3tej home

High-Interest Rate Dilemma Solver (Pay Debt vs. Invest)

When interest rates are high, the math changes. Should you use your extra $500 a month to aggressively pay down your 7% mortgage, or invest it in the market?

Your Debt Profile

Cash available ABOVE your minimum payment.

Investment Alternative

15% for long-term capital gains, or your standard bracket if using a high-yield savings account (HYSA).

Scenario Comparison (After Debt is Gone)

Scenario A: Aggressively Pay Debt

Put all extra cash towards the loan until paid off.
Months to Payoff 65 months
Net Worth at Payoff $0

Scenario B: Invest Extra Cash Instead

Pay only the minimum. Invest extra cash in the market.
Investment Balance (After Taxes) $38,000
Remaining Debt Balance -$35,000
Net Worth at Month 65 $3,000
Calculating...

About this tool

This calculator models the math behind the most common personal finance dilemma: take an extra 500 USD a month and pay down a 7.5 percent mortgage faster, or invest it in an S&P 500 index fund expected to return 8 percent. The comparison comes down to after-tax effective rates, which surface the otherwise hidden investment tax drag and emergency fund risk.

How it works

After-tax investment return = Investment return x (1 - Tax rate on gains)
Debt return                 = Debt APR (after-tax saving, no risk)

Scenario A (pay debt): months_to_payoff using -ln((m-bal*r)/m)/ln(1+r)
Scenario B (invest):   debt grows at minimum payment, cash compounds at after-tax rate
Net worth A at payoff  = 0
Net worth B at payoff  = Investment FV - Remaining debt balance
  • Debt APR: 22 to 29 percent on credit cards in 2026 (Fed G.19 January data), 6.5 to 7.5 percent on 30 year mortgages, 8 to 12 percent on personal loans.
  • Investment return: long term S&P 500 averages 10 percent nominal / 7 percent real; bond portfolios 4 to 5 percent; HYSA 4 to 5 percent in 2026.
  • Tax rate on gains: 15 percent LTCG for most investors (single income 47,025 to 518,900 USD); 20 percent above; 0 percent below 47,025 USD single in 2026.
  • Tax shelter: 401(k) match, HSA, and Roth IRA contributions sit above this dilemma; capture them first.

Worked example

A 35 year old single Florida engineer has a 50,000 USD remaining balance on a 7.5 percent mortgage and 500 USD per month of free cash. The S&P 500 is expected to return 8 percent; they are in the 22 percent federal bracket so LTCG tax is 15 percent.

  1. After-tax investment return: 8 percent x (1 - 0.15) = 6.8 percent. Below the 7.5 percent debt cost.
  2. Scenario A (pay debt): minimum payment 596 USD plus 500 USD extra = 1,096 USD per month payoff in roughly 65 months.
  3. Scenario B (invest): over the same 65 months, 500 USD per month at 6.8 percent compounded grows to 38,640 USD.
  4. Debt B remaining: minimum 596 USD payment reduces balance from 50,000 USD to ~35,940 USD over 65 months.
  5. Net worth A: 0 USD (debt gone, no investment built).
  6. Net worth B: 38,640 USD investment minus 35,940 USD debt = +2,700 USD.
Result: Even with 7.5 percent versus 6.8 percent after-tax expected return, the long compounding window favors investing by ~2,700 USD. Drop the assumed return to 7 percent (nets 5.95 percent after tax) and the math reverses: Scenario A wins. The closer the rates, the more emergency liquidity matters: cash kept in the brokerage is reachable; cash sunk into mortgage equity is not.

Decision matrix for 2026 debt types

After-tax cost rates are estimates assuming 22 percent federal bracket and 15 percent LTCG. Expected stock return is 7 percent real / 10 percent nominal long term.

Debt typeTypical APRAfter-tax costDecision
Credit card22-29%22-29%Always pay debt first
Personal loan10-14%10-14%Pay debt
Private student loan7-12%~5.5-9.5% (with deduction)Pay debt
Federal student loan5-8%~4-6.5% (with deduction)IDR + invest
Auto loan6-10%6-10%Pay debt above 7%
Mortgage 2025 vintage6.5-7.5%5.5-7.5%Toss up; emergency cash first
Mortgage 2020-2021 vintage2.75-3.5%2.75-3.5%Invest
0% promo (BNPL, intro APR)0% until expiry0%Invest; watch expiry trap

Common mistakes

  • Skipping employer match. A 50 to 100 percent match dwarfs every interest rate decision. Capture full match before doing anything else.
  • Comparing nominal returns. 8 percent investment return at the 22 percent bracket and 5 percent state = 5.84 percent after tax in a taxable account. The mortgage rate stays nominal.
  • Forgetting emergency fund. Sinking cash into a mortgage payoff reduces liquidity. If you lose income before the payoff completes you have an illiquid house and no cash.
  • Ignoring tax shelters. 401(k), HSA, and Roth IRA shield gains entirely. Investing pre-tax in a 401(k) at 22 percent bracket plus 5 percent state effectively boosts the comparable return to 8 / (1 - 0.27) = 11 percent.
  • Treating snowball as suboptimal. Mathematically avalanche (highest APR) wins by 1 to 3 percent. Behaviorally snowball wins by completion rate. Pick the one you will actually finish.
  • Forgetting BNPL trap. Buy Now Pay Later 0 percent only stays 0 percent if every payment lands on time; one miss can retroactively apply 30 percent APR from day one.

Related tools and glossary

BNPL vs Credit Card401(k) Rollover PlannerCompound InterestMarginal Tax Rate

Frequently asked questions

How do I decide between paying debt and investing?

Compare the after-tax debt cost to the after-tax expected investment return. A 7.5 percent mortgage (deductible only above the 30,000 USD MFJ standard deduction in 2026, so usually not for itemizers without large balances) saves a guaranteed 7.5 percent. A taxable brokerage with 8 percent expected return taxed at 15 percent long term capital gains nets 6.8 percent. The mortgage wins. Cards above 20 percent always win; 3 percent mortgages from 2020 to 2021 almost never do.

Does inflation actually help my debt?

Yes, on fixed rate debt. If you locked a 3 percent mortgage in 2021 and CPI averages 3 percent, the real interest rate is zero. Your nominal payment stays constant while wages and prices rise around it, so the debt becomes cheaper in real terms each year. Variable rate debt (credit cards, HELOCs, adjustable rate mortgages) does not get the same treatment because the rate resets with inflation.

Should I capture the 401(k) match before paying off debt?

Almost always yes. A 50 percent or 100 percent employer match is an instant 50 to 100 percent return that no debt can match. The standard ordering of operations: (1) capture full 401(k) match, (2) attack APR above 8 to 10 percent (credit cards, personal loans), (3) build 3 to 6 month emergency fund, (4) Roth IRA and HSA, (5) decide invest vs pay extra principal on mortgages and low rate loans.

What about the emotional cost of carrying debt?

Spreadsheets ignore stress and behavioral risk. Dave Ramsey's debt snowball method (smallest balance first) consistently outperforms the avalanche method (highest APR first) on completion rate even though it costs more in interest, because closing accounts produces visible wins. If carrying debt damages your sleep or marriage, paying off a 5 percent loan instead of investing at expected 8 percent is a rational psychological choice.

Sources and further reading

  • Federal Reserve G.19 Consumer Credit (March 2026), credit card APR averages 22 to 29 percent.
  • Freddie Mac Primary Mortgage Market Survey (April 2026), 30 year fixed averaging 6.6 percent.
  • IRS Publication 550 (2025), capital gains tax treatment and qualified dividend rates.
  • Vanguard (2024), How America Saves, 401(k) employer match prevalence (95 percent of plans offer some match).
  • Ramsey, Dave (2003), The Total Money Makeover, debt snowball methodology and Baby Steps ordering.

Last updated 2026-05-28.