Pay Off Debt or Invest? How to Run the Math on Your Exact Interest Rate

Every article on this topic falls back on the same shortcut: if your debt is above 6%, pay it off; if it's below, invest. Fidelity uses that heuristic. So does SmartAsset. So does most of the personal finance internet. The heuristic is not wrong. It's just incomplete, because it collapses three separate decisions into one number and hides the assumptions underneath.

This post walks through the actual math in real dollars. Not a heuristic, not a rule of thumb. Three questions, in order, applied to your specific APR, your specific expected return, and your specific employer match. By the end you'll know exactly where your next spare dollar should go — and you'll be able to defend the answer with numbers.

The Three-Question Framework (Do These In Order)

Before we get into the math, here's the sequence. Skip a step and you'll get the wrong answer.

1. Am I capturing the full employer 401(k) match? If no, stop everything and do that first.
2. Is my debt APR above the risk-adjusted expected return of my investments? If yes, pay off the debt. If no, invest.
3. What does the after-tax picture look like? Investment returns get taxed. Debt interest doesn't get a deduction (for most consumer debt). Adjust accordingly.

That's the whole framework. Everything below is the math that makes each step concrete.

Step 1: The Employer Match Is Not Optional

The employer match beats everything. It's not close.

A 50% match on the first 6% of salary means for every dollar you contribute, your employer adds 50 cents. That's an immediate 50-100% return on your contribution, depending on your plan's match formula. There is no debt on your balance sheet earning anywhere near that rate. Not even a 29.99% penalty-rate credit card.

Here's what the numbers look like in 2026. Vanguard's 2024 data shows the average employer match is 4.6%, the median is 4%, and the most common structure is a 50% match on the first 6% of pay, offered by 68% of plans.

So if you make $70,000 and contribute 6% ($4,200), your employer adds $2,100. That $2,100 is a return you cannot get anywhere else. Skipping the match to pay down a 24% APR credit card is mathematically wrong — you're leaving a 50% guaranteed return on the table to chase a 24% guaranteed return.

One caveat: if you're behind on minimum payments and going backward every month, that's a cash-flow crisis, not a math problem. Fix the cash-flow crisis first, then come back to the match. But if your minimums are covered and you have money left over, the match wins.

Step 2: Compare APR to Expected Return

Once the match is captured, the next dollar is a real comparison. Debt payoff on the one side, investment on the other. This is where the 6% rule comes from.

The logic is this: paying off a debt with an 18% interest rate is like getting an 18% return, without taking on any market risk. Debt payoff is a guaranteed return. There's no scenario where paying off a 22% APR card and then having the market drop 30% leaves you worse off. The return is locked in the moment you make the payment.

Investment returns are not guaranteed. The S&P 500 has averaged 10.4% over the 30-year period from January 1996 through December 2025, with the 20-year average at 11%. That's a real number, but it's an average across decades that included two major crashes. In any given three-year window, the return could be anywhere from -20% to +25%.

So the actual comparison is: guaranteed APR (debt payoff) vs. probabilistic long-run return (investing). The 6% rule exists because Fidelity's methodology says: if you have at least 10 years before retirement, a balanced portfolio with about 50% stocks, and a tax-advantaged account, then debts above 6% should be paid off first.

That threshold is not a universal law. It's the output of three specific assumptions. If you have a shorter time horizon, the threshold goes up (you can't wait out a bad decade). If you're all in on stocks, the threshold might go up slightly (higher expected return, higher variance). If your investments aren't tax-advantaged, the threshold definitely goes up (see step 3).

Step 3: Taxes Move the Line

Here's what most articles skip. The 10% long-run market return is a pre-tax number. Your credit card APR is an after-tax number (credit card interest is not deductible for most people).

Investment returns are reduced by taxes, so an 8% market gain might leave you with closer to 6-7% in your pocket once you account for long-term capital gains taxes. If you're investing in a taxable brokerage account and holding long enough for long-term capital gains treatment (15-20% federal, plus state), a nominal 10% return might net closer to 7.5-8.5%.

Compare that to a credit card at 22.15% — the average APR on cards accruing interest in Q2 2026, per Federal Reserve G.19 data. The card is a 22.15% guaranteed return, tax-free. The taxable brokerage is an 8% expected return with variance. There's no version of this math where investing wins.

Inside a Roth IRA or 401(k), returns compound tax-free, so the comparison stays closer to the nominal 10%. That's why the 6% rule is calibrated to tax-advantaged accounts. If your investing happens in a taxable account, mentally shift the threshold up by 2-3 percentage points.

A Worked Example in Real Dollars

Let's put a person into the framework. Call her Priya. She makes $85,000 a year and just got a $600/month raise. She's trying to decide where the $600 should go.

Her situation:

• 401(k): She contributes 3% of salary. Her employer matches 50% on the first 6%. She is leaving match on the table.
• Credit Card A: $8,400 balance at 24.99% APR
• Credit Card B: $3,200 balance at 19.99% APR (0% promo expired last month)
• Auto loan: $14,000 balance at 6.5% APR
• Student loans: $22,000 balance at 4.5% APR
• Brokerage account: Currently contributing $0. Considers herself "behind on investing."

Her instinct is to split the $600 three ways: some to investing, some to the credit cards, some to the auto loan. Feels balanced. Let's see what the framework actually says.

Applying Step 1: Match First

Priya is contributing 3% ($2,550/year, or ~$212/month) to her 401(k). To capture the full match, she needs to hit 6% ($5,100/year, or ~$425/month). The gap is $213/month.

So the first $213 of the $600 raise goes straight to 401(k) contribution. That unlocks another $1,275 in employer match per year ($5,100 × 25% = the additional match she's currently missing). That's a guaranteed 100% return on that specific $1,275 in incremental contribution. Nothing else on her balance sheet comes close.

Raise remaining: $600 - $213 = $387/month.

Applying Step 2: APR vs. Expected Return

Now Priya has $387/month to allocate. Compare each debt APR to her expected investment return.

Assume her investing happens inside the 401(k) (already tax-advantaged) or a Roth IRA. Expected long-run return: ~10% nominal, tax-free at withdrawal.

The answer is clean: the $387 goes to Credit Card A first, then Credit Card B once A is dead. The auto loan and student loans get minimums only. Any additional savings above the credit card payoff should go to investing, not to accelerating the low-rate debt.

Applying Step 3: Tax Check

Priya's investing is inside tax-advantaged accounts, so the nominal 10% return is a fair comparison. If she were also investing in a taxable brokerage on top of that, the 6.5% auto loan would become a closer call — a nominal 10% return minus ~20% in long-term capital gains taxes lands around 8%, which still beats 6.5% but narrows the margin.

The 4.5% student loans might also have a deduction (up to $2,500/year of student loan interest is deductible, subject to income limits). That effectively lowers the true cost of the debt to something like 3.4% for someone in the 24% bracket. Even more clearly not worth accelerating.

The Final Allocation

Priya's $600/month raise ends up allocated:

• $213/month to 401(k) contribution to capture full employer match
• $387/month to Credit Card A as extra principal until it's paid off
• Then $387/month rolls to Credit Card B until it's paid off
• Then $387/month rolls to Roth IRA investing (not to the auto loan or student loans)

That's the answer. Not a 50/50 split, not "a little of everything." A specific, defensible sequence based on the math.

What This Framework Doesn't Cover

Three edge cases worth naming:

Emergency fund. If Priya doesn't have at least $1,000-$2,000 in liquid savings, some of that $387 should go to a starter emergency fund first. Otherwise the next unexpected car repair goes back on the credit card and you're running in place. Most planners recommend a small emergency fund before aggressive debt payoff, then a full 3-6 month fund after the high-rate debt is dead.

Variable income. If Priya's income is inconsistent (freelance, commission, seasonal), the extra-payment number isn't $387/month — it's a range. Planning against the floor of that range and treating high-income months as windfalls is safer than planning against the average.

0% promo APR debt. Priya's Card B was on a 0% promo that just expired. If she still had six months of 0% left, the math would flip temporarily — during the promo, that debt is effectively 0% APR and shouldn't be prioritized over investing. But you have to model the cliff, because the moment the promo ends, the balance jumps to 19.99% and any remaining principal starts accruing at full rate. RealiPlan's planner models promo expirations explicitly, which is the one thing generic calculators tend to miss.

Where To Run Your Own Numbers

The framework is worthless if you don't apply it to your actual portfolio. Two concrete steps:

1. Confirm your employer match. Log into your 401(k) portal. Look up the match formula. If you're not hitting the full match, raise your contribution today, before doing anything else. This is the single highest-return action available to anyone reading this article.

2. Model your debt payoff with real numbers. Run your debts through the free calculator to see your specific debt-free date under snowball, avalanche, and hybrid strategies. Then compare that timeline against what investing the same monthly dollars would produce over the same period. The comparison is much less abstract once you're looking at your own APRs and your own payoff date.

For most people carrying credit card balances at current rates, the math is not close. The average APR on cards accruing interest hit 22.15% in Q2 2026, and 45% of adult cardholders carried a balance for at least one month in the past year. If you're in that 45%, paying off the card is not just "probably better than investing" — it's dramatically better, by 10-15 percentage points of guaranteed return.

The 6% rule points in the right direction. The three-question framework tells you exactly what to do. Match first, then compare APR to expected return, then check the tax angle. Do the math on your specific numbers and the answer stops feeling like a judgment call and starts feeling like arithmetic.

See your debt-free date and model your specific portfolio → or compare RealiPlan plans →.