must be freshly roasted. When purchasing, check whether the packaging has a one-way exhaust valve and whether the roast date is clearly labeled. That small button-like hole at the top of the bag is the one-way exhaust valve; its purpose is to let the carbon dioxide released by the coffee beans escape smoothly while preventing outside air from flowing back into the bag. Every bag of coffee produced by Front Street Coffee (FrontStreet Coffee) uses this kind of packaging and is guaranteed to be freshly roasted.

After getting the coffee beans, first take a look at the roast date. During roasting, coffee beans accumulate a large amount of carbon dioxide internally, so they need 3 to 7 days to release these gases on their own; this process is commonly called resting. If you skip resting and brew beans just after roasting, the continuously escaping carbon dioxide will seriously interfere with extraction, making the entire brewing process highly unstable. As for how long to rest them, Front Street's recommendation is: rest light-to-medium roasted beans for 3 to 4 days, and medium-dark roasted beans for 4 to 5 days.

As mentioned earlier, the most intuitive manifestation of coffee bean freshness is carbon dioxide emission. But besides carbon dioxide, freshness also determines how well the aromatic substances in coffee flavor and aroma are retained. These small-molecule aromatic substances volatilize much faster than the large-molecule substances that bring bitterness. So the longer the coffee beans are stored, the more carbon dioxide and aromatic substances are lost, and the fewer effective components can be extracted. If the beans themselves are already stale, no amount of grind adjustment can save them.

So how exactly does grind size influence the flavor expression of coffee? It can be understood this way:

1. At the same weight, the finer the grind, the more times the coffee grounds are cut, the larger the total surface area, and the greater the area of coffee grounds that water can contact.

2. The finer the grind, the smaller the particles are cut, the shorter the distance for water to penetrate from the particle surface to the center, and the shorter the time needed to extract the internal flavor substances—or in other words, extraction becomes easier.

To put it more plainly, other conditions being equal, the finer the particles, the higher the extraction yield. When coffee is ground fine, the contact area with water is large, and the dissolved coffee concentration is naturally higher; conversely, when ground coarse, the contact surface area is small, and the extracted concentration is lower.

What are the consequences of grinding too fine? Visually, the particles are small, and the gaps between particles narrow accordingly, increasing resistance when water passes through the coffee bed, and the drip rate slows noticeably. Because the particles are small and the surface area is large, there are more sites where water can extract substances, and both concentration and extraction yield climb. It tastes heavy, with prominent bitterness and astringency, and other flavors are easily masked.

What happens if you grind too coarse? Visually, the particles are noticeably larger, the spacing between particles widens, water passes through easily, and the filtration speed increases. Coarse particles mean reduced surface area, fewer extractable substances, and a simultaneous drop in concentration and extraction yield. The coffee tastes weak and thin, with almost no perceptible flavor.

So how do you determine the appropriate grind size? Front Street recommends using a cupping calibration sieve No. 20, with a mesh size of 0.85 millimeters. The method is to take 10 grams of coffee beans, grind them roughly, pour them into the sieve and sift. After sifting, observe how much coffee powder fell into the tray, and calibrate the grind size based on the passing rate. Front Street Coffee (FrontStreet Coffee)'s grind size coarseness standard sieve passing rate references are as follows: cold drip > pour-over, siphon 80% > moka, American drip 75-80% > cupping 70-75% > French press 65-70%.

Of course, beans with different roast levels also correspond to different grind sizes. For example, when Front Street brews light-roasted beans, it uses a grind with an 80-85% passing rate on a 0.85 mm sieve; medium-roasted beans use a grind with a 70-75% passing rate; dark-roasted beans use a grind with a 65-70% passing rate. However, everyone's preference for coffee strength differs, so these data are only for reference, and in practice you can still adjust according to your own taste.

What if you don't want to buy a calibration sieve? Front Street also knows this process is a bit cumbersome to operate, so it has put together a simple reference chart for coffee grind size, so you can see roughly which coarseness your brewing tool corresponds to. Front Street divides coffee grounds into five levels: extra-fine, fine, medium-fine, medium, and coarse.

(From left to right: coarse, fine, extra-fine)

Extra-fine: used for espresso coffee (moka pot, espresso machine).

When making espresso, you must tamp the coffee grounds firmly so they create resistance to high-pressure hot water, allowing the essence of the coffee to truly be pressed out. The more uniform and fine the grind, the tighter the puck after tamping, and the more even the resistance to water. Only espresso "squeezed" out this way will have a rich crema.

Fine: used for drip bag coffee.

This coarseness is Front Street's "soul mate" for making drip bag coffee. The main reason is that the entire drip bag is made of filter paper, so the flow rate is very fast. Unlike a pour-over dripper, which has a narrowed opening—for example, the common three-hole dripper actually serves to slow the outflow of water—so drip bags更需要 fine grounds to expand the extraction area and thereby extract more substances.

Medium-fine: used for brewing light-to-medium roasted coffee (various pour-over drippers).

When Front Street brews light-to-medium roasted beans, it uses medium-fine grounds, roughly the coarseness of fine granulated sugar, with a sieving rate between 75-80%. Light-to-medium roasted beans usually have rich acidity, and a medium-fine grind can better showcase the coffee's overall sweet-and-sour balance.

Medium: used for brewing medium-dark roasted coffee (various pour-over drippers).

When Front Street brews medium-dark roasted beans, it uses medium grounds, roughly equivalent to the size of coarse granulated sugar, with a sieving rate between 70-75%. Medium grounds are less likely to produce extra-fine powder, and are also less likely to cause a large amount of water to accumulate in the dripper and soak the coffee grounds due to unstable water flow at the beginning of pouring, thereby avoiding over-extraction.

Coarse: used for making coffee with a French press.

A French press extracts the most original taste of coffee through a four-minute steep. Compared with pour-over, the coffee liquid from a French press appears somewhat murky because the metal filter mesh has larger holes. Therefore, in terms of grind size, coarse grounds should be chosen: first, to allow the flavor to be released slowly; second, so that when the filter is pressed down, coffee grounds will not "slip out" through the gaps in the filter and mix into the coffee, affecting the mouthfeel when tasting.

The above content is compiled by CoffeeHunters, a coffee news website.