What Is Interest? Definition and Types

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Interest is the price of using someone else’s money for a period of time. A borrower pays it to a lender on top of the amount borrowed (the principal), and a saver or investor receives it when a bank, government or company holds their funds. Interest is usually stated as a percentage rate per year, while the interest itself is the dollar amount that rate produces on a given balance over a given time.

What Is Interest?

Interest has two sides that describe the same payment. For a borrower, it is an expense: the cost of having money now instead of later. For a lender, it is revenue: the reward for parting with money temporarily. A saver who deposits money is the lender; the bank uses deposits to fund loans and pays interest for the use of the balance. A cardholder carrying a balance is the borrower, and the issuer collects interest.

A rate means little without the balance and the time period it applies to. A 24% annual rate on a $100 balance for one month produces about $2 of interest; the same rate on $10,000 for a year produces $2,400.

In business and law, an ownership interest (or equity interest) is a share of ownership in a company or asset, and a “controlling interest” means owning enough shares to direct decisions. Neither has anything to do with borrowing costs, which are the subject of this article.

How Interest Works

Why lenders charge interest

A dollar available today is worth more than a dollar promised a year from now: it can be spent, invested or kept as a cushion, and the promised dollar may not arrive at all. The rate on a given loan reflects several pressures at once:

• Opportunity cost. Money lent to one borrower cannot be lent to another or invested elsewhere.
• Credit risk. Other things being equal, a borrower more likely to miss payments is charged a higher rate. This is why credit history and credit scores feed into the rates lenders offer.
• Term. Longer commitments tie up money for longer and expose the lender to more uncertainty about future rates and inflation.
• Collateral. A loan secured by a house or car gives the lender something to recover on default, which tends to lower the rate relative to unsecured debt such as credit cards. For mortgages, lenders also weigh the loan-to-value ratio.
• Inflation expectations. If prices are expected to rise, lenders need a higher nominal rate to come out ahead in real terms.
• The policy rate. Central banks influence the short-term rates banks charge each other, and that baseline passes through to many consumer and business rates.

Calculating interest

In its simplest form, interest is:

Interest = Principal × Rate × Time

Here the principal is the outstanding balance, the rate is expressed as a decimal per period (6% per year is 0.06), and time is measured in the same unit as the rate.

Because rates are usually quoted annually while interest is often charged monthly or daily, the annual rate is commonly divided into a periodic rate:

• Monthly periodic rate = annual rate ÷ 12. A 6% annual rate becomes 0.5% per month.
• Daily periodic rate = annual rate ÷ 365 (some lenders use 360). A 24% annual rate becomes about 0.0658% per day.

The periodic rate is then applied to the balance the agreement specifies: the outstanding principal on an installment loan, which shrinks as you pay, or the daily or average balance on a savings account, which grows with deposits and credited interest. Either way, the dollar amount of interest usually changes from period to period even when the rate stays the same.

Simple interest vs. compound interest

Simple interest is charged on the original principal only. It does not grow on itself.

Compound interest is charged on the principal plus any interest already added to the balance. Each period’s interest becomes part of the base for the next period. The general formula for the ending balance is:

A = P × (1 + r/n)^(n × t)

where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year and t is the number of years. Interest earned (or owed) is A minus P.

For savers, compounding means credited interest earns interest of its own. For borrowers, it works against them when unpaid interest is added to the balance. When accrued interest is formally rolled into the principal, it is called capitalized interest, and future interest is charged on the larger amount. This is common on some student loans after deferment or forbearance, and it can raise the monthly payment or extend the payoff time.

Fixed vs. variable rates

A fixed rate stays the same for a set period or the life of the loan, so the interest calculation is predictable. A variable (or adjustable, or floating) rate is tied to a benchmark index plus a margin and resets when the index moves. Many credit cards and home equity lines of credit carry variable rates, and adjustable-rate mortgages typically have a fixed introductory period followed by periodic resets. With a variable rate, the same balance can cost noticeably more after the benchmark rises.

Interpreting interest rates

The headline rate can be quoted in several ways, and the differences matter:

• Nominal rate. The stated annual rate before compounding is taken into account.
• APR (annual percentage rate). For loans, APR expresses the yearly cost of borrowing and can include certain fees, such as origination fees or discount points, in addition to the interest rate. It generally does not reflect intra-year compounding.
• APY (annual percentage yield) and effective annual rate. These include the effect of compounding within the year. A 6% nominal rate compounded monthly produces an APY of about 6.17%, because (1 + 0.06/12)^12 − 1 ≈ 0.0617. Deposit accounts in the U.S. typically advertise APY, which makes accounts with different compounding schedules comparable.
• Real rate. The nominal rate minus inflation, approximately. A savings account paying 5% when inflation runs at 3% leaves a real return of about 2% (precisely, 1.05 ÷ 1.03 − 1 ≈ 1.94%). If inflation exceeds the interest rate, the balance grows in dollars but loses purchasing power.

When comparing two loans, compare APRs on the same basis and term. When comparing two savings products, compare APYs. Mixing a loan’s interest rate with another loan’s APR, or one account’s nominal rate with another’s APY, produces a misleading comparison.

Types of Interest

Most interest terms describe either how interest is calculated or what stage it is at. The common ones:

• Simple, compound, fixed, variable and capitalized interest: covered above. Compounding can run daily, monthly, quarterly or annually.
• Accrued interest: interest earned or incurred but not yet paid. A borrower’s accrued interest is a liability (often recorded as interest payable on a company’s balance sheet); a lender’s is an asset (interest receivable). Bond buyers pay accrued interest to the seller when they buy between coupon dates.
• Prepaid interest: interest paid in advance, such as the interest mortgage borrowers often pay at closing to cover the days until the first full payment period.
• Penalty or default interest: a higher rate triggered by late payment or breach of the agreement, such as a penalty APR on a credit card or interest on overdue invoices or unpaid taxes.

Common Applications of Interest

Interest appears wherever money changes hands over time:

• Credit cards. Balances carried past the grace period accrue interest, at APRs generally higher than on secured loans because the debt has no collateral. Paying only the minimum leaves most of the balance in place, so interest keeps building.
• Mortgages. Home loans can run for 30 years, so total interest over the life of the loan can be large relative to the purchase price. Payments on a standard fixed-rate mortgage stay level, but the share going to interest falls over time as the principal is paid down.
• Auto loans. Shorter-term installment loans, commonly at fixed rates, from a bank, a credit union or the dealer’s own lender.
• Student loans. Interest may accrue while the borrower is in school or in deferment and may later be capitalized. During the COVID-19 pandemic, U.S. federal student loan payments were paused and the interest rate on eligible federal loans was set to 0% for the duration of the pause.
• Personal loans and lines of credit. Installment or revolving borrowing priced largely on the borrower’s credit profile.
• Savings accounts, money market accounts and certificates of deposit (CDs). Here the account holder earns interest. Credit unions often call the payment a dividend, but it functions as interest on deposits. CDs typically pay a fixed rate for leaving the money in place for a set term, often with a penalty for early withdrawal.
• Bonds and Treasury securities. The issuer pays interest, called a coupon, to bondholders, and repays the principal at maturity.
• Business invoices and taxes. Some businesses charge interest on overdue invoices, and tax authorities charge interest on unpaid balances.

Example of Interest

The following examples are hypothetical and use round numbers to show the mechanics. They are not offers or typical market rates.

Example 1: Simple vs. compound interest on $5,000 at 6% for three years

• Simple interest: $5,000 × 0.06 × 3 = $900 of interest. Ending balance: $5,900.
• Compounded annually: $5,000 × (1.06)³ ≈ $5,955.08, so $955.08 of interest.
• Compounded monthly: $5,000 × (1 + 0.06/12)^36 ≈ $5,983.40, so $983.40 of interest.

The gap between $900 and $983.40 is entirely interest earned on earlier interest. It widens with higher rates, longer periods and more frequent compounding.

Example 2: The first two payments on an auto loan

Assume a $20,000 loan at a 7.2% fixed annual rate, repaid in 60 equal monthly payments, with no fees. The monthly rate is 7.2% ÷ 12 = 0.6%, and the standard amortization formula gives a payment of about $397.91.

• Month 1: interest = $20,000 × 0.006 = $120.00. Principal repaid = $397.91 − $120.00 = $277.91. New balance: $19,722.09.
• Month 2: interest = $19,722.09 × 0.006 ≈ $118.33. Principal repaid ≈ $279.58.

The payment stays the same, but each month a little less goes to interest and a little more to principal. Over all 60 payments the borrower pays about $23,874.60, of which roughly $3,874.60 is interest. On amortizing loans like this, each payment typically covers the interest due for the period before reducing principal, which is why early payments do less to shrink the balance.

Example 3: Credit card interest for one billing cycle

Assume a card at 24% APR that charges interest daily on an average daily balance of $2,000, with no new purchases or payments during a 30-day cycle and no compounding within the cycle. The daily periodic rate is 24% ÷ 365 ≈ 0.0658%, so interest for the cycle is about $2,000 × 0.24 × 30 ÷ 365 ≈ $39.45. If that interest is added to the balance and not paid off, next cycle’s interest is calculated on roughly $2,039.45.

The rule of 72

For a quick estimate of how long a compounding balance takes to double, divide 72 by the annual interest rate in percent. At 6%, money doubles in about 72 ÷ 6 = 12 years; the precise figure with annual compounding is about 11.9 years. The rule is an approximation that works best for rates in the single digits, and it applies to debt as well: an unpaid balance compounding at 24% roughly doubles in three years.

Why Interest Matters

Interest moves money from people who do not need it right now to people who can put it to use, and it lets borrowers weigh the cost of buying a house, starting a business or financing equipment now against waiting.

For households, interest determines the true cost of large purchases and the growth of savings. For businesses, it sets the hurdle an investment must clear: a project expected to return less than the cost of borrowing to fund it destroys value. For banks, the difference between interest earned on loans and interest paid on deposits (the net interest margin) is a core source of revenue.

Paying Interest: Advantages and Disadvantages

Potential advantages for borrowers

• Access to money for large purchases, emergencies or opportunities that cannot wait.
• A record of on-time payments, which builds credit history and can lead to better rates later.
• Leverage: if borrowed money earns more than its interest cost, the borrower keeps the difference, as a developer does when rents return more than the loan rate.

Disadvantages for borrowers

• Interest is a real cash expense and is contractually owed regardless of how the purchase turns out.
• Compounding and capitalization can make debt grow faster than the borrower pays it down, especially at high rates.
• Heavy existing payments can limit the ability to qualify for new credit.
• Leverage works in both directions: if the investment returns less than the interest rate, losses are magnified.

Collecting Interest: Advantages and Disadvantages

Potential advantages for lenders and savers

• Regular, relatively passive cash flow on a set schedule, predictable when the rate is fixed and the borrower pays reliably.
• A use for capital that would otherwise sit unused, particularly money needed again at a known future date.

Disadvantages for lenders and savers

• Taxes. In the U.S., interest income is generally taxed as ordinary income at the rate of the bracket it falls into, with exceptions such as interest on many municipal bonds, which can be exempt from federal income tax.
• Default risk. A borrower who does not repay may leave the lender with little or no recourse. Deposits at FDIC-insured banks are protected up to the federal insurance limit; loans to individuals or companies have no such guarantee.
• Inflation risk. A fixed rate below the inflation rate means losing purchasing power.
• Opportunity cost. Money earning a fixed rate might have earned more elsewhere, though with different risks.
• Ethical and social scrutiny. High rates charged to borrowers in difficult circumstances draw criticism, as in debates over student loan rates, payday lending and credit card pricing.

A Brief History of Interest

Lending at interest is ancient, but for most of history many societies, from the ancient Mediterranean and Middle East to medieval Europe, condemned it as usury, which originally meant any interest on a loan, not only excessive interest. Aristotle held that money itself produces nothing, so making money from money was unnatural, and medieval Christian scholastic thought and Islamic jurisprudence treated lending at interest to people in need as sinful.

Attitudes shifted from the Renaissance onward as commerce expanded and merchants borrowed to finance trade. In the 18th and 19th centuries, economists including Adam Smith, Frédéric Bastiat and Carl Menger argued that interest is legitimate compensation for risk and for the lender’s sacrifice of present use of money.

Islamic law still prohibits riba, usually translated as interest or usury. Islamic banking instead uses profit and loss sharing, leasing or cost-plus sales, so the financier’s return comes from a real transaction rather than a fixed charge on money lent.

Many countries today regulate the cost of credit in other ways, such as disclosure rules and caps on certain rates under state usury laws in the U.S., rather than banning interest outright.

Interest and the Economy

Central banks use interest rates as their main tool for steering the economy. In the U.S., the Federal Reserve sets a target range for the federal funds rate, the rate banks charge each other for overnight loans. Changes in that target ripple outward into the prime rate, credit card APRs, adjustable-rate loans, deposit rates and, less directly, mortgage rates and bond yields.

• Lower rates make borrowing cheaper for home buyers, businesses and consumers. The trade-off is lower returns on savings and the risk that households and companies take on debt that becomes harder to carry when rates rise again.
• Higher rates make borrowing more expensive and saving more rewarding. Central banks raise rates to cool spending and bring down inflation, at the cost of slower growth.

The Federal Reserve cut its target to near zero in March 2020 in response to the COVID-19 pandemic, and deposit rates at many banks fell close to zero. As inflation rose after the pandemic, the Fed raised rates rapidly, making mortgages, car loans and card balances more expensive while savings yields climbed. In September 2024, with inflation easing and hiring slowing, it began cutting again.

Interest rates also affect asset prices. When rates rise, the fixed payments on existing bonds become less attractive and their prices fall, and future company earnings are discounted more heavily, which can weigh on stock indexes such as the S&P 500.

Limitations of Interest Rates as a Measure

• It may exclude fees. A loan with a low rate and high origination fees can cost more than one with a slightly higher rate and no fees. APR captures some of these costs, but not every charge, and it assumes the loan runs its full term.
• Variable rates are not promises. Today’s rate on a variable product says nothing about the rate after the next reset.
• Introductory rates expire. Promotional rates on cards and some loans revert to a higher standard rate after a set period, and some deferred-interest promotions charge back interest if the balance is not paid in full by the deadline.
• It does not measure risk. A high rate often reflects a higher risk of not being repaid, and the rate itself offers no protection.

Interest vs. Related Terms

• Interest vs. principal: principal is the amount borrowed or deposited; interest is the charge or income on top of it. A loan payment usually contains both.
• Interest vs. dividends: interest is a contractual payment on debt; dividends on stock are discretionary distributions of company profits.
• Interest vs. fees: fees are flat or one-time charges, such as late fees or annual fees, that do not depend on the balance over time, although some disclosures fold them into APR.

FAQs

The Bottom Line

Interest affects anyone borrowing on a credit card, mortgage, auto or student loan, and anyone saving in a bank account, CD or bond. The practical rule is to compare loans by APR on the same term and savings products by APY, then check fees, variable rate resets and inflation before judging the true cost or return. An unpaid balance compounding at 24% roughly doubles in three years, so compounding terms matter as much as the headline rate.