The `Iron Man` actor - who has 17-year-old son Indio from a former relationship with actress Deborah Falconer - wants to have a daughter with his wife of five years Susan, and can`t wait to get involved with changing dirty diapers again.
He said: "I think we should probably try to have a girl because I don`t want another male entity to have to compete; I don`t want Indio to feel there`s another boy in my life.
"I think I will be wrist-deep in doo-doo within 18 months. I`m calling it, right here."
Discussing why he wants to add to his family, Robert said he wants to do "what feels righteous".
He said: "What is the upside? The upside is doing what you`re supposed to do, what feels righteous. And that is another 18 years, legally, of thankless blood, sweat and tears."
He also joked about wanting to make sure he has a girl by trying various old superstitious methods.
He told America`s Playboy magazine: "I don`t know what we`d have to do. Do we have to put it in a spoon and hang upside-down? Of course that`s wrong, and I think, `Wait a minute, I don`t get to make that decision.`
"It`s the stupidest conversation ever because it`s like saying `Red or black?` You have a 50 percent chance of being right and a 50 percent chance of being wrong."