The basic formula: multiply your balance by the rate and the time

To figure out how much interest you'll earn, you need three pieces of information: your account balance, the annual interest rate (shown as APY or APR), and how long the money sits in the account. The simplest version is: Balance × Annual Rate ÷ 365 × Number of Days = Interest Earned.

Most savings accounts use daily compounding, which means the bank calculates interest on your balance each day, then adds that interest back into your account. The next day, you earn interest on the new, slightly larger balance. This is why the exact calculation matters — it's not just straightforward multiplication.

Banks are required to disclose the APY (Annual Percentage Yield) in writing before you open an account. This number already includes the effect of compounding, so it's more accurate than the base interest rate alone. You'll find it on the account agreement, the bank's website, or in the disclosure documents they send you.

Key Takeaways

  • The annual percentage yield (APY) is the number you need — it's what the bank must disclose and it already accounts for compounding.
  • To estimate monthly interest, divide the APY by 12, then multiply by your balance: (APY ÷ 12) × Balance = Approximate Monthly Interest.
  • Daily compounding means you earn interest on your interest, so the longer money stays in the account, the more you gain from compounding.
  • Your actual interest will vary slightly from month to month because it depends on your exact balance each day and the number of days in that month.

How daily compounding actually works

When a bank compounds daily, it divides the annual rate by 365, applies that tiny daily rate to your current balance, and adds the result back to your account. Tomorrow, the daily rate applies to that new balance — which is slightly larger because of yesterday's interest.

Over a year, this compounding effect adds real money. A $10,000 balance at 4.50% APY earns roughly $450 in interest if it stays untouched for 12 months. But that $450 is not split evenly across the months — you earn less in month one (because the balance is smaller) and more in month twelve (because compounding has been working the whole time).

The bank's system does this calculation automatically. You don't have to. But understanding that it happens helps explain why your monthly interest statements show slightly different amounts each month, and why leaving money in longer always produces more interest than withdrawing and redepositing.

Calculating interest for a specific time period

If you want to know how much interest you'll earn over three months, or six months, the math is slightly different. Banks use this formula: Balance × (APY ÷ 365) × Number of Days = Interest. The number of days matters because February has 28 (or 29), while other months have 30 or 31.

For a rough estimate without counting exact days, you can divide the APY by 12 to get the monthly rate, then multiply by your balance and the number of months. A $5,000 balance at 4.50% APY for six months would be roughly: ($5,000 × 0.045 ÷ 12) × 6 = $112.50. The actual amount will be slightly different because of the exact number of days and daily compounding, but this gets you close.

If your balance changes during the period — you deposit more money or withdraw some — the calculation becomes more complex. The bank tracks your daily balance and compounds on that, so some days earn interest on $5,000 and other days on $6,000. Your statement will show the exact total, but you can't calculate it yourself without knowing every deposit and withdrawal date.

Why APY matters more than the base rate

Banks sometimes advertise a base interest rate (called the APR or nominal rate) separately from the APY. The APY is always the larger number because it includes the effect of compounding. For savings accounts, always use the APY to compare accounts and calculate earnings — it's the honest number.

The difference between APR and APY is small on savings accounts but grows larger the more frequently interest compounds. A 4.50% APR compounded daily becomes roughly 4.60% APY. That extra 0.10% doesn't sound like much, but on $50,000 it's $50 per year that you'd miss if you only looked at the base rate.

Federal law requires banks to show you the APY before you open an account. If a bank shows only the base rate, ask for the APY in writing. It's the only number you need to compare one account to another.

What changes your interest earnings

Your actual interest depends on three things: the APY (which the bank sets and can change), your balance (which you control), and how long the money stays in the account. Banks can lower the APY at any time, though they must notify you first. If rates drop across the industry, your bank's rate will likely drop too.

Withdrawals reduce your balance and therefore your interest. If you withdraw $2,000 on day 15 of a month, you earn interest only on the smaller balance for the remaining days. Deposits increase your balance and increase your interest starting the day the deposit clears (not the day you make it — clearing takes one to two business days for transfers).

Some accounts have tiered rates, meaning you earn a higher APY if your balance is above a certain threshold. A bank might offer 4.50% APY on balances of $25,000 or more, but only 3.50% on smaller balances. Your interest rate depends on which tier your balance falls into each day.

Using online calculators vs. doing it yourself

Most banks and financial websites offer free savings calculators where you enter your balance, the APY, and the time period, and the tool shows you the interest earned. These are accurate and save you the arithmetic. They're useful for comparing what different accounts would earn or planning how long it takes to reach a savings goal.

Doing the math yourself is also straightforward if you have a calculator and the APY. The formula (Balance × APY ÷ 365 × Number of Days) works for any account at any bank. If you're comparing two accounts, calculating both by hand takes five minutes and gives you confidence in the numbers.

Your bank's statements show the actual interest you earned each month, so you can always verify the calculation after the fact. If the amount seems wrong — much lower than you expected — check whether the APY changed during the month or whether your balance was lower than you thought.

Common mistakes when calculating interest

The most common error is using the base interest rate instead of the APY. If a bank advertises "4.50% interest," that's usually the APY, but always confirm. Using a lower rate than the actual APY will make your calculation too pessimistic.

Another mistake is forgetting that interest compounds daily, not monthly or yearly. If you calculate interest as if it were straightforward (not compounded), you'll underestimate what you actually earn. The difference is small over short periods but meaningful over a year or more.

A third error is assuming your balance stays constant when it doesn't. If you deposit money mid-month or withdraw some, your actual interest will differ from a calculation based on a single balance. Your bank's statement is the source of truth for what you actually earned.

Frequently Asked Questions

How often does the bank add interest to my account?

Interest compounds daily, but most banks deposit the total monthly interest into your account once a month, usually on the last day or the first day of the next month. You can see the deposit on your statement. Some banks deposit interest quarterly or annually, depending on the account type — check your disclosure documents.

If I withdraw money mid-month, do I lose all the interest for that month?

No. You earn interest on your balance each day, so if you have $5,000 for 15 days and $3,000 for 15 days, you earn interest on both amounts for the days they were in the account. The bank calculates this automatically — you don't lose interest just because your balance changed.

Can the bank change my interest rate whenever it wants?

Yes, banks can change savings account rates at any time. They must notify you before the change takes effect, usually by email or mail. If rates drop, you'll earn less interest going forward. If you want to lock in a higher rate, consider a certificate of deposit (CD), which guarantees a fixed rate for a set time period.

Why does my interest vary from month to month?

Interest varies because your balance changes (deposits and withdrawals), the number of days in the month changes (February has fewer days), and compounding means you earn slightly more interest each month as your balance grows. All of these are normal and expected.

Is the interest I earn taxable?

Yes. Interest earned on savings accounts is taxable income. Banks report interest of $10 or more to the IRS on a 1099-INT form, and you must report it on your tax return. Keep your statements so you have a record of the total interest earned during the year.