401(k) loan vs. personal loan: the four hidden costs

Roughly 87% of US 401(k) plans permit participant loans according to the Vanguard How America Saves 2025 report (year-end 2024 data). Of plans that allow them, 13% of participants had an outstanding loan at year-end 2024, in line with 2023.

The federal rules under IRS Retirement Topics Loans set the ceiling: a participant may borrow up to 50% of the vested account balance, capped at $50,000. A vested balance below $20,000 unlocks a different floor that allows borrowing up to $10,000 even when 50% would yield less. Most plans set stricter internal caps than the IRS maximum.

The default repayment term is 5 years. Loans used to acquire a primary residence can extend to 15 years, though many plans cap residential loans at 10 years. Repayment runs via automatic payroll deduction in fixed installments. Interest rates are set by the plan and typically benchmark to the prime rate plus 1 to 2 percentage points. With prime at 6.75% as of the Federal Reserve H.15 release of May 12, 2026 (FOMC voted 8-4 to hold rates at 3.50-3.75% federal funds at the April 29, 2026 meeting), the working range for 401(k) loan APR in Q2 2026 is approximately 7.75% to 8.75%.

Interest paid on the loan goes back into the borrower's own 401(k) account. This is the "paying interest to yourself" framing that makes the product feel cheap. That framing is mechanically accurate for the interest portion. It says nothing about the principal sitting outside the market during the loan term — which is where the opportunity cost lives.

If employment ends during the loan term, the unpaid balance becomes a "loan offset." The Tax Cuts and Jobs Act of 2017, effective January 1, 2018, extended the rollover deadline from the previous 60-day rule to the tax filing due date (including extensions) of the year in which the offset occurs. If the balance is not rolled into an IRA or new qualified plan by that deadline, the unpaid amount converts to a taxable distribution. For borrowers under age 59½, the 10% early-withdrawal penalty applies on top of ordinary income tax.

Setup: a 40-year-old borrower has $80,000 vested in a 401(k) at a job with a 4% employer match. The borrower needs $20,000 and is comparing a 401(k) loan at 8.25% APR over 5 years against a personal loan at 12% APR over 5 years.

Hidden cost 1: opportunity cost on un-invested principal. The $20,000 leaves the investment account at loan origination and gets repaid in fixed installments over 60 months. Money outside the account earns nothing in the account. The S&P 500 has returned 10.33% nominal annually on average since 1957 per Macrotrends historical data (also widely cited via Damodaran NYU Stern historical returns dataset). Even at a conservative 7% nominal assumption (closer to a balanced 60/40 portfolio), the math is meaningful. Approximate forgone gains over 5 years on the full $20,000 if it had stayed invested: $20,000 × (1.10⁵ − 1) = $12,210 at 10% return, or $20,000 × (1.07⁵ − 1) = $8,051 at 7% return. The actual opportunity cost is slightly lower because the principal returns gradually through repayments, but a fair midpoint estimate is $8,000 to $10,000 over the loan term — none of which shows up in any 401(k) loan quote.

Hidden cost 2: double taxation on the interest paid back. Loan repayments come out of post-tax paycheck dollars. The interest portion of those repayments is contributed to the 401(k) account. When the borrower eventually withdraws those dollars at retirement, ordinary income tax applies again to everything in a traditional 401(k), including the dollars that originated as interest payments. The principal portion of repayments was already post-tax money going into the account, but principal would have had the same double-tax treatment in a non-loan world too (you contribute pre-tax originally and pay tax on withdrawal). The interest portion is the genuinely doubled component. Total interest on the example loan: approximately $4,478 over 5 years. Effective extra tax cost if the borrower's marginal rate at retirement is 22%: roughly $985. The number scales with marginal rate.

Hidden cost 3: separation-event acceleration risk. If the borrower changes jobs (voluntarily or not) during the 5-year term, the unpaid balance becomes a loan offset. The TCJA-extended grace period gives until the tax filing deadline of the offset year to roll the amount into another retirement account, but most departed employees cannot come up with $5,000 to $15,000 in cash on that timeline. The Boston College Center for Retirement Research reports that roughly 10% of plan loan borrowers default, with job separation being the single largest driver. BLS Job Openings and Labor Turnover Survey data suggests US worker average tenure is around 4 years, which means a 5-year loan has a meaningful probability of crossing a separation event. Probability over 5 years for a typical employee: 15-25%. If separation occurs at month 30 with a remaining balance of approximately $10,800 and the borrower cannot fund the rollover, the offset converts to taxable distribution plus 10% early-withdrawal penalty. At a 22% marginal rate, the tax + penalty hit is roughly $3,456. Expected value (20% probability × $3,456): $691. The expected value understates the harm — the actual scenario is binary and catastrophic when it triggers, not the smooth average the EV math implies.

Hidden cost 4: reduced contribution capacity and lost employer match. Some plans require participants to suspend or reduce 401(k) contributions while a loan is outstanding. Even when contributions are not mandatorily suspended, many borrowers reduce them voluntarily because the loan repayment crowds payroll. If the borrower in the example was contributing enough to capture the full 4% employer match on a $75,000 salary, that match represents $3,000 per year. Lose half the match for 3 of the 5 loan years through reduced contributions: $4,500 in forgone match. Worse, that $4,500 missed at age 40 would have grown to approximately $20,400 by age 65 at 7% real return. The contribution-suspension cost compounds across decades — not just the loan term.

Summary of the example loan total real cost. Principal repaid: $20,000. Interest paid: $4,478. Opportunity cost (midpoint estimate): $10,000. Lost employer match (5-year nominal): $4,500. Expected value of separation-event tax + penalty: $691. Total real cost: approximately $39,669 over the 5-year horizon, and the $4,500 of lost match continues to compound to roughly $20,400 in retirement-horizon dollars.

Compare the personal loan. $20,000 at 12% APR over 5 years: monthly payment of approximately $444.89, total payments of approximately $26,693, total interest of approximately $6,693. No opportunity cost on the 401(k) account because the 401(k) is untouched. No lost match. No separation-event risk on the personal loan itself. Total real cost: approximately $26,693. The 12% personal loan is roughly $13,000 cheaper than the 8.25% 401(k) loan when all four hidden costs are included.

The "paying interest to yourself" framing remains true for the interest portion of the loan, but the framing actively obscures the four costs above. A useful mental model: a 401(k) loan effectively has an embedded APR equivalent in the 20-30% range when normalized for opportunity cost and lost match — not the 8-9% nominal APR the loan documents quote.

The math reverses in a narrow set of cases.

Scenario 1: very short-term bridge with high payoff certainty. A borrower needs $5,000 for 90 days while waiting on a signed-and-dated bonus or commission check. The opportunity cost on 90 days of un-invested $5,000 at a 10% expected market return is approximately $125. The lost match is zero because contributions continue uninterrupted on a short term. Separation-event probability over 3 months is negligible for a typical employee. The personal loan equivalent at 12% APR for 3 months would charge approximately $150 in interest. The 401(k) loan wins by a small margin and avoids the credit pull. For very short terms with payoff certainty, the math works.

Scenario 2: subprime borrower facing 22%+ personal loan APR. A borrower with FICO 580-619 will be quoted personal loan APR in the 25-30% range. The 8.25% 401(k) loan APR plus four hidden costs (totaling roughly $15,000 in effective cost on a $20,000 / 5-year loan) compares against a 25% personal loan that would cost roughly $9,000 in pure interest. The 401(k) loan is still more expensive, but the gap narrows substantially. For borrowers with very poor credit who would face truly punitive personal loan rates, the 401(k) loan becomes a less-bad option — not a good one, but a lesser harm than 25%+ unsecured debt. The cleaner alternative remains avoiding the borrowing entirely, but if borrowing is unavoidable, the 401(k) route may be the lesser harm.

Scenario 3: repayment from a known non-paycheck source. A borrower expects a matured CD ($25,000 maturing in 8 months), a legal settlement with signed-and-dated payout, or proceeds from a real estate sale already in contract. The borrower takes a $15,000 401(k) loan that will be repaid in full from the incoming external funds, bypassing the payroll-deduction schedule. This eliminates the contribution-suspension cost (loan repaid before suspension matters), shortens the separation-event exposure to the gap window, and minimizes the opportunity cost. For these structurally short and externally funded repayments, the 401(k) loan often wins on math.

The common thread across the three scenarios: the loan is short, the repayment source is certain, and the contribution-capacity cost is small or zero. Outside those conditions, the math defaults back to the personal loan being cheaper.

Several characteristics push the math hard against the 401(k) loan:

Long-term loans (3-5 years) with stable employment unlikely. The opportunity cost compounds with time, and the separation-event probability rises with term length. A 5-year loan in an industry with above-average turnover (food service, retail, some tech roles, construction) carries 30%+ separation probability over the term. The expected-value cost of the acceleration risk balloons proportionally.

Younger borrowers under age 45 with long retirement horizon. The opportunity cost of un-invested principal compounds for the entire remaining horizon, not just the loan term. A $20,000 401(k) loan at age 35 that produces $10,000 in nominal forgone gains over 5 years also produces an additional $40,000+ in forgone compound growth between age 40 and retirement at 65. The full opportunity cost is multiples of the loan amount itself for young borrowers.

High-match plans (4%+ employer match) where contribution suspension forfeits significant match. The lost match is real dollars from the employer. Forfeit is permanent (most plans do not allow back-funding missed match). Every dollar of forgone match at age 35-45 compounds to several dollars at retirement, layering opportunity cost on top of lost-match.

Variable income or above-average industry turnover. Industries with high churn (hospitality, retail, contracting, early-stage tech, oil and gas operations) have separation probabilities well above the 15-25% range used in the example. A 35% probability of separation over a 5-year loan, with the same $3,456 cost-when-triggered, has expected value approaching $1,200.

A useful diagnostic: if a borrower cannot articulate which of the three minority scenarios applies, the math is against the 401(k) loan. The default answer for the typical borrower under age 50 with stable employment is the personal loan — even at higher nominal APR.

No single calculator natively models the four hidden costs combined. The QuickUse retirement calculator handles the long-term retirement projection cleanly, and the QuickUse loan calculator handles the amortization side of either loan type cleanly. The recommended workflow:

1. Model the 401(k) loan amortization in the loan calculator. Input loan amount, APR (the rate quoted by the plan portal), term (5 years standard). The calculator returns monthly payment and total interest.

2. Model the personal loan amortization separately. Same calculator, different APR (the pre-qualified rate from credit union or online lender), same term. Compare nominal total interest first.

3. Estimate opportunity cost manually. Loan amount × historical market return × term years × adjustment factor. For a 5-year loan, the adjustment factor is approximately 0.55 because principal returns gradually rather than sitting idle for the full term. Example: $20,000 × 10% × 5 × 0.55 = $5,500 at conservative return assumption; $20,000 × 10% × 5 × 0.55 actually undercounts because the historical S&P return is geometric mean over long periods, so a fair upper bound is closer to $10,000 using compound growth on the full balance.

4. Estimate lost employer match. Annual match × suspension share × loan years. Check the plan document for whether contributions can continue during loan repayment.

5. Estimate expected value of separation-event cost. Probability of separation over term (15-25% baseline, higher for variable industries) × (remaining balance midpoint × marginal tax + 10% if under 59½).

6. Sum total real costs of both options. Personal loan total = total interest only. 401(k) loan total = interest + opportunity cost + lost match + separation EV. Compare these totals, not the nominal APRs.

7. Project retirement impact using the retirement calculator. Input current 401(k) balance, expected annual contribution (reduced if contributions will be suspended during loan repayment), and retirement age. Run the projection twice: once at the planned contribution level, once at the reduced level. The difference is the long-horizon cost of the 401(k) loan beyond what the 5-year math captures.

The QuickUse loan calculator and retirement calculator together cover the inputs cleanly, but the synthesis (hidden cost 1 + 4 in particular) requires manual estimation. No aggregator calc surfaces opportunity cost on un-invested principal as a line item — and that omission is exactly the source of most of the misleading "low rate" advice in personal finance media around 401(k) loans.

A common misconception in personal finance writing attributes the 401(k) loan separation grace period extension to the SECURE 2.0 Act of 2022. That attribution is incorrect.

TCJA 2017 changed the rule. The Tax Cuts and Jobs Act, effective January 1, 2018, extended the post-separation rollover deadline from the previous 60-day rule to the tax filing due date (including extensions) of the year in which the loan offset occurs. This rule is sometimes called the Qualified Plan Loan Offset (QPLO) provision and is documented in IRS Notice 2018-74 and final regulations issued in 2020.

SECURE 2.0 changed different things. The SECURE 2.0 Act of 2022 created emergency withdrawal provisions (up to $1,000 per year penalty-free starting in 2024), added disaster-relief loan provisions with higher caps and extended terms, modified hardship withdrawal rules, and introduced auto-enrollment requirements for new plans. SECURE 2.0 did not modify the separation-event rule for ordinary participant loans.

Why the distinction matters in 2026. Borrowers reading personal finance media often see SECURE 2.0 mentioned alongside 401(k) loan flexibility and assume the grace period is "new." It has been the law since January 2018 — not a recent SECURE 2.0 update. The implication: a borrower considering a 401(k) loan in Q2 2026 already has the TCJA-extended grace period built in. There is no pending change to the separation-event rule. The 10% early-withdrawal penalty still applies if the offset is not rolled over by the tax filing deadline. Borrowers under age 59½ should plan for separation-event risk as a real cost, not an outdated concern.