Banks use your balance, the interest rate, and the number of days in the compounding period to calculate what you earn

The amount of interest a bank pays you depends on three things: how much money you have in the account, what rate the bank is offering, and how often the bank compounds (adds interest to your balance). Most banks compound daily or monthly. The more frequently they compound, the more interest you earn, because you start earning interest on your interest.

The basic formula is: Interest = Balance × Annual Rate ÷ Number of Compounding Periods Per Year. If you have $10,000 in an account earning 4.5% annual interest compounded monthly, the bank divides 4.5% by 12 to get the monthly rate (0.375%), then multiplies that by your balance. The next month, it calculates interest on the new balance—which now includes the interest from the previous month.

Different banks use slightly different methods to count the days in each period, which can change the final amount by a few cents. Some use a 360-day year, others use 365 days. The difference matters more on larger balances or over longer periods, but it is one reason two banks offering the same rate may pay you slightly different amounts.

Key Takeaways

  • Interest is calculated by multiplying your account balance by the annual interest rate, then dividing by how many times per year the bank compounds.
  • Daily compounding earns you more than monthly compounding because interest starts earning interest immediately.
  • Your balance changes every time you deposit or withdraw money, so the bank recalculates interest on the new balance each period.
  • Banks may use different day-counting methods (360-day vs. 365-day year), which slightly changes the final amount you receive.
  • The annual percentage yield (APY) shown on bank websites already accounts for compounding, so it is a more accurate picture of what you will earn than the stated interest rate alone.

How compounding frequency changes what you earn

The more often a bank compounds interest, the faster your money grows. If a bank compounds annually, it calculates interest once per year on your starting balance. If it compounds monthly, it calculates interest 12 times per year—and each time, the new interest is added to the balance before the next calculation. If it compounds daily, that happens 365 times per year.

Here is a concrete example. Say you have $5,000 earning 5% annual interest. With annual compounding, you earn $250 in year one. With monthly compounding, you earn about $256. With daily compounding, you earn about $257. The difference grows larger over time and with larger balances, because you are earning interest on a bigger pool each period.

Most savings accounts offered by banks and credit unions compound daily. Some older accounts or accounts with very low rates may compound monthly or quarterly. High-yield savings accounts almost always compound daily, which is one reason they pay noticeably more than traditional savings accounts at the same stated rate.

Why your balance matters more than the rate alone

The interest you receive is always a percentage of what you have in the account. A $50,000 balance earning 4% receives twice as much interest as a $25,000 balance earning the same rate. Every deposit you make increases the balance the bank uses to calculate interest going forward. Every withdrawal decreases it.

Banks calculate interest on your balance as it stands on each compounding date. If you deposit $1,000 on the first of the month and the bank compounds monthly, that $1,000 earns interest starting the next compounding date. If you withdraw $500 on the 15th, the bank uses the lower balance for the next calculation. Some banks use an average daily balance method instead, which smooths out the effect of deposits and withdrawals throughout the month.

This is why moving money into a savings account earlier in the month, rather than later, can earn you slightly more interest—the balance is higher for a longer portion of the compounding period. The difference is usually small, but it compounds over time.

The difference between APR and APY

APR (annual percentage rate) is the interest rate the bank advertises. APY (annual percentage yield) is what you actually earn when compounding is included. APY is always equal to or higher than APR, depending on how often the bank compounds.

Banks are required to show you the APY when they advertise a savings account, because it is the honest number—it reflects what will actually appear in your account over a year. If a bank shows you an APR of 4.5% with daily compounding, the APY might be 4.60% or higher. The difference comes entirely from compounding.

When comparing savings accounts, always look at the APY, not the APR. Two banks might advertise the same APR, but if one compounds daily and the other compounds monthly, the daily-compounding account will pay you more. The APY already accounts for this difference, so comparing APYs tells you which account actually pays more.

How banks count days in the compounding period

Banks use one of two methods to count the number of days in a year when calculating interest. The ordinary interest method uses 360 days. The exact interest method uses 365 days (or 366 in a leap year). Most banks use the exact method, but some still use 360 days, which slightly inflates the interest you earn.

The difference is small on most balances. On a $10,000 account earning 4% annual interest, using 360 days instead of 365 pays you about $5.50 more per year. On a $100,000 account, the difference is about $55 per year. Over decades, this compounds into real money, but for most people the choice of day-counting method matters far less than the compounding frequency and the rate itself.

You can ask your bank which method it uses, but you will not find this information on the account disclosure or website in most cases. It is buried in the fine print of the account agreement. If you are comparing two accounts and the rates and compounding frequency are identical, the one using 365-day accounting is marginally better.

What happens when you make deposits or withdrawals

Every time you deposit money, the bank adds it to your balance and includes it in the next interest calculation. Every time you withdraw, the balance decreases. The bank recalculates interest based on the new balance for the next compounding period.

Some accounts use an average daily balance method, which adds up your balance at the end of each day during the month, then divides by the number of days. This smooths out the effect of a large deposit or withdrawal on a single day. Other accounts use the daily balance method, which calculates interest on your actual balance each day. Daily balance compounding is more common and usually pays slightly more if your balance is growing.

If you withdraw money near the end of a compounding period, you lose the interest you would have earned on that amount. If you deposit money at the beginning of a period, it earns interest for the full period. This is why timing deposits and withdrawals can make a small difference to your total earnings, though the effect is usually measured in cents or dollars per month on typical balances.

How to find out what your bank is actually paying you

Your bank statement shows the interest you earned during the statement period. Add up all the interest deposits over a year and divide by your average balance to see what effective rate you actually received. This number should be close to the APY the bank advertised, though it may be slightly different if your balance changed significantly during the year.

You can also use an online savings calculator to estimate what you will earn. Enter your starting balance, the APY, and the compounding frequency, and the calculator shows you the projected balance after one year or more. This helps you compare accounts before you open one.

If your bank's actual interest payments are significantly lower than the advertised APY, contact the bank and ask why. The difference could be a calculation error, a rate change you were not notified about, or a misunderstanding about when the rate took effect. Banks make mistakes, and it is worth asking.

Frequently Asked Questions

Does my balance have to stay the same the whole month for me to earn interest?

No. The bank recalculates interest on your actual balance each compounding period. If you deposit $1,000 on day 5 and withdraw $500 on day 20, the bank uses the correct balance for each day when it calculates interest. You earn interest on every dollar for every day it sits in the account.

Why does my bank statement show a different interest amount than I calculated?

The most common reasons are that your balance changed during the month, the bank uses a day-counting method you did not account for, or the rate changed partway through the period. Check whether the bank compounds daily or monthly—daily compounding produces a different result than monthly. If the difference is more than a few dollars, contact your bank and ask them to explain the calculation.

If I move money to a different savings account, do I lose the interest I earned?

No. Interest that has already been added to your account is yours to keep. When you transfer money to another bank, the interest you earned up to that point stays with you. The new bank will start calculating interest on your new balance using its own rate and compounding method.

Is APY the same as the interest rate?

No. The interest rate (APR) is what the bank advertises. The APY is what you actually earn after compounding is included. APY is always equal to or higher than APR. When comparing accounts, use the APY to see which one pays more.

Can a bank change the interest rate on my savings account?

Yes. Banks can change rates on savings accounts at any time, though they usually give you notice. The rate you see advertised today may not be the rate you receive next month. Check your account statements and bank emails to stay aware of rate changes. Some accounts offer a promotional rate for a limited time, then drop to a lower rate afterward.