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

Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest back into your account. The formula is straightforward: your current balance multiplied by the annual interest rate, divided by 365 (or 360, depending on the bank), then multiplied by the number of days the money sat in the account.

Here is what that looks like in practice. If you have $10,000 in an account earning 4.5% annual interest, the bank divides 4.5% by 365 to get a daily rate of about 0.0123%. On day one, you earn roughly $1.23. On day two, you earn interest on $10,001.23 — not just the original $10,000. That extra penny of interest starts earning interest too. This is called compounding, and it is why the interest you actually receive is slightly higher than the simple rate suggests.

The timing matters. Interest accrues (builds up) daily, but most banks only deposit it into your account once a month or once a quarter. Until that deposit hits, the interest is sitting in the bank's system, earning more interest. When it finally posts to your account, you see a lump sum rather than daily deposits.

Key Takeaways

  • Banks calculate interest daily by multiplying your balance by the annual rate and dividing by 365, then compounding means that interest earns interest too.
  • The interest rate you see advertised is the annual percentage yield (APY), which already accounts for compounding over a full year.
  • Interest accrues every day but usually posts to your account monthly or quarterly, so you do not see daily deposits.
  • Moving money in or out changes your average daily balance, which changes how much interest you earn that month.
  • Different banks use slightly different methods (365 days versus 360 days), but the difference is small enough that you should focus on finding the highest APY instead.

Why the advertised rate is not the same as what you earn

The number the bank shows you — usually called the Annual Percentage Yield (APY) — is not the same as the interest rate the bank uses in its daily calculation. The APY is the rate you would earn over a full year if you never touched the money and interest kept compounding. It is higher than the base rate because it includes the effect of compounding.

For example, a bank might advertise 4.5% APY. That does not mean you earn exactly 4.5% of your balance once a year. It means that if you started with $10,000 and left it untouched for 365 days, you would have $10,450 at the end — a gain of $450. The daily calculation the bank actually uses is slightly lower, but compounding brings it up to that 4.5% figure by year's end.

This is why comparing accounts by APY makes sense. Two banks might use slightly different daily calculation methods, but if they both advertise the same APY, you will earn the same amount over a year.

How deposits and withdrawals affect your interest

When you add money to your account, that new balance starts earning interest immediately. When you withdraw money, the interest calculation shrinks. Most banks use the average daily balance method, which means they add up your balance at the end of each day during the month, then divide by the number of days. That average is what they use to calculate your monthly interest.

This matters if you are moving money around. If you deposit $5,000 on the first day of the month and leave it there, that $5,000 earns interest for all 30 days. If you deposit $5,000 on the 30th day of the month, it only earns interest for one day. The bank does not care when you deposit or withdraw — it only cares what your balance was each day.

Some banks offer a different method called minimum daily balance, where they use your lowest balance during the month to calculate interest. This method is less common and usually pays less, so check your account terms to see which one your bank uses.

The difference between simple interest and compound interest

Simple interest means the bank pays you interest only on your original deposit. If you put in $10,000 at 4.5% simple interest, you earn $450 a year, every year, forever. The interest never earns interest.

Compound interest means the bank pays you interest on your original deposit plus all the interest that has already been added. After the first month, you have $10,000 plus a small amount of interest. In month two, you earn interest on that larger amount. By the end of the year, you have earned more than $450 because the interest itself has been earning interest.

All savings accounts use compound interest, not simple interest. The compounding happens daily, even though the interest only posts to your account monthly or quarterly. This is one of the few ways banks work in your favor — the more often interest compounds, the more you earn.

Why some banks use 360 days instead of 365

A small number of banks calculate daily interest using 360 days instead of 365. This is called the 360-day year or banker's year. It makes the daily interest rate slightly higher, which sounds good, but the effect is tiny — about 1.4% more interest per year.

The 360-day method is an old banking convention that made hand calculations easier decades ago. Most modern banks have switched to 365 days because it is more accurate. Unless you are comparing two accounts with identical APYs and one uses 360 days, this difference should not drive your decision. Focus on the APY instead, which already accounts for the calculation method.

Your account statement or the bank's disclosure document will tell you which method they use, usually buried in the fine print under "how interest is calculated" or "interest calculation method."

How interest rates change and when they take effect

Banks change their interest rates based on what the Federal Reserve does with its benchmark rate. When the Fed raises rates, banks usually raise savings account rates within days or weeks. When the Fed cuts rates, banks cut savings account rates much faster — sometimes overnight.

The new rate applies to interest accrued after the change takes effect, not to interest that already posted to your account. If your rate drops on the 15th of the month, the interest you earned from the 1st to the 14th is locked in at the old rate. Interest from the 15th onward uses the new rate.

You can see your current rate on your bank's website, in your account settings, or by calling customer service. If the rate drops and you are unhappy, you can move your money to a different bank — there is no penalty for switching savings accounts, and no bank owns your money once it is there.

What happens to interest if you close your account mid-month

Interest accrues every day, but it only posts to your account on the bank's schedule — usually the last day of the month or the last day of the quarter. If you close your account before that date, you still receive the interest that has accrued up to that point, but you have to wait for the regular posting date to see it. Some banks will deposit it into your new account at another bank; others mail a check.

Check your account terms or call the bank before you close to find out how they handle accrued interest. Do not assume you will lose it — you will not. But do not assume it will post immediately either. It will follow the bank's normal schedule.

Frequently Asked Questions

Does my interest earn interest?

Yes. Once interest posts to your account, it becomes part of your balance and starts earning interest too. This is compounding. It happens automatically — you do not have to do anything. The APY the bank advertises already includes this effect over a full year.

Why is my interest so low if the APY is 4.5%?

You only earn the full APY if your money sits in the account for a full year without moving. If you deposited money partway through the month, or withdrew some, your average daily balance was lower, so you earned less. Also, interest accrues daily but posts monthly or quarterly, so you might not see the full year's worth of interest until several months have passed.

Can I negotiate a higher interest rate?

No. Banks set rates based on market conditions and the Federal Reserve, not on individual customers. You cannot call and ask for a better rate. Your only option is to move your money to a bank offering a higher rate.

What if I keep adding money throughout the month?

Each deposit starts earning interest immediately at the current rate. The bank calculates your average daily balance by adding up what you had each day and dividing by the number of days in the month. More deposits mean a higher average, which means more interest.

Do I have to pay taxes on savings account interest?

Yes. Interest is taxable income. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest. You report this on your tax return. This is separate from how the bank calculates the interest itself.