<p>Coffee brewing is simple physics and chemistry. In short, soluble compounds and oils migrate from the beans to the water. The extraction will depend on temperature, pressure and contact area between beans and water. There’s also a balance between desired and less desired flavor compounds (e.g. acidic and bitter) that a specific brewing method needs to manage, so you tweak the given parameters.</p>
<p>If you have a coarse grind, the total surface area is smaller, so to get a comparable extraction you need more time (minutes for hot water as in French press, hours for cold water as in cold brew). For finer and fine grinds, you brew shorter (pour over) and probably increase the pressure (espresso).</p>
<p>Of course you can also look at the problem in reverse, like in the question (“how much can method X extract”), and match the grind size to hit the optimum. However, there your assumption that e.g. an espresso machine can extract <em>more</em> is incorrect. The very short time plus the compact grounds means that comparatively little coffee flavor is extracted - but some would say that it’s the best parts ;-)</p>

CC BY-SA 4.0
2021-04-11T22:08:35.393Z
Surface area wouldn't seem to matter as much as the amount of of time the hot water is in contact with the grains.
Geremia
CC BY-SA 4.0
2022-01-15T18:22:08.413Z
Use an espresso grind in a pour-over setup or vice versa and it becomes blatantly obvious.
Stephie
CC BY-SA 4.0
2021-04-10T05:34:19.073Z
2021-04-10T05:34:19.073Z
Stephie
<p>There are two obvious reasons why course grind is better for a French press:</p>
<ol>
<li>All the water is added at once, so the brewing process is relatively fast.
With fine-grind, <em>your</em> perfect time before plunging might be 1 minute, and with course grind it might be 5 minutes.
If your timing is out by 15 seconds, it will cause a very noticeable difference with the fine-grind (±25%), while the course-grind will be far more forgiving (±5%).</li>
<li>Cleanup is so much easier, as the fine grounds are much more apt to become caught in the mesh and other parts.</li>
</ol>

CC BY-SA 4.0
2023-10-07T03:38:29.603Z
2023-10-07T03:38:29.603Z
Ray Butterworth
<p>Key to understanding this is the <strong>flow rate</strong> of the hot water: Fine grains = slow flow rate; coarse grains = fast flow rate.</p>
<p>Grain size determines how quickly the hot water flows, and thus how much time it is contact with the grains. Faster flow means the water has less time to be in contact with the grains and thus less time the water can extract the oils and other nutrients. (Certainly, surface area factors in, too; cf. <a href="https://coffee.stackexchange.com/a/5606/8287">Stephie's answer</a>.)</p>
<p>Methods where the grains are soaked (cold-brew, French press, pour-over) don't require fine grains. One could use fine grains in such methods (as in the Turkish method), but it's not necessary, and they would be harder to filter out.</p>
<p>Methods where water quickly flows by the grains also extract more quickly, because of higher temperature and pressure.</p>
<pre>
Grain Size     Flow Rate     Extraction Time     Methods
⬤             •             ⬤                  Cold brew, French press, etc.
•              ⬤            •                   Espresso, moka, etc.
</pre>
<hr>
<p>Kingston's <a href="https://isidore.co/calibre#panel=book_details&amp;book_id=7842" rel="nofollow noreferrer"><em>How to Make Coffee: The Science Behind the Bean</em></a>:</p>
<blockquote>
<p>When grinding for espresso at home, as a rule of thumb if the coffee is coming out too fast, use a <strong>finer grind to slow down the extraction</strong>; or, if the coffee is coming out too slowly, make the grind coarser.<br><br>If your coffee tastes a little <strong>sour</strong>, this is often due to <strong>underextraction</strong>. With methods of brewing other than espresso, using a finer grind will allow more coffee to be extracted during the brewing time. The same is true of <strong>bitter</strong> coffee—this could be due to your beans, equipment, or brewing method, but it may also be caused by <strong>overextraction</strong>. Make the grounds a little coarser and to slow down the extraction rate.</p>
</blockquote>
<p><a href="https://i.sstatic.net/8IIho.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8IIho.png" alt="Seven Primary Levels of Grind" /></a></p>

CC BY-SA 4.0
2021-04-10T06:24:11.067Z
Please try to address the question more specifically. While grind size is certainly related to extraction, I think someone who comes across the question would prefer a more direct answer. Please edit it to make the answer more explicit.
JJJ
CC BY-SA 4.0
2021-04-12T01:38:08.130Z
@JJJ Please see my edits.
Geremia
CC BY-SA 4.0
2021-04-12T02:55:10.077Z
I think flow rate is a confusing substitute for extraction time. Some other methods may go finer than normally recommended, but that changes the extraction time too. So it's not just that finer grains may cause clogging in some brewing methods, they can also make the extraction time unworkable. I think that applies for most brewing methods when using a Turkish coffee grind size.
JJJ
CC BY-SA 4.0
2021-04-12T16:58:36.447Z
@JJJ Extraction time is another way of looking at it. [_Ceteris paribus_](https://www.merriam-webster.com/dictionary/ceteris%20paribus), flow rate and extraction time are inversely proportional: (flow rate) ∝ 1/(extraction time).
Geremia
CC BY-SA 4.0
2021-04-10T01:18:48.617Z
2021-04-12T17:04:52.867Z
2021-04-12T17:04:52.867Z
Geremia
<p>Why do espresso grains have to be finely ground?
<br><sub>(I've seen some moka pot instructions say medium is good, too.)</sub></p>
<p>I thought espresso machines extracted the most out of the grains, so it would seem they wouldn't need to be finely ground—that fine-ground would be suited for French press (lower temps &amp; pressure, no?) and course ground for espresso (the exact opposite of what's recommended!).</p>

<espresso><espresso-machine><moka><french-press><ground-coffee>
Why do espresso grains have to be finely ground and French press grains coursely ground?
Geremia
# Why do espresso grains have to be finely ground and French press grains coursely ground?

(Asked by: username_0 on 2021-04-10)

Why do espresso grains have to be finely ground?
  
(I've seen some moka pot instructions say medium is good, too.)

I thought espresso machines extracted the most out of the grains, so it would seem they wouldn't need to be finely ground—that fine-ground would be suited for French press (lower temps & pressure, no?) and course ground for espresso (the exact opposite of what's recommended!).

**Tags:** espresso, espresso-machine, moka, french-press, ground-coffee


---

## 3 answers

### Answer by username_0 on 2021-04-10. Score: -3 | Accepted answer

Key to understanding this is the flow rate of the hot water: Fine grains = slow flow rate; coarse grains = fast flow rate.
Grain size determines how quickly the hot water flows, and thus how much time it is contact with the grains. Faster flow means the water has less time to be in contact with the grains and thus less time the water can extract the oils and other nutrients. (Certainly, surface area factors in, too; cf. username_1's answer.)
Methods where the grains are soaked (cold-brew, French press, pour-over) don't require fine grains. One could use fine grains in such methods (as in the Turkish method), but it's not necessary, and they would be harder to filter out.
Methods where water quickly flows by the grains also extract more quickly, because of higher temperature and pressure.

Grain Size     Flow Rate     Extraction Time     Methods
⬤             •             ⬤                  Cold brew, French press, etc.
•              ⬤            •                   Espresso, moka, etc.


Kingston's How to Make Coffee: The Science Behind the Bean:

When grinding for espresso at home, as a rule of thumb if the coffee is coming out too fast, use a finer grind to slow down the extraction; or, if the coffee is coming out too slowly, make the grind coarser.If your coffee tastes a little sour, this is often due to underextraction. With methods of brewing other than espresso, using a finer grind will allow more coffee to be extracted during the brewing time. The same is true of bitter coffee—this could be due to your beans, equipment, or brewing method, but it may also be caused by overextraction. Make the grounds a little coarser and to slow down the extraction rate.

> **Comments:**
> **username_3**: Please try to address the question more specifically. While grind size is certainly related to extraction, I think someone who comes across the question would prefer a more direct answer. Please edit it to make the answer more explicit.  
> **username_0**: @username_3 Please see my edits.  
> **username_3**: I think flow rate is a confusing substitute for extraction time. Some other methods may go finer than normally recommended, but that changes the extraction time too. So it's not just that finer grains may cause clogging in some brewing methods, they can also make the extraction time unworkable. I think that applies for most brewing methods when using a Turkish coffee grind size.  
> **username_0**: @username_3 Extraction time is another way of looking at it. [_Ceteris paribus_](https://www.merriam-webster.com/dictionary/ceteris%20paribus), flow rate and extraction time are inversely proportional: (flow rate) ∝ 1/(extraction time).  

---

### Answer by username_1 on 2021-04-10. Score: 3

Coffee brewing is simple physics and chemistry. In short, soluble compounds and oils migrate from the beans to the water. The extraction will depend on temperature, pressure and contact area between beans and water. There’s also a balance between desired and less desired flavor compounds (e.g. acidic and bitter) that a specific brewing method needs to manage, so you tweak the given parameters.

If you have a coarse grind, the total surface area is smaller, so to get a comparable extraction you need more time (minutes for hot water as in French press, hours for cold water as in cold brew). For finer and fine grinds, you brew shorter (pour over) and probably increase the pressure (espresso).

Of course you can also look at the problem in reverse, like in the question (“how much can method X extract”), and match the grind size to hit the optimum. However, there your assumption that e.g. an espresso machine can extract *more* is incorrect. The very short time plus the compact grounds means that comparatively little coffee flavor is extracted - but some would say that it’s the best parts ;-)

> **Comments:**
> **username_0**: Surface area wouldn't seem to matter as much as the amount of of time the hot water is in contact with the grains.  
> **username_1**: Use an espresso grind in a pour-over setup or vice versa and it becomes blatantly obvious.  

---

### Answer by username_2 on 2023-10-07. Score: 0

There are two obvious reasons why course grind is better for a French press:

1. All the water is added at once, so the brewing process is relatively fast.
   With fine-grind, *your* perfect time before plunging might be 1 minute, and with course grind it might be 5 minutes.
   If your timing is out by 15 seconds, it will cause a very noticeable difference with the fine-grind (±25%), while the course-grind will be far more forgiving (±5%).
2. Cleanup is so much easier, as the fine grounds are much more apt to become caught in the mesh and other parts.


---

