Paying off debt vs investing: which dollar is worth more?
Short answer
Paying off debt returns your interest rate, guaranteed and tax-free. Investing returns more on average but not reliably, so the practical rule is to clear anything above roughly 6% first and invest ahead of anything below about 4%.
Option A
Pay down debt
Send surplus cash to principal on loans, cards, or the mortgage.
Strengths
- Return equals the interest rate, with zero variance
- No tax owed on the benefit — it is avoided expense, not income
- Lowers fixed monthly obligations, which raises resilience
- Improves debt-to-income for future borrowing
Trade-offs
- Money becomes illiquid once it is in the loan
- Forgoes any employer 401(k) match you skip to do it
- Low-rate fixed debt is genuinely cheap and worth keeping
- No compounding beyond the rate you avoided
Option B
Invest the surplus
Route surplus cash into index funds or tax-advantaged retirement accounts.
Strengths
- Historical long-run equity returns run near 7% real
- Tax-advantaged space is use-it-or-lose-it each year
- An employer match is an instant 50–100% return
- Stays liquid in a taxable brokerage
Trade-offs
- Returns are an average, not a promise — sequence risk is real
- Gains are taxable outside retirement accounts
- Carrying high-rate debt while investing is negative arbitrage
- Behavioural risk of selling in a drawdown
Head-to-head
| Metric | Pay down debt | Invest the surplus |
|---|---|---|
| Return on 22% credit card payoffA | 22% guaranteed | ~7% expected |
| Return on 6.5% student loanEven | 6.5% guaranteed | ~7% expected |
| Return on 3.2% mortgageB | 3.2% guaranteed | ~7% expected |
| 401(k) match dollarsB | Forgone | 50–100% instant |
| VarianceA | Zero | High |
| Liquidity after the factB | Low | High in taxable |
| Tax on the benefitA | None | Capital gains outside IRA |
Badge marks which option wins that row: A = Pay down debt, B = Invest the surplus.
The line is not 7% — it is your rate versus a risk-adjusted return, which in practice sits near 6%.
Comparing a guaranteed 6% against an expected 7% is not a one-point win for investing; the 7% carries a standard deviation near 16% and a real chance of a decade of nothing. Adjust for that and the two are roughly even in the 5–7% band, which is why the answer flips to personal circumstance there: job stability, emergency fund depth, and how much a monthly payment weighs on you. Below 4% the arbitrage is wide enough to ignore the variance. Above 8% there is no argument at all.
Worked example: $1,000/month surplus, $18,000 of 19% card debt, 4% employer match available
- Step 1: capture the match first — 4% of a $90,000 salary = $3,600/year free
- Remaining surplus after match contribution = about $700/month
- Card at 19%: $18,000 paid at $700/month clears in about 29 months
- Interest paid on that path = roughly $4,300
- Investing that $700 instead at 7% for 29 months = roughly $22,000 grown to $23,500
- But the card accrues about $9,900 of interest over the same window
- Net difference favours payoff by roughly $5,600
Take the match, then kill the 19% debt. Investing around a 19% card loses about $5,600 over 29 months and adds risk on top of the loss.
The verdict
Choose Pay down debt
Pay down anything above 6%, always, before taxable investing.
Choose Invest the surplus
Invest ahead of sub-4% fixed debt, and always capture an employer match before either.
Or run both
The 4–6% band is a personal call — split the surplus and stop optimising a decision worth a few hundred dollars a year.
Frequently asked questions
Where does the emergency fund fit?
Before both, at least one month of expenses. Without it, the next surprise goes back onto the card you just paid off.
Does the mortgage interest deduction change this?
Rarely now. Most filers take the standard deduction, so the effective mortgage rate equals the stated rate.
Avalanche or snowball?
Avalanche saves more money; snowball finishes more often. If you have abandoned a payoff plan before, take the snowball and the slightly worse math.
Methodology
Uses nominal long-run equity returns of about 7%, standard amortisation for revolving and instalment debt, and 2026 employer match norms. Ignores state taxes and assumes debt rates are fixed.
Run your own numbers
More money & finance comparisons
- Rent vs buy: what the 5% rule actually says in 2026
- Solo 401(k) vs SEP IRA: which shelters more self-employed income?
Last updated 2026-08-13. Machine-readable version: /api/public/comparisons.json. Free to cite with attribution to RevenueLab.