How the debt vs invest calculator works
Updated November 2026
The choice, in plain words
Every spare dollar can only do one job at a time. Sent to a debt, it stops that balance from charging interest, at exactly the debt APR, with no risk. Sent to an investment, it can grow at the return you assume, which may or may not show up. Neither answer is right for everyone, because the rates differ from household to household. The honest way to choose is to run both paths with your own numbers and the same total money, which is what the calculator above does.
The method, month by month
Both paths run the same monthly steps, and the calculator above runs exactly these:
Step 1: set the monthly rates. The debt charges APR / 12 each month on the balance at the start of the month. Investments grow at your return / 12 each month. A contribution is added at the end of its month, so it earns nothing in its first month.
Step 2: run the debt-first path. Your base payment plus your extra all go to the debt until it is gone, and the last payment is just the smaller remainder. From the next month on, the whole amount is invested instead, for the rest of the horizon.
Step 3: run the invest-extra path. Only the base payment goes to the debt, and the extra is invested from the first month. If the base payment does not cover the month interest, the balance grows, and the simulation lets it grow: that is what your numbers really do. Once this path clears the debt too, its base payment joins the investing.
Step 4: read the score. At the horizon, each path scores its investment balance minus any debt still left. The higher score wins, and the difference between the scores is the headline. A win of $100 or less either way is called too close to call on math alone, because small changes in the return would flip it.
A worked example
Take the numbers this calculator opens with. A balance of $8,000 at 19.99 percent APR, a base payment of $200 a month, $200 extra a month, a 7 percent return, and a horizon of 10 years.
On the debt-first path, $400 a month clears the debt in month 25, the interest paid along the way is $1,811.67, and the freed $400 then invests for the rest of the run, ending at a net position of $50,911.71. On the invest-extra path, the $200 base payment needs until month 67 to clear the same debt, the interest paid is $5,287.81, and the net position ends at $47,149.08. Same $400 a month, same 10 years: paying the debt first leaves you $3,762.63 ahead, and $3,476.14 of that gap is simply interest the slower path paid and the fast path did not. Change any one input and every one of those numbers moves; that is what the fields above are for.
How to read your result
Debt first wins by more than $100: at a high APR this is the usual story. The interest saved is certain, and clearing the balance early frees the whole payment to invest for longer. Invest extra wins by more than $100: the usual case at a low APR, where the debt is cheap and the assumed return does the heavy lifting; the win only holds if that return actually shows up, year after year. Within $100 either way: call it a tie on math, then decide on the rest of life: the debt-first path ends the debt sooner and removes a bill, which many people count for more than a small paper gap.
What the math takes as given, and what it leaves out
The math holds your return steady every year, which real markets never do, and it works with one debt at one APR. It leaves out taxes on gains, investment fees, employer matching, minimum payments on any other debts, and the chance that extra money sent to a card creeps back as new spending. It also treats interest saved and market gains as the same kind of dollar; they are not. Interest saved is locked in the day the payment lands. Gains are a hope until the day you sell.
The same question, worked as a guide
The Kollaz Money guide Pay Off Debt or Invest Calculator: Get Your Answer in 60 Seconds walks through this same choice in plain words, with the reasoning that sits behind the rates and the break-even point between them.
The date underneath the decision
Both paths end with the debt gone; they differ on when. The free Know Your Date Calculator prices that timing on its own: at the payments you already make, the month your debt is gone for good.
Sources
- The debt side is standard loan amortization: interest each month is the balance at the start of the month times APR / 12, and payments land at the end of the month, with the final payment cut to the remainder. The growth side is the standard compound growth method, the same method behind the U.S. Securities and Exchange Commission compound interest calculator at investor.gov (checked 2026-11-10). The return here is the visitor's own assumption, set from 0 to 12 percent; it is never presented as a forecast.
- The paid product named on this page is the Debt vs Invest Calculator Excel (Kollaz, KM-029): the verdict, the debt-free month, the 5-year net worth under both plans, and the personal break-even rate are its stated method in the Kollaz catalog (catalog.json, checked 2026-11-10). The Debt Freedom Bundle (Kollaz, KM-037, CA$27.99) named in the gap panel pairs it with the Debt Payoff Calculator and the Impulse Buying Tracker, per the same catalog.
Educational information only, not financial advice. Consider a qualified professional for decisions about your specific situation.