Borrowing From Your Own 401(k): The Loan That Sounds Free
Borrowing from yourself sounds like a trick question
A 401(k) loan is a genuinely different kind of debt. There’s no credit check, no application, no lender to convince — you’re borrowing money you already own. You repay it through payroll deduction, typically over up to five years, and the interest you’re charged doesn’t go to a bank. It goes back into your own account.
That last detail is the pitch: “you’re just paying interest to yourself.” It’s true, as far as it goes. But “the interest goes back to you” is doing a lot of work to make this sound like a free lunch, and it isn’t one — for two separate reasons, plus a third risk that has nothing to do with interest rates at all.
See it for yourself
The chart races two versions of the same account. Both start together, at today’s balance. The teal line is what happens if you never touch it — the whole thing stays invested, compounding at the market’s return the entire time. The amber line is what actually happens if you borrow against it: the part you didn’t touch keeps compounding, but the part you borrowed only starts earning the market’s return again once you’ve paid it back — a little at a time, one monthly payment at a time. The two lines start equal and pull apart, because the borrowed money spends time outside the market before it works its way back in.
Things worth trying
- Read the gap at the default. Borrowing $20,000 at 6% against a $60,000 balance, repaid over 5 years while the market returns 7%: left alone, the balance reaches $85,058. Borrow instead, and it reaches $84,387 — $671 less. Nobody sent that money anywhere; it just spent five years compounding at 6% instead of 7% while it was on its way back in.
- Match the rate to the market. Drag “Rate you pay yourself” until it equals “What the market could otherwise earn” (7% and 7%). “Costs you in market growth” reads “Too close to call” — and it’s not just close. Check the exact math and the gap is zero to the penny: paying yourself the market’s own rate really is a wash. That’s the only case where the “you’re just paying yourself” pitch is completely true.
- Find the narrow win. Push “Rate you pay yourself” to 9% and “What the market could otherwise earn” down to 5%. Now the verdict flips: your account ends up $2,567 AHEAD of leaving it alone. This is real and the sim doesn’t hide it — if your self-charged rate genuinely beats the market over the loan’s life, borrowing wins on this one measure. It’s also not something you can predict; you’re guessing at five years of market returns against a rate your plan sets.
- Watch “Interest taxed a second time” the whole time you’re dragging. It never goes away, even in the “narrow win” case above — because it’s a completely separate cost from the market-growth race. Every dollar of interest you pay yourself comes from your paycheck AFTER tax, then sits inside a pre-tax account that will tax it again when you eventually withdraw it.
- Drag “Months until you’d change jobs” down to month 0. The card “Owe if you left that job then” jumps to $6,400 — tax and penalty on the ENTIRE $20,000, due almost immediately. Drag it back up past the loan’s own term and the number drops to ”—”: once the loan is repaid, changing jobs costs nothing extra.
- Overdraw the plan’s cap. Push “How much you borrow” to $45,000 against the default $60,000 balance. The note explains the clamp: plans typically cap a loan at the lesser of $50,000 or half your vested balance — here that’s $30,000, and the sim holds you to it.
The two costs hiding in “you’re just paying yourself”
First: the money is out of the market, and paying yourself doesn’t change that. The moment you take the loan, the borrowed principal stops earning whatever the market would have paid. You do pay it back — principal and interest — but each payment only starts compounding at the market’s rate again from the month it actually lands. If the market outperforms your loan’s rate (the common case: long-run market averages tend to run above typical plan-loan rates), that gap compounds against you the entire time the money’s out. The size of the gap depends entirely on how those two rates compare — which is exactly what the chart’s crossing point shows you.
Second: the interest is taxed twice. This is easy to miss because it has nothing to do with market returns. Loan repayments come out of your paycheck as ordinary payroll deductions, not pre-tax elective deferrals — so you’re repaying both principal and interest with money that’s already been taxed once. The principal roughly nets out against never having borrowed at all: it’s the same dollars, restored. But the interest is brand-new money that only exists in your account because you borrowed, and it’s sitting inside a traditional, pre-tax-deferred account that will tax it again, as ordinary income, whenever you eventually withdraw it. Taxed once leaving your paycheck, taxed again coming out in retirement — the “Interest taxed a second time” card puts a number on exactly that gap.
The risk that has nothing to do with any of that
Leaving your job can call the whole loan. Quit, get laid off, get fired — however it happens, most plans require the outstanding balance to be repaid quickly, commonly by your next tax-filing deadline. Miss that window and the balance becomes a “deemed distribution”: taxed as ordinary income, plus a 10% early-withdrawal penalty if you’re under 59½ — due almost immediately, not spread over years like the loan itself was. The earlier in the loan’s life you’d change jobs, the more you still owe and the bigger that bill.
This is the risk that has nothing to do with interest rates or market timing. You can pay yourself a great rate, beat the market over the loan’s whole term, and still get hit by this one if the job ends before the loan does.
How to actually decide
- Compare your loan rate to a realistic market return, not to a credit card. A 401(k) loan almost always beats a credit card’s rate — that’s not the real question. The real question is whether your plan’s rate beats what that money would otherwise have earned invested, and you can’t know that in advance; you’re making a bet on both rates for the life of the loan.
- Price the double taxation before you borrow, not after. It’s a real, quantifiable cost on top of the market-growth question, not a rounding error — and it applies even in scenarios where the growth math looks fine.
- Ask “what if I lost this job tomorrow” before you sign, not the day it happens. If the answer is “I couldn’t repay the balance by the tax deadline,” that’s the real risk this loan carries — bigger, in most cases, than either of the money questions above.
- A 401(k) loan is usually a better idea the closer it is to a true short-term bridge — a small amount, repaid fast, from a stable job — and a worse one the longer it runs and the shakier the job underneath it.
Key terms
- Vested balance — the portion of your 401(k) that’s fully yours, including any employer match that’s cleared its vesting schedule; it’s what plan-rule loan caps are based on.
- Deemed distribution — an unpaid loan balance treated as if you withdrew it: taxed as ordinary income, plus a 10% penalty if you’re under 59½.
- Double taxation (on the interest) — the interest you repay is after-tax money going into a pre-tax account, so that specific slice gets taxed twice: once now, once on withdrawal.
- Opportunity cost — what you gave up by not doing something else with the money — here, specifically the market growth the borrowed principal missed while it was out.
“You’re just paying interest to yourself” is true and mostly beside the point. The real costs are the market growth the money misses while it’s out, the interest that gets taxed twice on its way back, and a job change turning the whole remaining balance into an immediate tax bill. None of that means a 401(k) loan is always wrong — it can still be the least-bad option next to high-rate debt or a true emergency. It means “borrowing from myself” is not the same as “free,” and now you can put a number on the difference.