I built a Roth conversion optimizer because retirement is the phase I’m in. That sounds reasonable until you notice what it leaves out: by the time anyone needs a conversion plan, the big decision (traditional or Roth, every paycheck, for decades) has already been made. So I built the piece I was missing: a Roth vs traditional 401k calculator that optimizes the working years and the retirement years together, as one plan.
It’s free and needs no signup. You enter your salary, when you plan to stop working, what you spend, which accounts you have, and how your employer matches. It tells you how much to put in each account this year, and every year after that.

