401k

Most 401(k) plans let you borrow against your own balance — no credit check, no application, and the interest you're charged is paid right back into your own account. That last part makes it sound like borrowing from yourself is free. It isn't, for two separate reasons: the money you removed stops compounding at the market's rate while it's gone, even though you're paying yourself interest, and that interest is paid with after-tax paycheck dollars that land inside a pre-tax account — taxed once now, taxed again on withdrawal. On top of both, if you leave your job before the loan is repaid, the outstanding balance can come due almost immediately, taxed as a withdrawal plus a penalty. This lesson races your real account value against what it would be worth had you left it alone, so you can see exactly what borrowing from yourself costs — and the one narrow case where it doesn't.

Retirement accounts and the employer match cover the container; this lesson is the deep dive on the single most-asked question about that container — Roth or Traditional? Both let you contribute the exact same monthly dollar amount, the real choice on a payroll form. A Traditional contribution is pre-tax, so it compounds to the identical gross balance a Roth contribution does — a Roth contribution is already-taxed money that then grows completely tax-free, and neither path pays any tax on its growth along the way. The only thing that ever touches the money is the ONE tax event: never, for Roth; at your future rate, for Traditional. That sounds simple, but almost every back-of-envelope comparison gets it wrong, because contributing the same dollar amount to each plan is not actually an equal sacrifice — the Traditional contribution shrinks your taxable income, so it costs you less take-home pay today than the Roth contribution does. Unless that monthly tax saving gets invested too, a naive comparison makes Traditional look strictly worse than Roth no matter what the tax rates are, which is backwards. Invest it, and the comparison collapses to one exact number: your current tax rate minus your expected retirement tax rate. Equal rates make the two plans identical, to the penny — not approximately, exactly, and that holds regardless of how much you contribute, what it returns, or how long it grows; only the tax-rate relationship ever decides the winner. The simulator races three balances — Roth, Traditional with the tax break invested, and Traditional with the tax break spent — so the size of that naive mistake is visible in dollars, not just asserted. The durable lesson: it's a bet on your own future bracket, never a bet on time horizon or investment return, and whichever way you bet, invest the tax break or the bet isn't even being placed fairly.

Nearly everyone who leaves a job with money in an old 401(k) faces the same fork: roll it into an IRA or the new employer's plan, or cash it out. Cashing out is tempting — it's money in hand today — but it triggers ordinary income tax on the ENTIRE balance immediately, plus a 10% early-withdrawal penalty if you're under 55 (a narrower cutoff than the 59½ most people expect, because of a little-known exception called the Rule of 55). Unlike a 401(k) loan, which you repay, a cash-out is permanent: that money never goes back, so decades of future compounding are gone for good. This lesson races two after-tax paths from the day you leave the job to the age you plan to retire: rolling the full balance over, tax-deferred the whole way, against cashing out and reinvesting whatever's left after today's tax bill. Rolling over almost always wins, and by a lot — but not unconditionally, and the simulator shows the real, narrow exception too, not just the common case.