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

The basic formula 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 a month or quarter, and deposit the total into your account.

The reason banks break it into daily pieces is that your balance changes constantly. Money goes in when you deposit it, money goes out when you withdraw it. If a bank calculated interest only once a month on your ending balance, you'd lose interest on deposits you made early in the month. Daily calculation is fairer to you.

Here's a concrete example: suppose you have $1,000 in a savings account earning 4.5% annual interest. The bank divides 4.5% by 365 days, which gives roughly 0.0123% per day. On that $1,000, you earn about $0.12 per day. Over 30 days, that's roughly $3.70 in interest. The bank then deposits that $3.70 into your account.

Key Takeaways

  • Banks calculate daily interest by multiplying your balance by the annual rate, then dividing by 365 days in the year.
  • Interest is compounded when the bank adds earned interest back to your account, so the next day's calculation includes that interest too.
  • The stated annual percentage rate (APY) already accounts for compounding, so you can compare rates between banks directly.
  • Your actual interest earned depends on your balance each day, so deposits and withdrawals during the month change your total.

Why banks use daily calculation instead of monthly

If your bank calculated interest only once per month on your final balance, you'd lose money. Imagine you deposit $5,000 on the first day of the month, then withdraw it on the last day. A monthly calculation would give you zero interest, because your ending balance is zero. But you had use of that $5,000 for 29 days — you should earn something.

Daily calculation solves this. The bank tracks your balance every single day, calculates that day's interest, and keeps a running total. This way, every dollar you have on deposit earns interest for every day it sits there. When you withdraw money, you stop earning interest on that amount starting the next day.

How compounding multiplies your interest

Compounding means the bank adds the interest you've earned back into your account, so the next day's interest calculation includes that interest too. You earn interest on your interest.

Here's how it works in practice: on day one, you have $1,000 and earn $0.12 in interest. Your balance is now $1,000.12. On day two, the bank calculates interest on $1,000.12, not just the original $1,000. You earn slightly more than $0.12. This tiny extra amount compounds day after day.

Over a year, compounding makes a real difference. On a $1,000 deposit at 4.5% annual interest, daily compounding earns you about $46 instead of $45. That extra dollar comes entirely from earning interest on your interest. The more often interest compounds (daily is better than monthly, monthly is better than yearly), the more you earn.

The difference between APR and APY

APR stands for annual percentage rate — the raw interest rate the bank offers, before compounding. APY stands for annual percentage yield — the actual amount you'll earn in a year after compounding is included.

Banks are required to show you the APY, not the APR, when advertising savings accounts. This is the number that matters to you. If one bank advertises 4.5% APY and another advertises 4.5% APY, you'll earn the same amount over a year, regardless of how often they compound. The APY already does that math for you.

When you're comparing savings accounts at different banks, always compare the APY numbers. That's the only fair comparison, because it tells you the real return you'll get.

How your balance changes the interest you earn

Because interest is calculated daily on your actual balance, deposits and withdrawals during the month directly change how much interest you earn. A $500 deposit made on the 15th of the month earns interest for only 16 days that month, not 30. A $500 withdrawal made on the 10th means you stop earning interest on that $500 starting the 11th.

This is why the interest you earn each month varies. If you deposit a large sum early in the month, you'll earn more interest that month. If you withdraw money mid-month, you'll earn less. Over time, these daily changes average out, but in any single month your interest amount depends on when money moved in or out.

Some banks show you a running interest calculation in your online account. You can watch the interest accumulate day by day. Others show it only when it's deposited into your account, usually monthly or quarterly.

Why different banks offer different interest rates

Banks set their own interest rates based on what they're earning from lending money out. When the Federal Reserve raises its benchmark interest rate, banks have more room to offer higher rates on savings accounts. When the Fed lowers rates, banks lower their savings rates too.

Online banks often offer higher rates than brick-and-mortar banks because they have lower overhead costs — no physical branches to maintain. A bank with a 4.5% APY and a bank with a 2.0% APY are using the same calculation method, but one is offering you more of their earnings.

The rate you see advertised is the rate the bank is currently offering to new customers. Some banks lower their rates after you open an account. Read the account terms to see whether your rate is may provide or whether the bank can change it.

What happens when interest rates change

If your bank lowers the interest rate on your savings account, the new rate applies to interest calculated going forward. Your existing balance doesn't shrink — the bank isn't taking money away. But the daily interest you earn becomes smaller.

For example, if your rate drops from 4.5% APY to 3.5% APY, you'll earn roughly $0.10 per day on a $1,000 balance instead of $0.12. That's a real difference over a year, which is why some people move their money to a bank offering a higher rate. There's no penalty for moving your savings to a different bank.

Frequently Asked Questions

Do I earn interest on interest?

Yes. When the bank deposits your earned interest into your account, that interest becomes part of your balance. The next day's interest calculation includes it. This is compounding, and it means your money grows slightly faster than the stated rate alone would suggest.

How often do banks deposit interest into my account?

Most banks calculate interest daily but deposit it monthly or quarterly. Some deposit it more frequently. Check your account terms or online banking portal to see when your bank deposits interest. The frequency doesn't change how much you earn over a year — only the APY matters for that.

What if I withdraw money mid-month — do I lose all the interest?

No. You keep all the interest you've earned up to the day you withdraw. You stop earning interest on the withdrawn amount starting the next day. So if you withdraw $500 on the 15th, you've earned interest on that $500 for 14 days, and you keep that interest.

Why is my interest different every month?

Because your balance changes when you deposit or withdraw money. A larger balance earns more interest. If you deposit money early in the month, that month's interest is higher. If you withdraw mid-month, that month's interest is lower. Over a full year with a steady balance, the interest becomes predictable.

Can a bank change my interest rate without warning?

Yes, unless your account terms may provide a fixed rate for a specific period. Most savings accounts have variable rates that can change anytime. Banks must notify you before the change takes effect, usually by email or mail. If the new rate is too low, you can move your money to a different bank.