The Basic Formula for Savings Account Interest

The interest your bank pays you depends on three things: how much money you have in the account, what rate the bank offers, and how long the money sits there. The simplest way to calculate it is: Interest = Principal × Rate × Time.

Here is what each part means. Principal is the amount of money you deposited. Rate is the annual percentage yield (APY) the bank advertises—expressed as a decimal. Time is how long your money stays in the account, measured in years. If you leave $5,000 in an account earning 4.5% APY for one full year, you earn $5,000 × 0.045 × 1 = $225.

Most savings accounts use compound interest, which means the bank calculates interest on your original deposit plus any interest you have already earned. This happens on a schedule—daily, monthly, or quarterly—depending on what the bank offers. Compounding makes your money grow faster than simple interest, but the formula changes slightly to account for it.

Key Takeaways

  • Simple interest uses the formula Principal × Rate × Time, but most savings accounts compound interest instead, which pays you interest on your interest.
  • The compounding frequency—daily, monthly, or quarterly—matters because more frequent compounding means slightly more money in your account by year's end.
  • You can calculate compound interest using the formula A = P(1 + r/n)^(nt), where n is how many times per year interest compounds and t is the number of years.
  • Your bank statement or online account dashboard shows the interest you earned each month, so you do not have to calculate it yourself unless you want to verify the amount.
  • Comparing APY between banks tells you which account will earn you the most money, because APY already includes the effect of compounding.

How Compound Interest Works in Practice

When a bank compounds interest daily, it divides the annual rate by 365, calculates interest on your balance that day, and adds it to your account. The next day, it calculates interest on the new, slightly higher balance. By the end of the month, you have earned interest on your interest.

The formula for compound interest is: A = P(1 + r/n)^(nt). Here, A is your final amount, P is your principal, r is the annual rate as a decimal, n is how many times per year interest compounds, and t is the number of years. If you deposit $5,000 at 4.5% APY compounded daily for one year, the math looks like this: A = $5,000(1 + 0.045/365)^(365×1). That works out to about $5,230.38—roughly $5 more than simple interest would give you.

The difference grows larger over time and with bigger balances. Over five years, that same $5,000 at 4.5% compounded daily becomes about $6,272, compared to $6,125 with simple interest. Banks advertise APY instead of a plain interest rate precisely because APY shows you the real return after compounding is factored in.

Why APY Matters More Than the Base Rate

The base interest rate and the APY are not the same thing. The base rate is what the bank uses to calculate daily interest. The APY is what you actually earn after compounding happens all year long. A bank might advertise a 4.4% base rate but a 4.5% APY because of daily compounding.

When you compare savings accounts, always look at the APY, not the base rate. Two banks might offer similar base rates, but one compounds daily and the other compounds monthly. The daily-compounding account will earn you more money. The APY already includes the compounding effect, so comparing APYs between banks tells you which one will put the most money in your account.

Your bank is required to disclose the APY in writing before you open an account, usually in a document called a Truth in Savings disclosure or account terms. Online banks typically show the APY prominently on their website because they compete on rate, while traditional banks may bury it in the fine print.

Calculating Interest for Partial Years and Changing Balances

If you deposit money partway through the year or withdraw it before year's end, the calculation changes. Banks track your daily balance and compound interest based on how many days your money actually sat in the account. If you deposit $5,000 on July 1 and leave it there through December 31, that is roughly half a year, so you earn roughly half the annual interest.

When your balance changes—because you make deposits or withdrawals—the bank recalculates the daily interest on the new balance. This is why the interest you earn each month varies slightly. A month when you deposit extra money will earn more interest than a month when you withdraw. Your bank statement shows the exact interest earned each month, which already accounts for these changes.

If you want to estimate interest on a changing balance, you can calculate it month by month using the formula for each month's average balance, but this gets tedious. Most people simply check their statement or use their bank's online calculator, which does the work automatically.

Using Your Bank's Online Tools and Statements

You do not need to do the math yourself. Your bank's website or mobile app usually has a savings calculator where you enter your deposit amount, the APY, and how long you plan to keep the money. The calculator shows you how much you will earn. This is the fastest way to compare accounts or estimate your earnings.

Your monthly or quarterly statement also shows the interest you earned during that period. Look for a line item labeled "Interest Paid" or "Interest Earned." Add up these monthly amounts and you have your total interest for the year. This number is what actually went into your account, so it is the most reliable figure.

If you want to verify that your bank calculated interest correctly, you can use the compound interest formula with the APY shown on your statement. But most people find it easier to trust the statement and use the formula only when comparing different banks or planning how much to save.

The Difference Between APY and APR

APY and APR sound similar but mean different things. APY (annual percentage yield) is what you earn on money you deposit—it includes compounding. APR (annual percentage rate) is what you pay on money you borrow, like a credit card or loan. APR does not include compounding in the same way.

For savings accounts, always use APY. For loans and credit cards, always use APR. The two numbers are not interchangeable, and mixing them up will give you the wrong answer. Your bank will clearly label which one applies to your account.

What Affects How Much Interest You Earn

Three factors control your interest earnings: the APY the bank offers, how much money you deposit, and how long you leave it there. You control the last two. The APY changes based on what the Federal Reserve does with interest rates—when the Fed raises rates, banks usually raise APYs on savings accounts within weeks. When the Fed cuts rates, APYs fall.

You cannot control the Fed, but you can control where you bank. Online banks typically offer higher APYs than traditional banks because they have lower overhead costs. High-yield savings accounts at online banks often pay 4% to 5% APY, while brick-and-mortar banks might pay 0.01% to 0.5%. Over a year, the difference on a $10,000 deposit is hundreds of dollars.

Money market accounts and certificates of deposit (CDs) sometimes offer higher rates than regular savings accounts, but they come with restrictions—money market accounts may limit how many withdrawals you can make, and CDs lock your money away for a set term. A regular high-yield savings account gives you the best combination of rate and flexibility for most people.

Frequently Asked Questions

Do I have to pay taxes on savings account interest?

Yes. The interest you earn counts as taxable income. Your bank sends you a 1099-INT form each January showing how much interest you earned the previous year. You report this on your tax return. The amount is usually small unless you have a large balance or a very high APY.

What if my bank changes the APY after I open the account?

Banks can change APY at any time, usually with a few days' notice. They must notify you in writing before the change takes effect. If the rate drops, you can move your money to a different bank. If it rises, you benefit immediately. This is why it pays to shop around every few months—rates change constantly.

Is the interest I see on my statement the same as what the formula gives me?

It should be very close, but not exact. Banks use slightly different rounding methods and may calculate interest on your balance at a specific time each day rather than averaging it. The difference is usually a few cents. If your statement shows significantly less interest than your calculation, contact your bank to ask how they calculated it.

Can I earn interest on interest in a savings account?

Yes, that is what compounding does. The bank pays interest on your original deposit plus all the interest you have already earned. This is why compound interest grows faster than simple interest, especially over many years.

What is the best way to maximize savings account interest?

Deposit as much as you can, keep it there as long as possible, and choose an account with the highest APY available. Online banks and credit unions often offer better rates than traditional banks. Even a 1% difference in APY adds up to significant money over time on a large balance.