Banks multiply your balance by the interest rate, then divide by the number of days in a year

The math behind savings account interest is straightforward: your bank takes the money you have on deposit, multiplies it by the annual interest rate they've promised you, and divides by 365 (or sometimes 360). That gives you the interest you earn per day. Then they add up all those daily amounts for the month or quarter, and deposit the total into your account.

The catch is that your balance changes almost every day—you deposit money, you withdraw money—so the bank recalculates which balance they're using. Most banks use what's called the daily balance method: they track your balance at the end of each day, add all those daily balances together for the month, divide by the number of days, and then apply the interest rate to that average. Some banks use the ending balance method instead, which is simpler for them but usually pays you less, because they only look at what you have on the very last day of the month.

Key Takeaways

  • Banks calculate daily interest by multiplying your balance by the annual rate and dividing by 365, then they add up each day's interest for the month.
  • The daily balance method, used by most banks, averages your balance across the whole month before applying the rate—this usually pays more than the ending balance method.
  • Interest compounds, meaning you earn interest on the interest you already earned, and the frequency (daily, monthly, or quarterly) affects how much you make.
  • A higher APY (annual percentage yield) accounts for compounding, while APR does not, so APY is the number that tells you what you'll actually earn.
  • Your actual interest depends on how long money sits in the account and whether you make deposits or withdrawals during the month.

Why your balance matters more than the rate alone

Two people with the same interest rate can earn different amounts of interest because they don't have the same balance all month. If you deposit $5,000 on the first day and leave it untouched, the bank uses $5,000 for all 30 days. If you deposit $5,000 on the last day, the bank only uses that $5,000 for one day. The daily balance method catches this difference.

Here's a concrete example. Say your account earns 4.50% APY and you start with $10,000. On day 15, you withdraw $2,000. The bank calculates interest like this: the first 14 days use $10,000, the remaining 16 days use $8,000. They add those up (14 × $10,000 + 16 × $8,000 = $268,000), divide by 30 days to get an average balance of $8,933.33, then multiply by 4.50% and divide by 365. That gives you roughly $1.10 for the month. If you'd left the full $10,000 in all month, you'd earn about $1.50.

How compounding changes what you actually earn

Interest compounds when the bank adds your earned interest back into your account, and then you start earning interest on that interest. How often this happens—daily, monthly, or quarterly—changes your total earnings.

Daily compounding is the most common and pays the most. The bank calculates your daily interest, deposits it into your account, and the next day you earn interest on that deposit too. Monthly compounding means the bank waits until the end of the month to add all the interest at once. Quarterly compounding waits three months. The longer the bank waits to add your interest back, the less total interest you earn, because you're not earning interest on that interest yet.

The difference is small on small balances but real. On $10,000 at 4.50% APY, daily compounding versus monthly compounding might mean $45 versus $44 per year. On $100,000, it could mean $450 versus $440. This is why banks advertise the APY instead of just the interest rate—the APY already includes the effect of compounding, so it shows you what you'll actually make.

APY versus APR: which number to trust

APY (annual percentage yield) is the rate that includes compounding. It's the real number—what you'll actually earn in a year if you leave the money alone. APR (annual percentage rate) does not include compounding; it's just the raw interest rate. Banks are required to show you the APY on savings accounts, so that's the one you should compare between banks.

If a bank advertises "4.50% APY," you know that after one year, $10,000 will grow to $10,450 (before any deposits or withdrawals). If they only said "4.50% APR," the actual amount would be slightly less, because compounding hasn't been factored in. Always look for the APY label when you're deciding between accounts.

What happens when you deposit or withdraw mid-month

Every deposit and withdrawal changes your balance, which changes the interest you earn that day forward. If you deposit $1,000 on day 20 of a 30-day month, that $1,000 only earns interest for 11 days. If you withdraw $1,000 on day 10, you lose interest on that $1,000 for the remaining 20 days.

This is why the timing of large deposits and withdrawals matters. Depositing money early in the month means it earns interest for more days. Withdrawing money late in the month means you keep earning interest on it for longer. Banks don't penalize you for withdrawals the way they do with CDs, but the math still works in your favor if you keep money in the account longer.

Why different banks pay different amounts on the same balance

Two banks might both offer 4.50% APY, but one might use daily compounding and the other monthly compounding. One might use the daily balance method and the other the ending balance method. One might calculate interest on a 360-day year (which makes the daily rate slightly higher) and another on 365 days. These small differences add up.

Over a year on $50,000, choosing a bank with daily compounding instead of monthly compounding might earn you $5 to $10 more. That's not huge, but it's real money. More important is the APY itself—a bank offering 4.75% APY will pay you noticeably more than one offering 4.50%, no matter which method they use. When you're comparing accounts, focus on the APY first, then ask about compounding frequency if the APYs are close.

How to estimate what you'll earn

You don't need to do the full calculation yourself. A simple rule of thumb: take your balance, multiply by the APY, and divide by 12 to get a rough monthly interest amount. On $10,000 at 4.50% APY, that's $10,000 × 0.045 ÷ 12 = $37.50 per month. This isn't exact—the real amount depends on the exact number of days in the month and how your balance changed—but it's close enough to plan with.

Your bank's website usually shows you the exact interest earned each month in your account statement or online banking dashboard. You can also use a savings calculator (many banks provide them) where you enter your balance, the APY, and how often you plan to deposit or withdraw, and it shows you what you'll have after a year or five years.

Frequently Asked Questions

Does the bank round down the interest they owe me?

Banks round to the nearest cent, which means sometimes they round in your favor and sometimes they don't. Over a year, the rounding differences are tiny—usually a few cents. If you see a discrepancy of more than a few cents between what you calculated and what the bank paid, ask them to explain the calculation; they should be able to show you the exact daily balances and rates they used.

What if I move money between accounts at the same bank—does that affect interest?

Moving money between your own accounts at the same bank doesn't trigger any fees or penalties, and interest is calculated on each account separately based on its own balance. If you move $5,000 from checking to savings, your checking balance drops (and earns less interest) while your savings balance rises (and earns more). The bank treats it as a withdrawal from one account and a deposit to another.

Can the bank change the interest rate after I open the account?

Yes. Savings account rates are variable, meaning the bank can raise or lower them whenever they want, usually with a few days' notice. This is different from a CD, where the rate is locked in for the term. Banks raise rates when the Federal Reserve raises rates, and lower them when the Fed cuts rates. You're not locked in, so if another bank offers a higher rate, you can move your money.

Why do some banks show interest as a percentage and others as a dollar amount?

Banks must show you the APY (the percentage) because that's what the law requires. Some also show you the dollar amount you'll earn on a sample balance like $10,000, which makes it easier to compare. The percentage is the standardized number; the dollar amount is just a convenience. Always compare the APY percentages between banks, not the dollar amounts, because those depend on the sample balance the bank chose.

Does interest get taxed?

Yes. Interest you earn on a savings account is taxable income. If you earn more than $10 in interest in a year, the bank will send you a 1099-INT form in January showing how much you earned, and you'll report it on your tax return. This is why high-yield savings accounts are popular—the higher interest rate means more earnings, but also more taxable income. The interest itself isn't penalized; it's just treated like any other income.