Subjects and lessons to explore.

Law

Economics · Foundations

Interest Rates

9 min read 5 practice questions
9 key termsSourcesReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

An is the price of borrowing money and the reward for saving it, stated as a percent per year. It is set in the , where saving supplies funds and borrowing demands them, and central banks nudge short-term rates to steer the economy. The nominal rate is what you are quoted; the real rate subtracts inflation. And because a dollar later is worth less than a dollar now, interest rates are also how we compare money across time.

Why this matters

Interest rates set the cost of a mortgage, a car loan, and a credit-card balance, and the payoff on savings and bonds. They connect the present to the future: every choice to borrow, save, or invest turns on the rate. In macroeconomics the interest rate is the hinge between money, inflation, and the business cycle, which is why central-bank rate decisions dominate the financial news. Learning to separate nominal from real rates, and to discount future money to its , lets you judge whether a loan or a return is actually good once inflation and timing are accounted for, instead of being fooled by the headline number.

The college version

What an interest rate is

An interest rate is the price of using someone else's money for a period of time. To a borrower it is a cost: the extra you repay on top of the , the original sum borrowed. To a saver or lender it is a reward: the payment you receive for letting someone else use your funds and for waiting to spend. It is quoted as a percent of the principal per year, which lets you compare a $500 charge on a $10,000 loan (5 percent) with a $200 charge on a $2,000 loan (10 percent) on the same footing. The percent-per-year convention matters because interest compounds over time and because loans differ in size and length. Framed this way, the interest rate is simply a price like any other, and it responds to supply and demand in a market for funds.

Nominal versus real interest rates

The is the number you are quoted on a loan or a savings account. The adjusts that number for inflation, because inflation erodes the purchasing power of the dollars you are repaid. The working approximation, often called the after economist Irving Fisher, is that the real rate roughly equals the nominal rate minus expected inflation. If your savings account pays 4 percent while prices rise 5 percent, your real return is about negative 1 percent: you end the year with more dollars but slightly less buying power. This is why expected inflation gets built into nominal rates. It also explains a redistribution: when a loan's rate is locked in and inflation turns out higher than expected, lenders lose because they are repaid in cheaper dollars, and borrowers gain. The subtraction is an approximation; the exact relationship divides rather than subtracts, so at 6 percent nominal and 4 percent inflation the real rate is about 1.9 percent, close to the quick estimate of 2 percent.

Where market interest rates come from

In the market for loanable funds, savers are the suppliers and borrowers are the demanders. Households and firms that save (make financial investments) supply funds; households, firms, and governments that want to spend more than they currently have demand funds. The equilibrium interest rate is the price at which the quantity of funds supplied equals the quantity demanded. If the rate sits above equilibrium, savers offer more than borrowers want and the surplus pushes the rate down; below equilibrium, borrowers want more than savers offer and the shortage pushes it up. On top of this market, a central bank influences short-term rates. In the United States the Federal Reserve sets a target range for the , and a change there is quickly reflected in other short-term borrowing costs and feeds into longer-term rates. The Fed's own tools and decisions belong to the monetary-policy topic; here the point is only that policy is one force acting on the price of funds alongside private saving and borrowing.

Why different loans carry different rates

There is no single interest rate but a whole structure of them, and the differences are systematic. Default risk is central: lenders weigh the return against the chance of not being repaid, so a borrower more likely to default must offer a higher rate to compensate, a gap called the . Term, or maturity, matters because tying money up longer exposes the lender to more uncertainty, so long loans often carry different rates than short ones. Inflation expectations are folded in: if lenders expect high inflation over the life of a loan, they demand a higher nominal rate to protect their real return. Liquidity plays a role too, since an asset that is hard to sell quickly usually must pay more to attract lenders. This is why a government bond, a mortgage, and a credit card carry very different rates at the same moment: they differ in risk, length, and how easily the lender can get their money back.

Present value: comparing money across time

Because interest can be earned, a dollar today is worth more than a dollar in the future: today's dollar can be lent out and grow. Present value, or present discounted value, runs that logic backward to ask what a future payment is worth now. The formula divides the future amount by (1 + interest rate) raised to the number of years. At a 10 percent rate, $110 a year from now is worth exactly $100 today, because $100 invested at 10 percent grows to $110. Two consequences follow. First, money further in the future is discounted more heavily. Second, a higher interest rate lowers present value: a two-year bond paying $240 then $3,240 is worth $3,000 when discounted at 8 percent but only about $2,846 at 11 percent, even though the promised payments never changed. Present value is the tool for comparing costs and benefits that arrive at different times, which is why it underlies bond pricing, loan decisions, and investment appraisal.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Money has a rental price. If you borrow money, you pay rent on it; if you lend or save money, you collect rent. That rent, measured as cents per dollar per year, is the interest rate. When lots of people want to borrow and few want to lend, the rent goes up; when savers are plentiful and borrowers scarce, it goes down. There is also a catch called inflation: if prices climb 5 cents on the dollar while your savings earn 4 cents, you actually fell a little behind. And money you get later is worth less than money now, because money now could be earning rent in the meantime.

Picture it like this

An interest rate is the rental price of money, just like a daily rate to rent a bike. Borrowers are renters paying to use it; savers are the owners collecting the fee for lending it out and waiting.

Where the picture stops working

A rented bike comes back as the same bike, but inflation means the dollars a lender gets back can be worth less than the dollars they lent, so the real return can differ from the sticker rate. The bike analogy also hides risk: unlike a bike-rental fee, part of a loan's interest is compensation for the chance the borrower never returns the money at all.

Worked example

Suppose a savings account pays a 6 percent nominal rate this year and inflation is 4 percent. The real return, by the Fisher approximation, is about 6 minus 4 equals 2 percent, so your purchasing power grows roughly 2 percent (the exact figure is 1.9 percent). Now use present value to compare money across time at a 5 percent market rate: a promised $1,000 one year from now is worth $1,000 divided by 1.05, or $952.38, today; the same $1,000 three years out is worth $1,000 divided by 1.05 cubed, or $863.84. The further off the payment and the higher the rate, the less it is worth now, which is exactly why rising interest rates push bond prices down.

Key takeaway

The interest rate is the price of money over time, set where saving meets borrowing and nudged by the central bank. Always read it in real terms (nominal minus inflation) and remember that a dollar later is worth less than a dollar now.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

In the market for loanable funds, which groups are on the supply side and the demand side, and what does the equilibrium interest rate do?

Choose an answer, then check it.
Question 2 of 3intermediate

A one-year savings account pays a nominal interest rate of 3 percent, and inflation over the year turns out to be 5 percent. Using the Fisher approximation, what is the approximate real interest rate?

Choose an answer, then check it.
Question 3 of 3advanced

Two borrowers take one-year loans on the same day, but one pays a noticeably higher interest rate than the other. Which factor best explains a higher rate for that borrower?

Choose an answer, then check it.
Practice all 5

Keep learning

Ready to build on this? Continue to the next lesson.

Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Define the interest rate as the price of borrowing and the reward for saving, expressed as a percent per year
  • Distinguish nominal from real interest rates and apply the Fisher relationship (real approximately equals nominal minus expected inflation)
  • Explain how the equilibrium interest rate is set in the market for loanable funds and how a central bank influences short-term rates
  • Explain why different loans carry different rates in terms of default risk, term, inflation expectations, and liquidity
  • Apply present value to show why a dollar received later is worth less than a dollar now

Common mistakes

  • Treating the quoted (nominal) rate as your true gain.

    Subtract inflation to get the real rate. Earning 4 percent while prices rise 5 percent is a real loss of about 1 percent in purchasing power.

  • Assuming there is one interest rate for everything.

    Rates form a structure. Default risk, term, inflation expectations, and liquidity make a credit card, a mortgage, and a Treasury bond carry very different rates at the same time.

  • Thinking a dollar promised in the future equals a dollar today.

    Discount it. A future dollar is worth less now because today's dollar could earn interest in the meantime; present value quantifies the gap.

  • Believing higher interest rates always mean savers are unambiguously better off.

    What matters is the real rate. High nominal rates driven by high inflation can still leave a saver's purchasing power flat or falling.

  • Confusing the central bank's policy rate with all interest rates.

    The Fed targets a short-term rate; mortgage, corporate, and long-term rates are influenced by it but also reflect risk, term, and market expectations.

Easily confused

Nominal interest rate vs. Real interest rate

Nominal is the quoted number; real subtracts inflation to show the actual change in purchasing power.

The interest rate a saver earns vs. The interest rate a borrower pays

They are two sides of the same price of funds; the saver receives it as a reward, the borrower pays it as a cost, and lender-side risk and costs put a wedge between them.

Future value vs. Present value

Future value grows money forward with interest; present value discounts future money back to what it is worth today.

Key vocabulary

Interest rate
The price of borrowing money and the reward for saving or lending it, expressed as a percent of the principal per year.
Principal
The original amount of money borrowed or saved, on which interest is calculated.
Nominal interest rate
The stated, unadjusted interest rate on a loan or account, before accounting for inflation.
Real interest rate
The interest rate adjusted for inflation; it approximately equals the nominal rate minus expected inflation and measures the change in purchasing power.
Fisher relationship
The approximation that the real interest rate equals the nominal interest rate minus expected inflation, named after economist Irving Fisher.
Market for loanable funds
The market in which savers supply funds and borrowers demand them, and whose equilibrium price is the interest rate.
Federal funds rate
The short-term interest rate the U.S. Federal Reserve targets to influence broader borrowing costs; set as a range.
Risk premium
The extra interest a lender charges to compensate for the chance that a borrower defaults.
Present value
What a future payment is worth today, found by dividing the future amount by (1 + interest rate) raised to the number of years; also called present discounted value.

Sources & references

  1. Principles of Macroeconomics 3e, Section 4.2: Demand and Supply in Financial Markets — OpenStax (Rice University)
  2. Principles of Economics 3e, Appendix C: Present Discounted Value — OpenStax (Rice University)
  3. Principles of Economics 2e, Section 22.4: The Confusion Over Inflation — OpenStax (Rice University)
  4. Monetary Policy: What Are Its Goals? How Does It Work? — Board of Governors of the Federal Reserve System

EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.

Researched 2026-08-19

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.