A $20,000 401(k) Loan for Your Down Payment vs. Waiting 2 Years to Save It: The Real 10-Year Math
A 401(k) loan looks like free money on paper: you borrow from yourself, pay yourself back with interest, and the whole thing seems to net out to zero. It doesn't. The real cost of a 401(k) loan isn't the interest rate at all — it's the market growth that $20,000 doesn't earn while it's sitting outside your account as a loan to you instead of an investment. The alternative, waiting to save that $20,000 the old-fashioned way, has its own real cost: months or years of rent that builds no equity. Here's what both paths actually cost over 10 years, using the current numbers rather than rules of thumb.
1. What Borrowing From Your 401(k) Actually Involves
Under IRS rules, most 401(k) plans that allow loans cap them at the lesser of $50,000 or 50% of your vested account balance (if half your vested balance is under $10,000, some plans let you borrow up to $10,000, though plans aren't required to offer that exception). You then repay the loan, typically through payroll deduction, over up to five years for a general-purpose loan — longer if it's specifically for a primary residence. The interest rate is usually the prime rate plus 1 to 2 percentage points, and unlike a bank loan, that interest is credited back into your own account rather than paid to a lender.
That last detail is exactly why people assume a 401(k) loan is free. It isn't, for one specific reason explained below: the $20,000 you borrowed stops being invested the moment it leaves your account, and it doesn't start earning market returns again until you've paid it back in full.
2. What "Waiting to Save It Instead" Actually Costs
The alternative to borrowing is saving a separate $20,000 in cash while continuing to rent, then buying once you have it. That path has a real, countable cost too: the rent you keep paying in the meantime builds no equity at all, unlike a mortgage payment, part of which goes toward principal every month. Apartment List's national rent report put the median U.S. rent at $1,381 a month as of its October 2025 report, the most recent figure available at this writing; actual rents in many metro areas run well above that. At that national median, two years of rent comes to roughly $33,144 in payments that build no ownership stake whatsoever. Saving $20,000 in two years also means setting aside about $833 a month beyond ordinary expenses, which is its own real tradeoff against other goals.
3. The 401(k) Loan's Real Price: Lost Growth, Not Interest
Here's the mechanism. The S&P 500's long-run historical average annual return is commonly cited around 10% nominal. A typical 401(k) loan rate (prime plus roughly 1 point) runs around 8% in the current rate environment. While your $20,000 is out as a loan, it's earning you 8% (credited back to your own account) instead of being invested and earning whatever the market actually returns. If the market returns its long-run average of 10% during those years, you didn't break even by "paying yourself interest" — you gave up the 2-percentage-point gap between what the loan credited you and what the market would have paid, for every year the loan was outstanding.
This is the piece most 401(k)-loan explainers skip. The interest you pay yourself is not the cost. The cost is the difference between that rate and what the money would have earned had it stayed invested.
4. Running the Numbers on a $20,000 Loan Over 10 Years
Assume you take a $20,000 loan now, repay it over 5 years, and your 401(k) earns 8% while the loan is outstanding (the rate credited to you) versus a 10% long-run market average the rest of the time. Compare that to never taking the loan at all and leaving the same $20,000 invested at 10% uninterrupted for the full 10 years.
| Scenario | Value at year 5 (end of loan) | Value at year 10 |
|---|---|---|
| 401(k) loan taken (8% while out, then 10%) | $29,387 | $47,327 |
| No loan, invested at 10% the whole time | $32,210 | $51,875 |
| Difference (cost of the loan) | $2,824 | $4,547 (about 22.7% of the original $20,000) |
Assumptions: $20,000 principal, 8% crediting rate during the 5-year repayment period, 10% market return before and after that period, no new contributions or withdrawals modeled, compounding annually. Actual 401(k) loan rates and plan terms vary; check your specific plan document.
Set against that $4,547 ten-year cost, two years of rent at the national median rent comes to roughly $33,144 — a much larger number, but a different kind of cost: it's cash spent now with nothing built up, rather than retirement growth given up gradually. Whether that tradeoff favors borrowing or waiting depends heavily on your actual rent, your actual 401(k) balance, and how badly a 2-year delay would push you into a higher-price, higher-rate market by the time you buy. A $20,000 loan on a modest 401(k) balance also eats into the diversification and liquidity of that account in a way a bigger rent bill doesn't.
5. The Risk a Calculator Won't Show You: Job Loss
The math above assumes you keep your job and keep repaying on schedule. If you leave your job — voluntarily or not — while a 401(k) loan is outstanding, the plan sponsor can require the full remaining balance to be repaid immediately. If you can't repay it, the unpaid balance gets reported to the IRS as a taxable distribution on Form 1099-R. You do get a window to avoid that: current IRS guidance lets you roll the outstanding balance into an IRA or another eligible retirement plan by the due date (including extensions) for your federal tax return for the year the loan is treated as a distribution. Miss that window and the balance is taxed as ordinary income, plus a 10% early-withdrawal penalty if you're under 59½. That risk doesn't show up in any year-10 comparison table, but it's real, and it's the scenario that turns a 401(k) loan from "a bit of lost growth" into a genuinely expensive mistake.
6. Where This Could Go the Other Way
None of this means waiting is automatically the better choice. If home prices or mortgage rates are rising faster than 2 percentage points a year in your market, the cost of delaying a purchase by two years can swamp the $4,547 lost-growth estimate above many times over — that's a real, documented pattern in markets that saw fast appreciation. A 401(k) loan also doesn't show up on a credit report and doesn't affect your debt-to-income ratio the way a second mortgage or personal loan would, which can matter for qualifying at all. And if your plan's crediting rate happens to run close to or above long-run market returns in a particular stretch, the "lost growth" gap shrinks or disappears. The honest conclusion is that the 401(k)-loan route has a quantifiable, moderate cost that is easy to underestimate because it's invisible on a pay stub, while the wait-and-save route has a cost that's visible every month in a rent payment but carries no risk to retirement savings or job-loss exposure.
7. A Few Things the Headline Numbers Leave Out
A few details change the math enough to matter in individual cases. First, most plans let you keep contributing to your 401(k) while a loan is outstanding, so you aren't necessarily missing out on an employer match during the loan period — check your specific plan, because a minority of plans do suspend contributions for borrowers. Second, the 2026 IRS employee deferral limit is $24,500 (up from $23,500 in 2025), with an $8,000 catch-up for savers 50 and older, so a 401(k) loan doesn't reduce how much you're allowed to contribute going forward, only what's currently invested. Third, the 8% loan-crediting assumption used above is a reasonable estimate in the current rate environment, but it moves with the prime rate; plans typically set the loan rate at origination and don't adjust it for the life of that loan, so a loan taken when prime is higher locks in a higher crediting rate for its whole term, which narrows the gap against market returns. Fourth, the "wait and save" path assumes you can actually save the full $20,000 in two years without dipping into a 0% APR credit card or other debt to get there faster; if saving that fast requires its own borrowing, the comparison needs an additional cost line that this article doesn't model.
None of these details change the basic shape of the tradeoff: a 401(k) loan trades a quantifiable, moderate amount of future retirement growth for immediate cash with no new debt payment outside your existing paycheck deduction, while saving first trades two years of non-equity-building rent and slower access to homeownership for a retirement account that keeps compounding untouched. Which one costs more in your specific case depends on your actual rent, your actual plan's loan rate, and what mortgage rates and home prices do in your market over the next two years — none of which a generic calculator can know in advance.
Related reading
- HSA vs. Extra Mortgage Payments: Which $4,400 a Year Builds More by 2036?
- A $48,000 Down Payment Fund: High-Yield Savings vs. the Stock Market, the Real 5-Year Math
- Pay Down Principal to Drop PMI Early, or Invest That $10,000 Instead? The Real Math
Tip: Whichever path you pick, the decision rests on the repayment line fitting into your monthly budget for the full ten years. A budget planner makes that payment visible next to everything else you owe, and a financial calculator lets you rerun the compounding math with your own numbers instead of ours. (These are Amazon Associate links — we may earn a small commission on qualifying purchases.)
Sources
- IRS, Retirement Topics – Plan Loans
- Axios, "IRS raises 401(k) limits by largest amount in two years" (Nov. 13, 2025)
- Apartment List National Rent Report, cited Oct. 2025 median rent figure
This article is general information, not personalized financial, tax, or legal advice. Market returns, 401(k) plan terms, and loan rates vary and are not guaranteed; past performance does not predict future returns. Consult your plan administrator or a qualified financial professional before borrowing against a retirement account.
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