How Interest Works on a Savings Account

The Basic Mechanics: What Interest Actually Is

When you deposit money into a savings account, the bank doesn't simply store it in a vault. It lends that money to other customers and businesses as mortgages, car loans, and lines of credit. As payment for the use of your funds, the bank credits your account with interest — a percentage of your deposited balance.

The starting point is the nominal interest rate (sometimes called the stated rate), expressed as an annual percentage. If a bank advertises a 4% annual rate and you deposit $1,000, the simple calculation would suggest you'd earn $40 over the year. In practice, however, the story is a bit richer because of how and when the bank applies that rate — which brings us to compounding.

For a broader look at how savings accounts fit alongside other account types, see our guide to bank account types.

Simple vs. Compound Interest: The Difference That Matters

Simple interest is calculated only on your original deposit (the principal). If you deposited $1,000 at a 4% simple annual rate, you'd earn exactly $40 every year — no more, no less, regardless of how long you keep the account open.

Compound interest, by contrast, is calculated on your principal plus any interest already credited to your account. So in year one you earn $40; in year two, your balance is $1,040 and interest is calculated on that higher figure — yielding $41.60. The difference feels small at first, but it accelerates meaningfully over years and decades.

Virtually all US savings accounts use compound interest, not simple interest. The key variable is compounding frequency — how often the bank adds earned interest to your balance. Common frequencies include:

• Daily — interest calculated every day and added to your balance
• Monthly — interest calculated and credited once per month
• Quarterly — credited four times per year

Daily compounding produces the highest effective return for a given stated rate. Compound interest's full long-term power becomes especially evident over multi-decade time horizons.

“Compound interest is the foundation of long-term wealth accumulation — the earlier you understand it, the more time it has to work in your favor.”

— Consumer Financial Protection Bureau, U.S. federal agency for consumer financial education

APY: The Number You Should Actually Compare

Because compounding frequency affects your real earnings, a single interest rate isn't enough to compare accounts fairly. That's why US banks are required by the Truth in Savings Act to disclose the Annual Percentage Yield (APY) — a standardized figure that bakes in the effect of compounding over one year.

APY is calculated as:

APY = (1 + r/n)^n − 1

Where r is the annual interest rate and n is the number of compounding periods per year. A 4% rate compounded daily produces an APY of approximately 4.08%, while the same rate compounded monthly produces roughly 4.07%. The gap narrows as compounding periods increase, but the concept is the same: APY always equals or exceeds the stated rate.

When comparing savings accounts, use APY — not the nominal rate — as your benchmark. Note that APY differs conceptually from APR, which measures borrowing costs. Our article on APR vs. interest rate explains that distinction in detail.

What Influences the Rate Your Bank Offers

Savings account rates are not fixed by law — banks set their own rates within a market shaped by several forces:

• Federal Reserve policy: When the Fed raises the federal funds rate, banks generally pass some of that increase on to depositors (and vice versa when rates fall).
• Bank type: Online-only banks typically offer higher APYs than traditional branches because they operate with lower overhead costs.
• Account structure: Some accounts use tiered rates — paying a higher APY on balances above a certain threshold.
• Promotional rates: Introductory APYs may be temporarily elevated; always check what the rate reverts to after any promotional period ends.

If earning competitive interest is a priority, you may also want to explore how savings accounts compare to money market accounts, which can sometimes offer higher rates in exchange for different features or requirements.

This article is for general informational and educational purposes only. It does not constitute personalized financial, tax, or legal advice. For guidance specific to your situation, consult a qualified financial adviser or tax professional.