Borrowing from your 401(k) feels like paying yourself interest instead of a bank—and in one sense it is. You repay principal plus interest to your own account. The hidden cost is opportunity cost: money removed from investments may miss market gains, and if you leave your job, many plans demand full repayment within months or treat the balance as a taxable distribution with penalties. This guide explains the rules, the math, and when the free 401(k) loan cost calculator is worth running before you sign the loan paperwork.
Not a free lunch 401(k) loan interest is not tax-deductible like mortgage interest, and you repay with after-tax dollars that will be taxed again on withdrawal in traditional accounts. The real comparison is loan cost plus missed market return versus alternatives (HELOC, personal loan, or delaying the expense).
IRS limits and plan rules (overview)
The IRS caps 401(k) loans at the lesser of $50,000 or 50% of your vested account balance (with special rules if you had another loan in the past 12 months). Plans set their own minimum and maximum, number of loans allowed, and interest rate—often prime + 1%. You must repay through payroll deduction on a schedule not exceeding five years, except for loans used to buy your primary home. Not every employer plan allows loans at all.
Opportunity cost in plain numbers
Example: you borrow $20,000 from a 401(k) that would have earned 7% annually while the loan charges you 5% paid back to yourself. You might think you save 2%, but the $20,000 is not invested during the loan—if the market returns 7% in a strong year, you lag by roughly that spread on the borrowed amount. The calculator models loan interest paid to your account versus a hypothetical stay-invested path using your assumptions.
Job change risk: the cliff most people miss
If you quit, are laid off, or retire with an outstanding 401(k) loan, many plans require full repayment within 60–90 days. Miss the deadline and the outstanding balance becomes a distribution: income tax plus 10% early withdrawal penalty if you are under 59½ (with limited exceptions). That risk makes 401(k) loans dangerous for emergency funds when your industry has layoffs or you are job-hunting.
401(k) loan vs hardship withdrawal vs taxable account
401(k) loan: no immediate tax if repaid on schedule; opportunity cost and job-change cliff.
Hardship withdrawal: permanent tax + penalty; cannot be repaid to the plan.
Taxable brokerage or HYSA: no cliff, but you give up tax-advantaged growth on that slice.
HELOC or personal loan: interest paid to lender, but investments stay invested.
Double taxation nuance on traditional 401(k)
Loan repayments come from paycheck after income tax. When you later withdraw in retirement from a traditional 401(k), those dollars are taxed again as ordinary income. Roth 401(k) loans have different recovery rules—consult your plan document. This is one reason planners often rank 401(k) loans below external financing for large purchases.
When a 401(k) loan can be reasonable
Short repayment horizon (12–24 months) and stable employment you control.
Borrowing to eliminate high-interest credit card debt after cutting spending.
The amount is small relative to balance and you would otherwise stop contributing.
Alternatives are unavailable or more expensive after honest comparison in the calculator.
Checklist before you borrow
Read the summary plan description for repayment on termination.
Model opportunity cost at conservative and base return assumptions.
Confirm you can still afford regular 401(k) contributions during repayment.
Compare to a Roth IRA or taxable emergency fund for future needs.
If buying a home, compare to mortgage or HELOC options with longer terms.