At first glance, portafilters might seem interchangeable, and you can switch portafilters between various espresso machines. However, a portafilter will only work if it fits the exact spot on the group head. Measuring a portafilter is easy as long as you’re using the right tools. You can use a machine caliber to measure the interior dimensions of the portafilter. After measuring the diameter, you should transform the measurement from inches to decimals and then to mm by multiplying the number by 2.54. If the portafilter basket is tapered, you should measure the narrowest diameter at the basket's bottom. The most standard size of portafilters is either 58 mm or 54 mm.