401k Loan Opportunity Cost: How Much Growth You Lose

Understanding 401k Loan Opportunity Cost

When you borrow from your 401k, you’re not just paying interest — you’re losing out on potential investment growth. This opportunity cost is the most overlooked expense of 401k loans.

What Is Opportunity Cost?

Opportunity cost is the return you could have earned if you left the money invested instead of borrowing it.

Example: You borrow $50,000 for 5 years:

• If the market returns 10% annually, that $50,000 would have grown to $80,526
• Your actual opportunity cost: $30,526 in missed growth
• Plus, the growth on that growth in subsequent years

The Compound Effect

The real damage comes from compounding. When you miss growth, you also miss the growth-on-growth:

Years After Loan	$50,000 at 7%	$50,000 at 10%
5 (end of loan)	$70,128	$80,526
10 years	$98,358	$129,687
20 years	$193,484	$336,375
30 years	$380,613	$872,470

That 5-year loan doesn’t just cost you 5 years of growth — it costs you 30 years of compound returns on the missed growth.

Does Paying Interest Back Help?

Some argue that paying interest back to your 401k offsets the opportunity cost. Let’s compare:

Scenario: $50,000 loan at 9.5%, 5 years

• Interest paid back to account: $13,064
• Missed market growth at 10%: $30,526
• Net loss: $17,462

Even after accounting for the interest you pay yourself, you’re still behind by over $17,000.

Tax-Efficiency Loss

There’s another hidden cost: the interest you pay back to your 401k is made with after-tax dollars but will be taxed again when you withdraw in retirement. This is the double-taxation problem.

• Interest paid: $13,064 (after-tax money)
• Future tax on that interest (24% bracket): $3,135
• Effective double-tax cost: $3,135

Factors That Affect Opportunity Cost

1. Amount borrowed — larger loans = more missed growth
2. Loan term — longer terms = more missed compounding
3. Market returns — higher returns = higher opportunity cost
4. Your age — younger borrowers have more years of missed compounding
5. Repayment speed — faster repayment = less missed growth

When Opportunity Cost Is Acceptable

• Market is in a sustained downturn (less growth to miss)
• The loan prevents a higher-cost alternative (e.g., 25% credit card)
• You’re close to retirement (fewer years of compounding)
• The loan enables a high-return investment (e.g., below-market home purchase)

Use our 401k calculator to see your personalized opportunity cost analysis. For more context on the tax implications, read our 401k loan double taxation analysis. To compare rates, see our 401k loan interest rate guide. And if you’re considering alternatives, our 401k loan vs personal loan comparison is worth a look.