You pay tax on the money before it goes in. After that, everything it earns is yours to keep, and nothing comes out taxed at 65. Put in your age and what you can add each year, and see what that turns into. Then try starting five years earlier.
We compound once a year and add your contribution at the end of each year until you turn 65, holding the return steady the whole way. Real markets never do that, so read the result as a direction rather than a promise. If your income is inside the phase-out band, we shrink the contribution the way the IRS does. Nothing you type leaves this page.