Don’t you normally need an undergrad to start a PhD in the US as well?

In the UK an undergrad is 3 years in most cases, then 3 years for a PhD, so 6 years total.

Some institutions will technically admit you for a PhD with no prior degree at all, but nobody does this in practice.

It's less common now in the UK to admit people to PhD programs directly from undergrad. In many cases you'd be expected to do a one-year masters first, although that's not a universal requirement.

Nowhere near universal. It’s common to go from undergrad to PhD in the U.K.

Tbh most Master’s, esp MSc, don’t add anything.

Varies a lot from field to field.

>Don’t you normally need an undergrad to start a PhD in the US as well?

Yes, but not a graduate degree.

For comparison, at least in Austria, you need to have finished both a Bachelor of Science and a Master of Science in order to be able to start a PhD.

That's usually not the case in the USA (and some common law countries), where you only need the equivalent of a Bachelor of Science in order to start the PhD. You then do a small version of the Master of Science in parallel with the PhD.

So the UK is probably the best model - 3 years undergrad, 3-4 years PhD max.

Oh, that explains why I see more Americans going for PhD (successfully or not) than, say, Germans. It never made sense to me because I was used to it being the thing you can do after an MSc/MA and those already seemed like a questionable choice for many jobs.

Yes, this is true. You cannot start directly with a US 4-year Bachelor's, you would need a 1-year Master's. I suppose this can be a deterrent for many, but I still see many people with MSc degrees that complain (understandably) about all the hurdles of US PhDs and could benefit from considering Europe.

This is actually not a hard requirement, you can start after your bachelor's. Not sure about having no degree at all like a sibling comment mentioned, maybe that's possible too.

That's not necessarily true in the UK. Many masters courses, like mine, take a total of four years. In some cases you can even get lucky and go straight from undergraduate to a PhD. I know at least one person who did that.

It might depend on the field, but in Math/CS at a number of good unis, the standard path is a 4-year integrated MSc (you get a BSc after 3), then apply to PhD.

It is possible to do the MSc over 5-6 years and spend some time working, whether in parallel with studies as a teaching assistant, or taking a term/year out to get some industry experience. But in terms of credits gained, an MSc is 4 years net of studies.