"if you rollover a Roth 401k into a Roth IRA, is your basis the amount of the rollover, or the amount of the original contributions to the 401k not including gains?"
It depends. If the distribution from the Roth 401(k) would be a treated as a qualified distribution if not rolled over, the entire gross amount of the distribution becomes contribution basis in the Roth IRA. Otherwise, only the basis from the Roth 401(k) becomes basis in the Roth IRA. The amount of basis that transfers to the Roth IRA should be reflected in box 5 of the code-H Form 1099-R.
