New on FinoLib · Volume V: Personal Finance
New on FinoLib, my finance education website: I have published an article called Credit Card Interest Rates Explained: How Interest Is Charged, from Volume V: Personal Finance. It is part 2 of a 2-part guide. Here is what the article explains, written as a short expert briefing; the full piece, with every worked example, is one click away.
A closer look
Not all debt is created equal, and the common advice to “avoid debt” is really shorthand for a more precise idea: debt is worth taking on when it finances something that grows in value or earning power faster than the debt’s own interest rate costs, and worth avoiding when it finances something that loses value the moment it’s bought.
For a household paying down several debts at once, two strategies dominate — one mathematically optimal, the other behaviorally optimal, and the “right” one depends on which failure mode a given person is actually more at risk of.
The avalanche saves $3,712.98 − $3,050.77 = $662.21 and two months; the snowball delivers its first closed account 17 months sooner. That is the whole trade in the table above: a few hundred dollars against a much earlier sign of progress.
A worked example
Worked Example — Same Debts, Two Orders
$600 a month for three debts: a store card, $1,200 at 18% (minimum $35); a Visa, $7,800 at 26% (minimum $195); a personal loan, $3,500 at 12% (fixed payment $115). Minimums are held fixed for simplicity; the other $600 − $35 − $195 − $115 = $255 goes to the target debt, and each freed-up payment rolls to the next target.
Under the hood
Under the Hood: Why Card Interest Costs So Much, and How It Is Charged
The price. Most card APRs are variable: the agreement sets the rate as an index, almost always the prime rate, plus a margin fixed for the account. The prime rate follows the Fed: it was 6.75% before the September 16, 2026, increase and 7.00% after it, so a variable card rate rises by the same quarter point on the schedule its agreement sets. The CFPB found the average APR margin on revolving accounts reached 14.3% in 2023, the highest on record, and that nearly half of the 2013–2023 rise in card rates came from wider margins rather than a higher prime rate, even as charge-off rates fell.
Decision rule
If you carry balances on more than one debt, then pay every minimum and send every extra dollar to the highest-APR balance. Exception: a balance you can clear with three months or less of extra payments may go first; the detour is too short to cost much. Override: a deferred-interest balance whose promotion ends within six months goes first, because missing that date charges interest back to the purchase date.
The costliest mistake
Walking into a mortgage application with a score one pricing tier lower than it needed to be — because balances were high on the statement date, an error went unchecked, or a new card was opened. Lenders price by score band; assume one band down costs half a percentage point (illustrative; tiers vary by lender). On the $201,600, 30-year loan from the 5.1 example: payment = L × i ÷ [1 − (1 + i)^−360], with i = rate ÷ 12.
Why It Matters
A minimum payment is calculated to be just barely more than the interest accruing that month — its entire design purpose, from the card issuer’s side, is to keep a balance outstanding for as long as legally possible. Paying any fixed amount meaningfully above the minimum, even a modest one, changes the entire trajectory of the debt precisely because so little of a minimum payment goes toward principal in the first place.
Analogy
Vol. II’s Part 2 covers leverage in a leveraged buyout — borrowed money amplifies returns when the asset it buys grows faster than the debt’s interest cost, and amplifies losses when it doesn’t. A household’s own debt works on exactly the same logic, just at a smaller scale: a mortgage is leverage on a home that has historically appreciated; credit card debt on a dinner already eaten is leverage on nothing at all.
Recap
A FICO score, generally 300 to 850, rests mostly on payment history (35%) and amounts owed (30%), the two factors you control on a short timeline, and it is calculated from three separate bureau reports that you can pull free at AnnualCreditReport.com and dispute free, with copies of your proof, under the Fair Credit Reporting Act.
What the full article covers
The full article includes worked examples, a look under the hood at the mechanics, a clear decision rule, the one mistake that costs people the most, a recap, a reveal-on-click quiz and the official sources behind every figure.
Volume V: Personal Finance
Read the full article on FinoLib
The complete article, Credit Card Interest Rates Explained: How Interest Is Charged, with every worked example, diagram, quiz and source, is free to read in Volume V, Personal Finance on finolib.com.
Explore the Finance Education Series: Vol. II — Capital Markets · Vol. I — Global Economy · Vol. III — Finance Encyclopedia · Vol. IV — Finance Careers · Vol. V — Personal Finance