The 33-year-old actress is currently engaged to Muse front-man Matt Bellamy, but does not want to wed quickly because her first marriage to musician Chris Robinson - with whom she had now eight-year-old son Ryder - was "extremely tumultuous".
She said: "I shot `Almost Famous` at 20, and I got married at 21. I had Ryder at 24 so, you know, a lot happened in my 20s.
"I was working a lot, and there were times that were extremely tumultuous. So 30 kind of felt like `Oh, OK.` I had a different kind of confidence.
"We`ll do it eventually, but I want it to be right. I don`t want to just do it, you know what I mean?"
Kate - who has 15-month-old son Bingham with Matt - spends much of her time jetting between the UK and US with her family and can only do it because she "plans", but does find it difficult with her eldest son in school.
She told Harper`s Bazaar magazine: "It is two very different lifestyles, but I feel really lucky to be able to expose the kids to both. But it takes planning, and for somebody like me who wants to fly by the seat of my pants all the time? I can be such a gypsy, but with the kids in school, I`m learning how to be more organised."