The math gives you a clear answer. Here's how to read it.
Compare the guaranteed cost of debt — its interest rate — with the expected return from investing. If your debt costs more than investing earns, pay off the debt first. If investing reliably earns more than your debt costs, invest.
The logic is clean: paying off a debt with a 20% interest rate is mathematically identical to earning a guaranteed 20% return on an investment. No index fund or savings account offers that reliably.
Most debt falls into one of three clear categories. Here's how to read the table: "pay off first" means the debt's guaranteed cost exceeds what you'd realistically expect to earn investing.
| Debt type | Typical rate | Verdict |
|---|---|---|
| Credit card | 20–29% | Pay off first. Always. |
| Personal loan | 10–20% | Pay off before investing. |
| Car loan | 5–10% | Borderline — lean toward paying off |
| Student loan (federal) | 5–8% | Borderline — either works |
| Mortgage | 6–8% | Investing likely earns more long-term |
| Mortgage (old, low rate) | 2–4% | Invest — the math is clear |
The S&P 500 has averaged roughly 10% annually over the past century — about 7% after inflation. That's a compelling long-term return. But "long-term" is doing a lot of work in that sentence: any given year can be down 30% or more. In 2022, a diversified equity portfolio lost around 20%. In 2008–2009, it lost over 50% from peak to trough.
Paying off debt, by contrast, is a guaranteed return equal to the interest rate. There's no volatility, no sequence-of-returns risk, no bad year where your "return" was actually a loss.
When running the comparison, use 6–7% as a conservative investing benchmark, not 10%. That's a more realistic expectation after fees and taxes, and it builds in a margin of safety.
There is one exception that overrides almost everything else: the 401k employer match.
If your employer matches your contributions — say, 50 cents for every dollar up to 6% of your salary — that's a guaranteed 50% return on the matched portion before your money even hits the market. A 100% match is a guaranteed 100% return on day one.
Always contribute enough to capture the full employer match before paying down anything except extremely high-rate debt. This is the one situation where "invest before paying debt" is almost always the right call.
When money is tight and you have competing priorities, work through this list in order:
The math is useful, but it's not everything. Being debt-free has real value that doesn't show up in a spreadsheet: reduced stress, greater flexibility, better sleep. Financial decisions you make consistently over years matter more than theoretically optimal ones you abandon because they feel wrong.
If carrying debt keeps you up at night, paying it off faster has a genuine return even if the numbers say otherwise. Conversely, if the idea of not investing during your peak earning years makes you anxious, that's a real cost too.
Neither choice is wrong in the borderline zone. Pick the one you'll stick to.
If you're paying off multiple debts at once, two strategies compete:
The avalanche saves more money. The snowball keeps more people on track. Research suggests the snowball's motivational benefits cause more people to actually become debt-free. Pick the one you'll follow through on — a good plan executed beats a perfect plan abandoned.
It depends on your rate. Federal student loans typically run 5–8% (UK student loans work differently — repayments are income-contingent). That's in the borderline zone — either approach works mathematically. A common strategy: contribute enough to get the full employer match, then split extra money between loan payoff and investing.
If your loans are above 8%, lean toward paying them down first. If below 5%, lean toward investing. In the middle, doing both simultaneously is perfectly reasonable.
Pay the minimum on all cards to avoid fees and credit damage, then direct every spare dollar at the highest-rate card first (the debt avalanche). Once that's gone, roll that payment into the next highest rate.
If motivation is your challenge, the debt snowball — paying the smallest balance first regardless of rate — can help you build momentum, even if it costs a little more in total interest.
Usually not, if your mortgage rate is below 6–7%. Historically, a diversified equity portfolio returns more than that over long periods. However, if your rate is above 7%, the math gets closer.
There's also a non-financial argument: owning your home outright provides security and flexibility that has real value even if the numbers technically favour investing. Both choices are defensible — see our dedicated guide on mortgage vs. investing for the full breakdown.
The mortgage interest deduction reduces the effective cost of your mortgage, but only if you itemise deductions. Since the 2017 tax law roughly doubled the standard deduction, most Americans no longer itemise — so they don't benefit from this deduction at all.
For those who do itemise, the deduction reduces your mortgage cost by your marginal tax rate. A 7% mortgage at a 22% bracket has an effective cost of about 5.5%. Worth factoring in, but rarely a game-changer in the overall invest-vs-pay-off calculation.