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

The simplest way to understand savings account interest: the bank pays you a percentage of the money you keep with them. That percentage is called the interest rate. The bank calculates how much you earn by taking your account balance, multiplying it by the interest rate, and dividing by 365 (or sometimes 360) days. The result is how much interest you earn that day. The bank repeats this calculation every single day, adds up all those daily amounts, and deposits the total into your account.

This daily calculation matters because your balance changes constantly. When you deposit money, your balance goes up, so you earn more interest that day. When you withdraw money, your balance goes down, so you earn less. The bank is always recalculating based on what you actually have in the account.

Key Takeaways

  • Banks calculate interest daily by multiplying your current balance by the annual interest rate and dividing by 365 days.
  • The interest earned each day depends on how much money is actually in your account that day, so deposits increase your daily earnings and withdrawals decrease them.
  • Interest is usually added to your account monthly, quarterly, or annually, even though it is calculated every day.
  • A higher interest rate and a larger balance both mean more money earned, but the difference between 0.01% and 5% can be hundreds of dollars per year on the same balance.
  • Some banks use 360 days instead of 365 to calculate interest, which slightly reduces what you earn, so it is worth asking which method your bank uses.

The actual formula banks use

Here is the math written out. Banks use this formula every single day:

Daily Interest = (Account Balance × Annual Interest Rate) ÷ 365

Let's say you have $10,000 in a savings account earning 4.5% interest per year. On a day when your balance is exactly $10,000, the bank calculates: ($10,000 × 0.045) ÷ 365 = $1.23. You earn $1.23 that day. Tomorrow, if your balance is still $10,000, you earn another $1.23. If you deposit $5,000 on day three, your new balance is $15,000, so that day you earn ($15,000 × 0.045) ÷ 365 = $1.85.

At the end of the month, the bank adds up all those daily amounts. If you earned $1.23 for 20 days and $1.85 for 10 days, your monthly interest would be about $37.30. That $37.30 gets deposited into your account, and next month the bank starts calculating interest on the new, larger balance.

Why the bank calculates every day instead of once a year

Banks could wait until the end of the year and calculate interest once, but that would be unfair to you. If you deposited money in January and withdrew it in June, you would earn interest on money you only held for six months. Daily calculation means you only earn interest on the money you actually had, for the days you actually had it.

Daily calculation also means that when the bank adds interest to your account, that interest itself starts earning interest the next day. This is called compounding. If you earn $37.30 in interest in month one, that $37.30 is now part of your balance. In month two, the bank calculates interest on your original balance plus the $37.30, so you earn slightly more. Over a year or several years, compounding makes a real difference.

How often the bank actually deposits the interest you earned

The bank calculates interest every day, but it does not deposit it every day. Instead, it adds up all the daily interest and deposits it on a schedule. Most banks deposit interest monthly, some do it quarterly (every three months), and a few do it annually (once a year). Your account statement or the bank's website will tell you which schedule your account uses.

The schedule matters because interest that has been calculated but not yet deposited is not yet part of your balance. Until the bank actually deposits it, it is not earning interest itself. So if your bank deposits interest monthly, you get the compounding benefit 12 times a year. If it deposits quarterly, you get it only 4 times. Over many years, monthly deposits mean more money in your pocket.

The difference between 360-day and 365-day calculations

Most banks use 365 days in their formula, but some use 360. This is a small difference that adds up. Using 360 days instead of 365 makes the daily interest slightly larger, which sounds good—but it is actually a trick that benefits the bank, not you.

Here is why: if a bank uses 360 days, it is saying "we will divide your annual rate by 360 instead of 365." That makes each day's interest bigger. But a year has 365 days, not 360. So the bank is paying you for 360 days of interest but keeping 5 extra days of your money without paying you. Over a year, this costs you a small amount. On a $10,000 balance at 4.5%, the difference is roughly $6 per year. On larger balances or higher rates, it is more.

When you are comparing banks, ask which method they use. Banks using 365 days are treating you fairly. Some banks will tell you this is the "actual/365" method. If a bank will not tell you, that is a sign to ask why.

Why the interest rate on your account matters so much

The interest rate is the percentage the bank pays you. A 0.01% rate means you earn almost nothing. A 5% rate means you earn much more on the same balance. The difference is enormous.

On a $10,000 balance held for one year, 0.01% interest earns you $1. The same $10,000 at 5% earns you $500. That is a $499 difference for doing nothing except choosing the right bank. On a $50,000 balance, the difference is $2,495 per year.

Interest rates change constantly. Banks raise them when the Federal Reserve raises its rates, and lower them when the Fed lowers its rates. Some banks raise rates faster than others, and some lower them faster. If you have been with the same bank for years and the rate has not changed, it is worth checking what other banks are offering. You can move your money to a bank with a higher rate and earn more interest on the same balance.

What happens when you withdraw money before interest is deposited

The bank has calculated your interest every day, but it has not deposited it yet. If you withdraw money before the deposit date, you lose the interest that was calculated but not yet added to your account.

For example, suppose your bank deposits interest on the last day of each month. On the 25th of the month, the bank has calculated $40 in interest, but it has not deposited it yet. If you withdraw $5,000 on the 26th, you withdraw from your balance, but the $40 interest is still sitting in the bank's system, not in your account. On the 30th, the bank deposits the $40—but now it is earning interest on a smaller balance because you withdrew money.

This is not a penalty; it is just how the math works. You earned the interest for the days you held the money. But if you withdraw before the deposit date, you have to wait until the next deposit date to see that interest in your account.

Frequently Asked Questions

Do I earn interest on interest?

Yes, once the bank deposits interest into your account, that interest becomes part of your balance and starts earning interest the next day. This is compounding. The more often the bank deposits interest (monthly is better than quarterly), the more you earn from compounding.

What if I have multiple deposits and withdrawals in one month?

The bank calculates interest on your actual balance each day, so every deposit and withdrawal changes the amount you earn that day. You do not have to do anything—the bank tracks this automatically and deposits the total interest at the end of the period.

Does the interest rate ever change?

Yes. Banks change interest rates based on what the Federal Reserve does and what other banks are offering. Your bank will tell you if your rate changes, usually by email or a notice in your account. Some accounts have fixed rates that do not change; others have variable rates that can go up or down.

Is the interest I earn taxable?

Yes. Interest is income, and you owe taxes on it. Your bank will send you a form called a 1099-INT at the end of the year showing how much interest you earned. You report this on your tax return. If you earned less than $10 in interest, the bank may not send the form, but you still owe taxes on it.

Why do some banks offer much higher interest rates than others?

Online banks and credit unions often offer higher rates than large brick-and-mortar banks because they have lower costs. They do not pay for physical branches, so they pass some of that savings to customers through higher interest rates. The trade-off is that you manage your account online instead of visiting a branch in person.