The portafilter is the handled basket holder you lock into the group. Its diameter sets the size of the coffee puck and, importantly, which baskets, tampers and distribution tools will fit your machine.
You will mostly see 58mm, but several popular home machines use smaller sizes, and it is worth knowing the difference before you spend money on accessories.
58mm: the commercial standard
58mm is the size used on commercial machines and the vast majority of prosumer ones. Choosing it gives you the widest possible choice of precision baskets, tampers, distribution tools and bottomless portafilters.
If you like the idea of tinkering with accessories later, 58mm keeps every door open.
Smaller sizes (54mm, 57mm)
Some home machines use 54mm (notably Breville/Sage) or 57mm. They make perfectly good espresso — the size itself is not a quality issue — but the accessory ecosystem is smaller and you have to match parts to your exact size.
It is the kind of detail that is easy to overlook and annoying to discover after you have bought the wrong tamper.
What it means when you're buying
If accessory choice and resale matter to you, 58mm is the safe pick. If you are happy with a specific machine that uses 54mm or 57mm, just buy tampers and baskets in that exact size from the start. Either way, check the portafilter size on the spec sheet before ordering extras.
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Frequently asked questions
Does portafilter size affect taste?
Not directly. It changes puck dimensions and accessory compatibility, but a well-prepared shot is excellent at any of the common sizes.
Why is 58mm so common?
It is the commercial standard, so the entire accessory industry is built around it. That network effect is why most prosumer machines adopt it.
Can I use a 58mm tamper on a 54mm machine?
No — the tamper must match the basket diameter. Always buy accessories in your machine's exact size.