Interest rates are the price of money — the cost of borrowing or the reward for saving/investing. They are typically expressed as an annual percentage and are the most fundamental variable in all of finance. Every financial product, from savings accounts to mortgages to bonds, is built around interest rates. Understanding how interest rates are set, how they move, and how they affect different financial products is essential for making sound borrowing, saving, and investment decisions.
Central banks set base rates (the federal funds rate in the US, the Bank Rate in the UK, the ECB deposit rate in Europe) that influence all other rates in the economy. When the Fed raises rates, borrowing becomes more expensive (higher mortgage and loan rates) but saving becomes more rewarding (higher savings account yields). The Fed uses this mechanism to manage the economy: raising rates to cool inflation (by reducing borrowing and spending) and lowering rates to stimulate a slow economy (by making borrowing cheaper and investment more attractive). The 2022-2023 rate hiking cycle raised the fed funds rate from near 0% to 5.25-5.5% — the fastest pace since the 1980s — in response to 40-year high inflation.
Interest rates come in several essential distinctions: nominal (stated rate), real (adjusted for inflation: Real rate ≈ Nominal rate − Inflation rate), fixed (stays constant for the loan/investment term), variable/adjustable (changes with a benchmark index like SOFR), APR (annual percentage rate — includes fees for loans), and APY (annual percentage yield — accounts for compounding for savings). The real interest rate is the most economically meaningful — a 5% nominal rate with 4% inflation is a 1% real rate; a 5% nominal rate with 1% inflation is a 4% real rate. Borrowers prefer high inflation (it erodes the real burden of debt); savers prefer low inflation (it preserves the real value of returns).
The yield curve and what it signals about economic expectations. The yield curve plots interest rates for bonds of the same credit quality but different maturities — typically from 3-month Treasury bills to 30-year Treasury bonds. Normally, the curve slopes upward (longer maturities pay higher rates to compensate for longer exposure). An inverted yield curve (short-term rates higher than long-term rates) has preceded every US recession since 1955 and is considered the most reliable leading recession indicator available. An inverted curve signals that markets expect the Fed to cut rates in the future — typically because a recession is anticipated.
The difference between APR and APY is crucial for comparing financial products. APR (Annual Percentage Rate) is the simple annual rate used for quoting loans and credit cards. APY (Annual Percentage Yield) accounts for the effect of compounding within the year. For savings products: a savings account paying 0.50% monthly (or 6% APR) has an APY of (1 + 0.06/12)^12 − 1 = 6.17%. The more frequently interest compounds, the higher the effective APY vs. the stated APR. For borrowing: focus on APR (which includes fees under TILA regulations for loans). For saving: focus on APY (which shows what you actually earn annually). Marketing materials often use APR for borrowing products and APY for savings products to make both look more attractive.
How interest rates affect bond prices — the inverse relationship. One of the most misunderstood relationships in finance: when interest rates rise, existing bond prices fall, and vice versa. A bond paying 3% interest loses market value when new bonds are issued at 5% — no one will pay full price for a 3% bond when they can buy 5% bonds. The magnitude of this price impact is measured by duration: a bond with 10-year duration loses approximately 10% in value for each 1% rise in interest rates. This is why bond funds can lose money even though bonds are "safe" — in 2022, the total US bond market (AGG) fell -13% as interest rates surged. Shorter duration bonds (and bond funds) are less sensitive to rate changes.
Interest rates and stock valuations: the discounting effect. Interest rates affect stock prices through multiple channels. Most directly: higher rates increase the discount rate applied to future earnings in discounted cash flow (DCF) models — the present value of a company's future earnings falls when interest rates rise, reducing fair value estimates. This explains why growth stocks (whose value is concentrated in distant future earnings) are more sensitive to rate changes than value stocks (current earnings dominate valuation). In 2022, many speculative tech/growth stocks fell 60-80% even while their businesses remained intact — the primary driver was rate-driven multiple compression from near-0% to 5%+ discount rates. Higher rates also make bonds more competitive with stocks for income, reducing the relative attractiveness of equity dividends.