A 401(k) loan pays interest to yourself and can still cost you in missed market growth or a job change.
Free 401(k) loan calculator from Fynvorax.
Practical tip Start with round numbers from your bank or contract. You can refine decimals after the first pass.
What to enter
Have these handy before you start. Names vary by tool, but the ideas are the same.
Target monthly or annual income in retirement
Current savings and monthly contributions
Expected return, inflation, and retirement age
Years you expect withdrawals to last
Quick workflow
Set the income you want in today's money.
Add what you already save and expected returns.
Stress-test with lower return or higher inflation.
Free calculator Model borrowing from your 401(k) versus leaving the balance invested in the 401(k) loan calculator.
Worked example (illustrative)
You borrow $25,000 from a $120,000 401(k) at 8% loan rate while investments might earn 9% long term. You repay yourself interest, but the borrowed amount is out of the market—if the fund returns 12% that year, opportunity cost can exceed the interest you pay back. Double taxation on interest in some plans adds drag. Use the 401(k) loan calculator with your balance, loan size, and expected return—not plan-specific advice.
Common search questions
401k loan calculator — opportunity cost vs interest repaid to yourself.
401k loans calculator — job change may accelerate full repayment.
Borrow from 401k — compare to taxable loan or emergency fund first.
Open the tool when you already have a quote, a deadline, or a what-if and need a clear number for retirement.
Situations people use it for
Checking if your current savings path hits a target retirement age
Modeling a late start with higher contributions
Seeing how inflation changes the nest egg you thought you had
How to read the result
Treat the output as a structured estimate. Small input changes should move the result in a direction that makes sense.
Run a conservative case first. If it still works, you have margin.
Change one field at a time so you know what actually moved the number.
Save the scenario with a name you will recognize later ("2027 refi", "15-year plan").
This model assumes: Free 401(k) loan calculator from Fynvorax.
Common slip-ups
Ignoring healthcare or one-off costs in the spending number
Assuming you can withdraw 4% in every market year without stress-testing
Forgetting employer match when modeling contributions
Try it with your numbers
Open the 401(k) Loan Cost calculator, enter your case, and keep a screenshot or PDF if you will revisit the decision. Everything runs in your browser; your inputs are not sent to our servers for the math.