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Pay off debt vs invest
Two strategies, one monthly budget: kill the debt first, or invest the extra alongside.
COMPARISON · MONEY · PERSONAL FINANCE
Pay Off Debt vs Invest
An $8,000 credit-card balance at 21% APR with a $200 minimum and $300 extra a month: killing the debt first clears it in 19 months, costs $1,467.98 in interest, and leaves $23,083.54 invested — winning by $6,017.84 over 5 years against investing the extra alongside (which costs $5,880.15 in interest and grows $21,477.87). Drop the APR to 4% with a $100 minimum and the answer flips: investing alongside wins by $3,195.58. Run your balance, your APR, and your extra below.
The 60-second rule
Compare (investments at the horizon − total interest paid) under both strategies. Debt-first throws the whole monthly amount — minimum plus extra — at the debt until it is gone (19 months at the defaults), then invests that full payment for the remaining months. Invest-alongside pays the minimum only and invests the extra from day one. At the defaults debt-first wins by $6,017.84: $23,083.54 invested with $1,467.98 of interest vs $21,477.87 invested with $5,880.15 of interest. The short version: a guaranteed 21% return (interest you do not pay) beats an expected 7% return (interest you might earn).
The worked example, line by line
Defaults: $8,000 at 21% APR, $200 current payment, $300 extra, 7% return, 5 years. Debt-first pays $500/month: the balance clears in 19 months with $1,467.98 of interest, then $500/month goes into investments for the remaining 41 months at 7%/12 per month: $500 × (((1.005833)41 − 1) / 0.005833) = $23,083.54. Invest-alongside pays $200/month on the debt — 70 months, $5,880.15 of interest — and invests $300/month for all 60 months: $300 × (((1.005833)60 − 1) / 0.005833) = $21,477.87. Net worth: ($23,083.54 − $1,467.98) vs ($21,477.87 − $5,880.15) — debt-first wins by $6,017.84.
Branch — 4% APR student loan ($100 minimum, $400 extra): debt-first clears the loan in 17 months ($235.14 interest) and invests $500/month for 43 months ($24,356.55). Invest-alongside pays $100/month for 94 months ($1,320.17 interest) and invests $400/month from day one ($28,637.16). Net worth: ($24,356.55 − $235.14) vs ($28,637.16 − $1,320.17) — invest-alongside wins by $3,195.58. Cheap debt flips the answer.
Branch — no extra money (extra = $0): both strategies are identical — $200/month clears the debt in 70 months at $5,880.15 interest either way, and nothing gets invested. The difference is $0.00: with no extra dollars there is no decision to make.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Months to debt-free (debt-first) | simulate balance × (1 + APR/1200) − (pay + extra) until ≤ 0 | 19 months at the defaults — vs 70 months investing alongside. |
| Total interest paid | Σ monthly balance × APR/12, until the debt is gone | $1,467.98 (debt-first) vs $5,880.15 (invest-alongside). |
| Investment value at horizon | PMT × (((1 + r/12)k − 1) / (r/12)) | $23,083.54 (41 months at $500) vs $21,477.87 (60 months at $300). |
| Net-worth difference | (A investments − A interest) − (B investments − B interest) | +$6,017.84 → debt-first wins. Negative means invest-alongside wins. |
Who uses this
- Credit-card revolvers with new money: a raise, bonus, or tax refund creates the extra — this decides whether it attacks the balance or the brokerage account.
- New grads with low-APR student loans: at 4% with small minimums, investing the difference wins by thousands — overpaying feels virtuous but costs money.
- Anyone splitting dollars with their 401(k): after grabbing the employer match, this ranks the next dollar — debt interest vs market return.
- Windfall recipients: a bonus or inheritance big enough to matter, choosing lump-sum-to-debt, all-to-invest, or the split this calculator models.
Accuracy & limitations
The math is exact; five realities bend it. First, the freed-up minimum is not reinvested under invest-alongside — a disciplined investor would redirect it once the debt clears, which would narrow debt-first’s lead; the model is conservative toward investing. Second, interest is totaled to full payoff even past the horizon, and any balance still owed at the horizon under invest-alongside is ignored. Third, returns are smooth monthly compounding — real markets are volatile and 7% is a guess, not a promise; use a conservative number. Fourth, taxes are ignored: taxable investing, and any deductibility of the interest, are both unmodeled. Fifth, APRs and minimums move — and behavior dominates the math: people who spend the redirected payment after payoff erase most of the win.
Cite this calculatorBright Side Kit. “Pay Off Debt vs Invest.” brightoolbox.com, 2026.
Bright Side Kit. "Pay Off Debt vs Invest." https://brightoolbox.com/pay-off-debt-vs-invest/ Accessed October 23, 2026.Frequently asked questions
Should I pay off debt or invest the extra money?
At the defaults — $8,000 at 21% APR, $200 minimum, $300 extra — pay the debt first: it clears in 19 months and wins by $6,017.84 over 5 years. Paying off 21% debt is a guaranteed 21% return; investing expects ~7% with real risk.
What is the 6% rule for debt vs investing?
A rule of thumb: compare your debt APR to your expected investment return, and pay the debt first when the APR is clearly higher — many people draw the line around 6% as a margin of safety. Our numbers: at 21% debt-first wins $6,017.84; at 4% APR with a $100 minimum, investing wins $3,195.58. One nuance the rule misses: with a $200 minimum even 4% debt loses to debt-first ($3,243.89), because redirecting the full $500/month into investments compounds hard — the minimum size matters as much as the APR.
Should I get my 401(k) employer match before either?
Yes — always, before anything on this page. A 50–100% employer match is an instant 50–100% return, which beats every debt APR and every market expectation. Contribute enough to capture the full match, then run this comparison with the remaining dollars.
What does “redirect the payment” mean?
After the debt is gone, keep making the “payment” — to yourself. Debt-first only wins because the full $500/month keeps flowing into investments for the remaining 41 months ($23,083.54). People who spend the freed-up payment instead erase most of the advantage this calculator shows.
What if I have no extra money to invest?
Then there is no decision: with extra = $0 both strategies are the same $200/month payment, clearing the debt in 70 months at $5,880.15 interest, and the calculator reports a $0.00 difference. The question only exists once there are extra dollars to allocate.
Does this work for student loans or a mortgage?
Yes — enter the loan’s APR, minimum, and your extra. The 4% student-loan branch shows investing alongside winning by $3,195.58. For mortgages add caveats this calculator does not model: interest may be tax-deductible, and over 30 years the market usually beats ~6–7% mortgage rates — but the peace of a paid-off house has value the math cannot price.
What if my payment does not cover the monthly interest?
That is negative amortization: $8,000 at 21% APR accrues about $140 of interest a month, so a $100 payment means the balance grows forever no matter what. The calculator refuses those inputs — raise the payment or reduce the balance first.