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Should you pay off a debt or invest?
Paying down a loan returns exactly its interest rate, guaranteed. Investing might do better. Here is how to compare them honestly — after tax, after the match, and after admitting which number is a forecast.
You have a spare $500 a month. It can go against a loan or into the market. Almost every version of this argument goes wrong in the same place: the two sides are compared as if both numbers were facts, when only one of them is.
What paying down debt actually returns
Put $500 against a 7% loan and you have removed, with certainty, every future interest charge that $500 would have generated at 7%. That is a 7% return, guaranteed, with no variance at all.
It is also, in most cases, tax-free. There is no tax on interest you did not pay. Compare that with an investment: to net 7% after tax you need to earn more than 7%, and how much more depends on the account and the holding period.
The honest comparison is therefore:
That framing settles a lot of cases on its own. A 22% card is not a close call against any equity forecast anyone will defend out loud.
The tax side, properly
Two adjustments in opposite directions.
Investment returns are taxed. Long-term capital gains are taxed in brackets, not at one flat rate. For 2026, the 0% band runs up to $49,450 of taxable income filing single and $98,900 married filing jointly; above those, 15% applies up to $545,500 and $613,700 respectively, and 20% above that. On top, the net investment income tax of 3.8% applies once modified AGI exceeds $200,000 single or $250,000 married. In a tax-deferred or Roth account, none of this applies — which is exactly why the account type matters more than the comparison people usually have.
Some debt interest is deductible, which lowers its real rate. Mortgage interest on acquisition debt can be, and student loan interest is deductible up to $2,500 a year above the line. But for mortgage interest you have to itemize, and the standard deduction for 2026 is $16,100 single and $32,200 married filing jointly. Most households do not clear that with mortgage interest alone, so for most people a 6.5% mortgage really does cost 6.5%.
Credit card and auto loan interest is not deductible at all.
The pay off debt or invest calculator runs both paths after tax, which is the only version of the comparison worth looking at.
The order that actually applies
Before the comparison is even relevant, two things come first.
Any employer retirement match. A 50% or 100% match is an immediate return that no debt rate competes with. Leaving it on the table to pay down a 6% loan is a losing trade by a wide margin. Capture the full match first, always.
An emergency fund. Money paid into a loan is gone — you cannot get it back when the transmission fails. Clearing every spare dollar into debt and then meeting the next emergency with a credit card means you refinanced 6% debt into 24% debt and paid for the privilege.
After those two, the comparison is live.
Where the line usually falls
- Above about 8%, paying down almost always wins. The guaranteed return is at or above what a diversified portfolio is expected to deliver before tax, and it comes with none of the risk.
- Below about 4%, investing usually wins over long horizons. A 3.25% mortgage is cheap money and there is a good case for keeping it.
- In between, it is genuinely close, and the deciding factors are not financial: how long the horizon is, how much variance you can live with, and what a paid-off loan is worth to you in peace of mind.
That middle band is where people argue, and it is precisely the band where the difference is small enough not to be worth arguing about.
The part the spreadsheet cannot show
A guaranteed 6% and an expected 6% are not the same thing, even though a calculator prints them the same way. The expected one comes with the possibility of a 40% drawdown in year three, and a payment schedule that does not care.
Conversely, money in a loan is illiquid. Paying an extra $30,000 into a mortgage and then needing $30,000 puts you back at a lender's mercy on that day's terms. Investments can be sold.
Neither of those shows up in a rate comparison, and both are real.
What to do with this
- Capture the full employer match. Non-negotiable.
- Fund the emergency buffer. Three months is a floor, not a target.
- Clear anything above 8% before considering the market at all.
- Compare after tax, using the account you would actually invest in — a 401(k) and a taxable brokerage give very different answers.
- Treat the investment return as a range, not a number. Run it at 4% and at 9% on the calculator and see whether the answer changes. If it flips, the honest conclusion is that you do not know, and the certain option has a real claim.
The arithmetic gets you a long way here. Where it runs out, the tiebreaker is usually that one side of the comparison is a promise and the other is a hope.
Sources: capital gains brackets and the net investment income tax threshold are read from the same constants the calculators use, sourced to IRS Revenue Procedure 2025-32 and IRS, net investment income tax. Home mortgage interest and the standard deduction from IRS Publication 936.