How Debt Works: Principal, Interest, and Why Time Matters So Much

Two people can owe the same amount on a card at the same interest rate, and one can pay more than three times the interest of the other. What separates them is the size of the monthly payment, and through it, time.¹

How debt works comes down to three numbers: the principal you borrow, the interest rate charged on whatever is still owed, and the time the balance stays unpaid. Interest is charged again on the remaining balance every day or month, so anything that stretches the time, such as a longer term or a smaller payment, raises the total cost.

How debt works: principal, interest and time

Every debt has the same three parts. The principal is the amount you borrow; interest is the price of borrowing it, and the higher the rate, the more you pay over the life of the loan, as the US Consumer Financial Protection Bureau (CFPB) puts it.² Time is the third part: on a typical US car loan, the CFPB notes, interest is worked out daily or monthly on the balance still outstanding, so each month a balance stays unpaid adds another round of interest.³

A worked example from Canada’s financial regulator shows how much the third part matters. In an example from the Financial Consumer Agency of Canada (FCAC), a C$2,000 card balance at 18 percent took 47 months and C$793 in interest to clear at C$60 a month, against 14 months and C$231 at C$160 a month.¹ Same balance, same rate: only the time changed.

1. 1Principalthe amount you borrow
2. 2Interest ratethe price, charged on what is still owed
3. 3Timehow many rounds of interest the balance faces
4. 4Total costprincipal plus all the interest and fees

No one owns this framework: it is the ordinary arithmetic of lending, which the CFPB describes piece by piece. Research suggests people tend to misjudge how interest grows over time.

Simple versus compound interest, and why the gap grows

In the worked example below, simple interest means interest charged only on the original principal. Compound interest, as the US Securities and Exchange Commission (SEC) describes it for savers, earns interest on the money and on the interest it has already earned; a borrower faces the same arithmetic in reverse.⁴ Card interest can compound daily: the CFPB says some issuers multiply a daily rate by the amount owed at the end of each day and add the result to the balance.⁵

The word “simple” on a loan contract can mislead. For US car loans, the CFPB uses “simple interest” to mean interest calculated daily or monthly on the balance still owed, far more common than precomputed interest, where the total interest is fixed at the start and spread across the payments. With precomputed interest, extra payments do not reduce the interest owed, though some unearned interest may be refunded.³

People tend to underestimate this kind of growth. In a 2009 paper in The Journal of Finance, Victor Stango and Jonathan Zinman described exponential growth bias, the habit of treating exponential growth as if it were a straight line, and linked it to underestimating the interest rate implied by a loan’s terms. According to the abstract, the only part we read, households with more of the bias borrowed more and saved less, a link rather than proof that the bias caused it.⁶

APR, APY and the effective rate: which number to compare

In the US, the annual percentage rate (APR) is the interest rate plus additional fees the lender charges, such as origination charges. The federal Truth in Lending Act requires lenders to disclose it, and the CFPB, in its car-loan guidance, advises comparing APRs with APRs, never an APR with a plain interest rate.² For credit cards, the CFPB says, the interest rate is typically stated as a yearly rate, and that yearly rate is the APR.⁷

Annual percentage yield (APY) is the savings-side figure. Under the US Truth in Savings rules (the CFPB’s Regulation DD), it measures the total interest paid on an account based on the interest rate and how often interest compounds.⁸ For a debt, the same idea gives an effective annual rate. Where a card uses a daily rate, it is the APR divided by the number of days the issuer counts in a year, and it compounds.⁵ By our arithmetic, a balance left untouched for a year at a 20 percent APR compounded daily grows by about 22 percent, not 20.

Card rates run high in the US and Australia, according to three official sources checked in September 2026:

The Federal Reserve’s figure divides total finance charges by the balances they were charged on, so it already reflects how interest was charged; the average stated APR across all card accounts was a little lower.⁹ Virtually no investment returns as much as an 18 percent card rate, the SEC says, which is why it suggests clearing expensive debt before you invest.¹⁰ Moneysmart’s Australian figure comes from the Reserve Bank of Australia.¹¹

Amortization: the same payment, a shifting split

An amortizing loan has a fixed monthly payment, but a larger share of early payments goes to interest and a larger share of later ones to principal, so the balance falls slowly at first and faster near the end, the CFPB explains. A longer term means lower monthly payments but more interest overall.¹²

Take an illustrative US$10,000 loan at a fixed 10 percent a year, repaid in equal monthly payments over five years (our arithmetic, interest charged monthly on the remaining balance, no fees). Over the first year, more than a third of what you pay goes to interest; in the fifth, about 5 percent. The payment never changes, but early on much of it only covers the interest, which is why the balance falls slowly at first.

1. Year 1: about 36% of the year’s payments go to interest, because the balance is at its largest
2. Year 5: about 5% of the year’s payments go to interest; the rest reduces the balance
3. Every bar is the same height: the monthly payment never changes (US$212.47 in this illustrative US$10,000 loan at 10% over 5 years)

interest principal (illustrative arithmetic, not a real loan)

Stretching the term lowers the payment but raises the total interest. The table uses the same illustrative loan, fixed rates and monthly payments; real loans add fees, and rates depend on the lender, the borrower and the date.

At the highest rate and longest term in the table, the interest comes to close to nine-tenths of the amount borrowed. Paying principal down faster cuts the interest, the CFPB notes for car loans, but payments generally go first to any fees, then to interest, and only then to principal. It suggests asking the lender or servicer whether extra money can go to principal.¹³

Minimum payments stretch the time the most

Paying only a card’s minimum stretches the time, because the minimum often shrinks as the balance shrinks. In Canada, the FCAC’s guidance (updated October 2025) says the minimum is typically a small flat amount plus interest and fees, or the higher of typically C$10 or 3 percent of the balance; since August 2025 it has been 5 percent for Quebec residents.¹

The FCAC’s example holds the payment at C$60. By our arithmetic, if the payment instead follows that C$10-or-3-percent minimum down, recalculated after each month’s interest, with no new purchases or fees, the same balance at the same rate takes about 162 months, well over a decade, and costs about C$1,750 in interest.

In the US, Regulation Z requires card statements to carry a bold “Minimum Payment Warning” saying that paying only the minimum means paying more interest and taking longer to pay off the balance.¹⁴ Issuers must also show how long the current balance would take to clear at the minimum, and the monthly payment that would clear it in 36 months, assuming no new purchases, the CFPB explains.¹⁵

In the UK, the Financial Conduct Authority (FCA) says you are in persistent debt if, over 18 months, you have paid more in interest, fees and charges than you have repaid of the balance, and your card provider must then contact you and offer help. If you are still in persistent debt at 36 months, the provider must offer ways to repay faster, and if you cannot afford that, it must take steps such as reducing, waiving or cancelling interest, fees or charges.¹⁶ When it announced the rules in 2018 the FCA said people in persistent debt paid on average about £2.50 in interest and charges for every £1 of borrowing repaid.¹⁷

The study

Moderate evidence

Who pays at the minimum: a quarter of the US card market (Keys and Wang, 2019)

Across a data set covering a quarter of the US general-purpose credit card market, 29 percent of accounts regularly paid at or near the minimum. When issuers changed their minimum-payment formulas, at least a fifth of those near-minimum payers, nearly one account in ten overall, shifted payments in a way liquidity constraints alone did not explain and that the authors attribute to anchoring on the minimum.¹⁸

On the authors’ reading, the minimum works as a suggestion as well as a floor for some borrowers. The caveats: the 2016 working-paper version, which had not been peer reviewed, puts the data at 2008 to 2013 for US cards, and the study infers anchoring from how payments moved rather than measuring it. In that working paper, fewer than 1 percent of accounts adopted the three-year payoff amount that US statements began showing after the 2009 CARD Act.¹⁹

What shortens the time on a debt

US, Canadian and Australian regulators point to a few levers: pay more than the minimum, clear card balances by the due date, choose an order for paying off several debts and talk to lenders early. The FCAC says even a small increase in the monthly payment shortens the payoff a lot.¹

Payoff order is debated: Australia’s Moneysmart presents the smallest debt first, for motivation, and the highest rate first as alternatives.¹¹ On arithmetic alone, with the same payments and no fees, the highest rate first costs the least interest, but in a 2012 study of a debt-settlement firm’s clients, David Gal and Blakeley McShane linked closing whole accounts, whatever their size, to getting out of debt (observational; we read the abstract only).²¹ We found no randomized field trial comparing these levers. A free debt adviser can talk through the choice. Making a budget from real statements shows how much can go to debt each month.

When debt gets hard to manage: free help, by urgency

Government and regulator sources in the US, the UK, Canada and Australia all point to free debt advice, and several urge acting early. We have sorted their advice by how soon to act.

• Urgent: if money worries come with thoughts of suicide or self-harm, in the US call or text 988, which is free, confidential and open at any hour.²² In a life-threatening situation in the US, call 911.²³ In the UK, call Samaritans on 116 123, or call 999 or go to A&E if a life is at risk or you cannot keep yourself safe; in England, the NHS also points to 111, mental health option, for urgent help.²⁴ Elsewhere, call your local emergency number.
• Next few weeks: if repayments crowd out essentials or a UK persistent-debt letter arrives, contact free help. In the US, the CFPB says credit counseling organizations are usually non-profits offering free or low-cost advice, but counselors may charge fees for some services, and the Federal Trade Commission has found that some organizations offering debt management plans have defrauded people; get a price quote in writing and check the organization with your state attorney general and consumer protection agency.²⁵ In the UK, the government’s debt guidance points to MoneyHelper for free debt advice services.²⁶ In Canada, the FCAC notes that both non-profit and for-profit firms offer credit counselling and that no reputable agency charges for a first meeting; licensed insolvency trustees typically assess your situation for free.²⁷ In Australia, the free National Debt Helpline is on 1800 007 007, weekdays; Moneysmart says the earlier you get help, the more options you will have, and that anyone facing legal action should get free legal advice straight away.²⁸ Elsewhere, look for free non-profit or government debt advice and your own financial regulator.
• Before your next big loan: compare APR with APR, and see a regulated adviser for decisions that depend on your own circumstances.²

The bottom line

A debt’s cost is set by three things, the principal, the rate and the time, and time is the easiest to misjudge: the same card balance cost C$793 in interest over nearly four years in the Canadian regulator’s example, and C$231 over 14 months with a bigger payment.¹ Compare loans by APR, read the minimum-payment box on a US card statement as a warning rather than a target, and if repayments are crowding out essentials, contact a free debt adviser early.