What Are Interest Rates? Types and What They Mean for Borrowers

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An interest rate is the price of borrowing money, or the return for lending it, expressed as a percentage of the principal over a set period, usually one year. A borrower who takes a loan at a 6% annual rate owes the lender 6 cents a year for every dollar outstanding. A saver who earns 4% on a deposit is, in effect, lending money to the bank and collecting 4 cents a year per dollar. The same number therefore measures a cost on one side of the transaction and a return on the other.

What Is an Interest Rate?

Interest is the dollar amount paid for the use of money. The interest rate is that amount expressed as a percentage of the principal, the sum borrowed or deposited. On a loan, excluding fees and other charges, the difference between what the borrower repays in total and the principal is interest.

Lenders charge interest because lending has a cost. Money handed to a borrower cannot be invested elsewhere during the loan period, which is the lender’s opportunity cost. The lender also takes the risk that the borrower will not repay, and that inflation will erode the value of the dollars that come back. The interest rate compensates for all three.

Interest rates apply to almost every borrowing and lending arrangement: home and car loans, tuition and credit cards for households, financing for business property and expansion, and government bonds. On the other side, banks and credit unions pay interest on savings accounts, money market accounts and certificates of deposit (CDs).

How Interest Rates Work

The rate on most loans and deposits is quoted on an annual basis. How that annual rate turns into dollars depends on whether interest is simple or compound and whether the principal is paid down over time.

Simple interest

Simple interest is calculated on the original principal only:

Simple interest = Principal × Annual rate × Time (in years)

Hypothetical example: a borrower takes a one-year, $300,000 loan at 4% simple interest. Interest is $300,000 × 0.04 × 1 = $12,000, so the borrower repays $312,000 at the end of the year.

If the same $300,000 stayed fully outstanding for 30 years at 4% simple interest, interest would be $300,000 × 0.04 × 30 = $360,000, or $12,000 a year. That figure assumes none of the principal is repaid until the end, which is how some bonds and interest-only loans work but not how a standard mortgage works.

Compound interest

Compound interest, often called interest on interest, is calculated on the principal plus all interest accumulated in earlier periods. At the end of the first year, the borrower owes principal plus one year of interest. In the second year, interest is charged on that larger amount, and so on.

Compound interest = P × [(1 + r)^n − 1]

where P is the principal, r is the rate per compounding period and n is the number of compounding periods.

Hypothetical example: a three-year, $10,000 loan at 5% compounded annually, with nothing paid until maturity:

$10,000 × [(1.05)^3 − 1] = $10,000 × (1.157625 − 1) = $1,576.25

Simple interest on the same loan would be $1,500. Over short periods the two methods give similar results. Over long periods the gap widens sharply. Applying annual compounding to the $300,000, 4%, 30-year case with no payments along the way gives $300,000 × [(1.04)^30 − 1], or about $673,019 in interest, compared with $360,000 under simple interest.

Compounding frequency also matters. Interest can compound annually, monthly or daily. The more often it compounds, the more interest accrues for the same quoted annual rate.

Amortizing loans: interest on the remaining balance

Most mortgages, auto loans and personal loans are amortizing. Each fixed monthly payment covers that month’s interest on the outstanding balance, and the rest reduces principal. Unpaid interest is generally not added to the balance as long as payments are made on time, so neither of the two calculations above describes what the borrower actually pays.

The monthly payment on an amortizing loan is:

Payment = P × i / [1 − (1 + i)^−N]

where i is the monthly rate (annual rate ÷ 12) and N is the number of monthly payments.

Hypothetical example: a $300,000, 30-year fixed-rate mortgage at 4%. The monthly rate is 0.04 ÷ 12 ≈ 0.003333, and N is 360. The payment works out to about $1,432 a month. Over 360 payments the borrower pays about $515,600, of which roughly $215,600 is interest. That is far less than the $360,000 simple-interest figure and the $673,019 compound figure, because the balance falls with every payment.

Early in the loan most of each payment goes to interest, since the balance is at its largest. Later, the interest share shrinks and more goes to principal. Extra principal payments reduce the balance on which future interest is charged, which is why prepaying an amortizing loan saves interest.

Interest Rate vs. APR vs. APY

Three figures are often confused because all are expressed as annual percentages.

Interest rate (note rate). The rate used to calculate interest on the principal. On a loan document, this is the rate that determines the payment.

APR (annual percentage rate). For consumer loans in the U.S., lenders disclose an APR that adds certain fees and costs, such as origination fees or mortgage points, to the interest rate and expresses the total as an annual rate. A mortgage might carry a 6.0% interest rate and a 6.3% APR because of upfront charges. APR is the better figure for comparing loan offers, but it generally does not reflect compounding.

APY (annual percentage yield). The figure banks and credit unions quote on savings accounts and CDs. It includes the effect of compounding within the year:

APY = (1 + r / n)^n − 1

where r is the stated annual rate and n is the number of compounding periods per year.

Hypothetical example: a deposit paying 5% compounded monthly has an APY of (1 + 0.05/12)^12 − 1 ≈ 5.12%.

Credit cards show why the distinction matters. Card rates are quoted as an APR, but interest is typically charged on a monthly or daily periodic rate. A 24% APR applied as 2% a month and left to compound would cost (1.02)^12 − 1 ≈ 26.8% over a year on a balance carried in full. The quoted APR understates the effective cost when interest compounds.

Types of Interest Rates

Beyond simple and compound interest, borrowers encounter several categories of rates.

Fixed rate. The rate stays the same for the life of the loan or for a defined period. Payments on a fixed-rate amortizing loan do not change, which makes budgeting predictable.

Variable (floating or adjustable) rate. The rate is tied to a benchmark index plus a margin and resets periodically. An adjustable-rate mortgage, a home equity line of credit or most credit cards can see payments rise or fall as the benchmark moves. Variable loans often start lower than comparable fixed loans, with the borrower taking on the risk of future increases.

Nominal vs. real rate. The nominal rate is the quoted rate. The real rate adjusts for inflation, approximately:

Real rate ≈ Nominal rate − Inflation rate

A savings account paying 4% while consumer prices rise 3% leaves the saver with roughly 1% more purchasing power. If inflation runs above the rate, the real return is negative even though the account balance grows. Inflation is usually measured with the Consumer Price Index.

Policy rate. The rate a central bank targets or sets. In the U.S., the Federal Reserve sets a target range for the federal funds rate, the rate at which banks lend reserves to each other overnight.

Base rate and prime rate. A base rate is the reference rate a lender builds loan pricing on. Most borrowers pay it plus a margin, so it is a floor for pricing rather than an average of what borrowers pay. In the U.S. the prime rate plays this role for many credit cards, lines of credit and business loans, and it has long moved with the federal funds target, conventionally about 3 percentage points above it.

Discount rate. The rate the Federal Reserve charges banks that borrow directly from it.

Coupon rate. The fixed interest rate a bond pays on its face value.

Deposit rates. The rates paid on savings accounts, money market accounts and CDs, typically quoted as APY.

How Interest Rates Are Determined

Economy-wide factors

Central bank policy anchors short-term rates. When the Federal Reserve raises its policy rate, banks’ funding costs rise, and rates on loans and deposits generally follow, though not one for one and not instantly. Banks also price loans using their own funding costs, the fees they charge and the terms of each product.

Inflation and inflation expectations push rates up because lenders want to be repaid in dollars that have not lost too much value. Not every rate rises automatically when inflation appears, but persistent inflation tends to lift market rates and often prompts central banks to raise policy rates.

Supply and demand for credit also matter. When many borrowers compete for a limited pool of savings, rates rise; when savings are plentiful and borrowing demand is weak, rates fall.

Borrower and loan factors

Credit risk. A loan considered low risk carries a lower rate; a high-risk loan carries a higher one. Lenders assess risk mainly through credit scores and credit history, along with income and existing debt, so borrowers with stronger credit profiles qualify for lower rates.

Collateral. Secured loans, such as mortgages and auto loans backed by the property or vehicle, usually cost less than unsecured debt like credit cards, because the lender can recover part of the loss by seizing the collateral.

Down payment and loan-to-value. A larger down payment means the lender has less money at risk relative to the asset’s value.

Term. Longer loans generally carry higher rates, as discussed below.

Fees and points. Some lenders offer a lower rate in exchange for upfront payments, or a higher rate with lower closing costs. This trade-off is one reason APR exists.

Why 30-year loans cost more than 15-year loans

A 30-year mortgage usually carries a higher rate than a 15-year mortgage. Over a longer term there is more time for the borrower’s finances to deteriorate, for inflation to erode the value of repayments and for market rates to move against the lender. The lender’s money is also tied up longer, raising the opportunity cost. Lenders price those added risks into the rate.

The longer term also means interest accrues on a larger balance for more years. A 15-year loan has higher monthly payments, but the combination of a lower rate and faster principal repayment typically means much less total interest.

How the Fed Uses Interest Rates

The Federal Reserve, like other central banks, uses interest rates as its main monetary policy tool. By raising the cost of overnight borrowing between banks, it influences rates throughout the economy, including those on personal loans, business loans and mortgages.

Higher rates make borrowing more expensive, which tends to slow spending financed by debt, such as home and car purchases and business investment. That cools demand in an overheating economy and puts downward pressure on inflation. If rates rise far enough, the slowdown can contribute to a contraction in output measured by GDP.

Lower rates do the opposite. Cheaper credit for qualified borrowers encourages spending and investment, including riskier investments, and can support economic expansion. Kept too low for too long, however, rates can feed excess demand and inflation, which is why central banks shift between easing and tightening as conditions change.

Interest Rates and Bond Prices

A bond typically pays a fixed coupon over its life, so its market price moves in the opposite direction from prevailing interest rates.

Hypothetical example: a bond with a $1,000 face value and a 5% coupon pays $50 a year. If new bonds pay 10%, or $100 a year per $1,000, the older bond’s price must fall until its yield is competitive. If rates drop to 1%, new bonds pay only $10 a year, the older bond’s $50 coupon becomes attractive, and buyers bid its price above face value.

The longer a bond has until maturity, the more its price reacts to a given change in rates, because the fixed coupon is locked in for more years. This is also why bond funds can lose value when rates rise, even though each bond still pays its scheduled interest.

Who Benefits From High or Low Rates

Whether high rates are good or bad depends on which side of the transaction someone is on.

• Borrowers generally prefer low rates. A company planning to finance expansion with debt, or a household shopping for a mortgage, pays less when rates are low.
• Savers and lenders earn more when rates are high, provided the rate exceeds inflation.
• Banks earn the spread between what they charge borrowers and what they pay depositors. Hypothetical example: a bank that pays depositors 5% and lends those funds out at 8% earns a 3-percentage-point spread before costs and loan losses. In effect, savers lend to the bank, and the bank lends to borrowers.
• Investors feel rate changes through asset prices. Rising rates lower the value of existing bonds and can weigh on stock valuations, since future earnings are discounted at a higher rate.

Limitations of the Quoted Rate

The headline interest rate is not the whole cost of borrowing.

Fees sit outside the rate. APR captures some fees but not all, and it assumes the loan is held to the end of its term. Refinancing or paying off early changes the effective cost of upfront fees.

Variable rates change. The rate on an adjustable loan is only the starting point. The terms that matter are the index it follows, the margin added, how often it resets and any caps on increases.

Personal rates differ from averages. Published average rates describe a typical borrower in a typical loan. An individual’s rate depends on credit score, collateral, term and fees, so averages are a reference point rather than a quote. Checking a credit report before applying, through Credit Karma or the credit bureaus, shows the information lenders will price from.

FAQs

The Bottom Line

Borrowers, savers and investors all feel interest rates, each from a different side. Borrowers should compare offers by APR, weigh fixed against variable terms and expect their own rate to depend on credit, collateral, term and fees. Savers should compare APY and check that it beats inflation, since only the real rate adds purchasing power.