The Moment
You have a 3.5% mortgage, a 4% car loan, or federal student loans at 3-5%. You also have money to invest. The question: should you accelerate debt payoff or invest the money?
This is the opposite of the high-interest debt question. At sub-4%, the math is clear — but the psychology is more nuanced.
The Math
$10,000 toward a 3.5% mortgage: Guaranteed return: 3.5%/year. After 10 years: $14,106 in saved interest.
$10,000 invested in index funds at 7% average: Expected return: 7%/year. After 10 years: $19,672 (expected).
Difference: ~$5,500 more from investing (expected, not guaranteed).
The gap is large enough that investing wins in most scenarios. Even in below-average market decades (5% returns), investing at 5% still beats paying off a 3.5% debt.
When to invest (the math answer): - All debt is below 5% - You have captured your 401(k) match - Your emergency fund is funded - You can tolerate market volatility without selling
When to pay off debt (the behavioral answer): - The debt causes you stress regardless of the rate - You would sleep better with zero debt - You are within 2-3 years of retirement and want to eliminate the mortgage payment - You have low risk tolerance and the certainty of debt payoff feels better than uncertain investment returns
Run Your Numbers
Compare debt payoff returns vs investing returns.
Compound Growth Projector
What to explore next
- →Should I make extra mortgage payments or invest?
- →What if my debt is 5-7% — is it a toss-up?
- →How do I balance debt payoff with retirement savings?
Frequently Asked Questions
Should I pay off my mortgage early?
At sub-4%, mathematically no — investing the extra payments produces more wealth. But if paying off the mortgage eliminates your largest monthly expense and provides peace of mind, the behavioral benefit has real value. There is no wrong answer in the sub-4% zone; it is a personal preference question, not a math question.