The second-placed Championship club suffered a 2-1 defeat to Middlesbrough on Saturday and Tomkins believes that the Hammers should take full advantage when they face Leicester City this evening.
"We are still confident," said Tomkins. "Our away record is really good and our home form, as we showed the other day in beating Brighton 6-0, is definitely better as well.
"Of course we can go into the play-offs and win them. We will concentrate on the rest of the season first, though, because we could still go up automatically."