Underdosing the portafilter is an espresso crime punishable by weak, underextracted coffee. It is too often overlooked as a cause of poor extraction, with many people believing it is solely the grind that needs adjusting when their shots are not flowing well.
Not filling the basket high enough means the coffee won’t stay compressed and the water will flow through too fast. The tell-tale sign of this phenomenon is a soft, mushy mess in the basket after extraction as opposed to a firm, clean puck that invariably comes out in one piece. Another clue is the pressure gauge not reaching 7 to 9 bars during the extraction.
When researching on the internet it is all too easy to take someone’s recommendation of, let’s say 16g as a definitive dose weight. The problem is that there is such a wide range in portafilter basket sizes out there that 16g of coffee producing a good shot in one basket will quite easily result in a disastrous shot for a different basket.
To confuse matters even more, the portafilter baskets are usually designated by the manufacturers to hold a particular weight. However, this weight is based on the Italian style of extraction which generally uses a coarser grind. When the particles are larger they don’t fit as snugly together as fine particles do, so a full basket of coarsely ground coffee will weigh less than the same volume of finely ground coffee. Thus, what is technically called a 14g basket may need 17g to pull perfect shots, depending on your coffee beans and grind size.
So then, how does one know how much to dose? The best thing to do is not get fixed on a weight to start with. Fill up the portafilter until there is a healthy mound of coffee sitting on top. Level it off, filling in the edges so that the grounds are evenly distributed before tamping. After tamping the coffee ‘puck’ should sit about a quarter of an inch below the top of the basket (though this level varies from machine to machine – use it only as a rough guideline).
If your shot then pours too slow (>30 seconds to achieve 1.5oz) you can adjust the grind to be coarser; if it pours too fast (<25 seconds) then make the grind finer. Bear in mind that each time you adjust the grind, the weight of coffee is going to naturally change too. This is another reason not to dose by weight.
After a little trial and error you should get a feel for how much to dose to achieve a great extraction. Once you reach this point try to always keep your dosing level and technique consistent. This means the only variable you now need to worry about is adjusting the grind.