Decision Framework
Invest or Pay Off Debt? The Interest Rate Rule
Paying off debt is a guaranteed return equal to the interest rate. Investing is an expected return of ~8–10%. Here's how to use that to make the decision.
The Speedrun Take
Paying off a 20% credit card is a guaranteed 20% return — no investment can touch that. But paying off a 3% mortgage early instead of investing is often wrong. The crossover is around 6–7% — above that, pay off debt. Below that, invest. With one exception: always capture the full employer 401k match first. That's a 50–100% guaranteed return.
The Core Math Framework
The decision comes down to two returns side by side:
Guaranteed Return
Paying off debt at X% APR locks in a guaranteed X% return. No market risk, no volatility, no sequence-of-returns problem. Zero chance the return is negative.
Example: 20% CC → guaranteed 20% return
Expected Return
Investing in a broad index fund historically returns ~8–10% average annual over 30 years. Expected — not guaranteed. Any given year could be +30% or -40%.
Rule: if debt rate > ~7% → pay it off. If < ~7% → invest.
Why 7%?
The 7% threshold sits below the expected long-term market return (~8–10%), accounting for inflation and uncertainty. It's approximate — adjust based on your risk tolerance. If you can't stomach market volatility, lower your threshold. If you're comfortable with risk, you might push it to 8%.
The Employer Match Exception (ALWAYS First)
This comes before any debt payoff decision — except minimum payments.
401k employer match = 50–100% instant guaranteed return
A 50% match on your 6% contribution means you put in 6%, your employer adds 3% — that's a 50% return before any market growth happens. This beats any debt interest rate mathematically.
Even with credit card debt: capture the full match
Yes, even at 22% APR. The match return is higher. Contribute enough to get every dollar of employer match, then attack the credit card with everything else.
Decision by Debt Type
Apply the 7% crossover rule to your actual debt rates.
| Debt Type | Typical APR | Decision |
|---|---|---|
| Credit cards | 18–29% | Pay off immediately. Nothing beats this guaranteed return. |
| Personal loans | 8–25% | Pay off aggressively if above 8–10% |
| Car loans | 4–9% | Judgment call: above 7% pay off, below 7% invest |
| Student loans (federal) | 5–8% | If above 7%, consider extra payments. Income-driven repayment first. |
| Student loans (private) | 3–12% | Variable — apply the 7% rule |
| Mortgage | 3–8% | Usually below crossover — invest rather than prepay |
| HELOC | Variable | Treat like current rate |
The Simultaneous Strategy (for Rates Near the Crossover)
At 5–7% debt, you don't have to choose — you can split.
The Split Approach
Direct 50% of extra dollars to debt payoff, 50% to investing. You don't know if markets will return 10% or 3% over the next 5 years — hedging is rational at borderline rates.
The Psychology Factor
Some people need to see debt balances shrink to stay motivated. That psychological value is real. If watching the debt disappear keeps you engaged, weighting more toward payoff is defensible even at 5%.
“But What If the Market Drops?” — The Risk Argument
Risk-free, guaranteed
The return is locked in the moment you pay. It never goes negative. Your net worth improves by exactly the balance paid, no uncertainty.
Expected, not guaranteed
Markets can drop 30–40% in a year. Historically they recover — but the timeline matters. If you'd panic-sell during a crash, that expected 8–10% return never materializes.
The honest answer
Factor in your emotional risk tolerance, not just math. If you can't stomach seeing your portfolio down 30% while carrying debt, paying off debt above 5% provides guaranteed returns without volatility. Sleeping at night has financial value too.
Student Loan Special Cases
Federal loans: check IDR and forgiveness first
Income-Driven Repayment (IDR) caps monthly payments based on income and forgives remaining balances after 20–25 years. If you're on IDR, aggressive payoff may be suboptimal — you might pay off a balance that would have been forgiven anyway.
PSLF: minimum payments are optimal
Public Service Loan Forgiveness forgives remaining balances after 10 years of qualifying payments. If you qualify, making minimum payments and investing the rest is mathematically superior — the government pays the balance.
Tax deductibility reduces effective rate
Student loan interest is deductible up to $2,500/year if your income is below the threshold. This lowers your effective interest rate. A 6% loan with a 22% marginal tax rate costs closer to ~4.7% after the deduction. Apply the 7% rule to the effective rate.
The Order of Operations
Follow this sequence with every dollar of surplus cash flow.
Employer 401k match
50–100% guaranteed return — always first. Capture every dollar of free match money before paying extra on anything.
Pay minimums on ALL debt
No exceptions. Missing minimums triggers late fees, credit damage, and penalty rates that cost more than any strategy saves.
Pay off debt >7% APR aggressively
Every extra dollar goes here. The guaranteed return exceeds expected market returns — this is mathematically correct.
Fund HSA if eligible
Triple tax advantage: pre-tax contributions, tax-free growth, tax-free withdrawals for medical. Better than a Roth IRA for eligible expenses.
Max Roth IRA
$7,000/year (2024). Tax-free growth for decades. Roth beats traditional for most people early in their career.
Max 401k
$23,000/year (2024) in pre-tax or Roth 401k. After match and Roth IRA, this is the next bucket to fill.
Pay off debt 5–7% (judgment call)
This range overlaps with long-term market returns. Splitting extra dollars between payoff and investing is defensible either way.
Taxable brokerage
No limits, no penalties. Index funds in a taxable account beat holding cash after all tax-advantaged space is used.
Pay off debt <5% (optional)
Mortgages and low-rate loans often fall here. Invest instead — expected returns exceed the cost of cheap debt.
Ready to Invest the Rest?
Once debt is under control, open a brokerage or IRA at Fidelity and put every extra dollar to work in a zero-fee index fund.
Real-World Scenarios
Three examples applying the framework.
Person A
$8,000 CC debt at 22%, $3,000/month cash flowPay off CC in 3 months, then invest
A 22% guaranteed return is untouchable. Clear it fast, then redirect that payment to investments.
Person B
$30,000 student loans at 5%, employer matches 4%Capture match, then minimum loans, invest the rest
5% is below the crossover. The employer match is a guaranteed 50%+ return. Minimum payments on loans, invest everything else.
Person C
$200k mortgage at 3.5%, $500 extra/month availableInvest the $500 in index funds, not prepay mortgage
3.5% mortgage is cheap debt. Over 30 years, index funds are expected to return 8–10%. Let the math compound.
Related
Next Step
See the Full Investment Order
You know when to invest vs pay debt. Now see the exact order of operations for every dollar you invest — match, HSA, Roth IRA, 401k, and beyond.
See the Investment Order