The basic math: how banks calculate what they owe you

Banks calculate interest on your savings account by multiplying your balance by the interest rate, then dividing by the number of days in a year. The formula is: Interest = (Balance × Annual Rate) ÷ 365. Most banks do this calculation daily, which means they look at what you had in the account each day, work out that day's interest, and add it to your balance. That daily interest then earns interest the next day — this is called compounding.

The timing matters more than you might think. If you deposit $5,000 on January 15 into an account earning 4.5% annual interest, the bank starts calculating interest that day. If you withdraw $2,000 on February 1, the interest calculation drops to the remaining $3,000 from that point forward. You do not earn interest on money that is not in the account.

Banks publish their interest rate as an Annual Percentage Rate (APR) or Annual Percentage Yield (APY). APR is the straightforward rate before compounding. APY is the rate after compounding is factored in — it is always equal to or higher than the APR. When you see a savings account advertised at "4.5%", that is usually the APY, which is what actually matters to you.

Key Takeaways

  • Banks calculate interest daily by multiplying your daily balance by the annual rate and dividing by 365, then add that amount to your account.
  • Compounding means the interest you earn each day also earns interest the next day, so your balance grows faster than straightforward math would suggest.
  • APY (Annual Percentage Yield) is the rate you should compare between accounts because it includes the effect of compounding; APR does not.
  • The day you deposit money, interest starts accruing; the day you withdraw it, interest stops — partial months are calculated proportionally.
  • Different banks compound at different frequencies (daily, monthly, quarterly), but daily compounding is most common and gives you the highest return.

Why compounding makes a real difference

Compounding is the reason your money grows faster than you might expect. On day one, you earn interest on your principal. On day two, you earn interest on your principal plus the interest from day one. This snowball effect accelerates over time.

Here is a concrete example. You deposit $10,000 into an account earning 4.5% APY, compounded daily. On day one, the bank calculates: ($10,000 × 0.045) ÷ 365 = $1.23. That $1.23 is added to your balance. On day two, the bank calculates interest on $10,001.23, not $10,000. The difference is tiny on day two, but after a year, compounding adds roughly $230 more than straightforward interest would. After five years, the difference is over $1,200.

The frequency of compounding matters, but less than the rate itself. Daily compounding beats monthly compounding, which beats quarterly compounding. However, the difference between daily and monthly compounding on a $10,000 balance at 4.5% is roughly $10 per year. The difference between a 4.5% account and a 2% account is roughly $250 per year. Shop for the highest rate first; compounding frequency is a tiebreaker.

How to calculate your expected interest for a specific period

If you want to know roughly how much interest you will earn in a month or a year without waiting for the bank to tell you, you can use a simplified version of the formula. For a quick estimate: (Balance × Annual Rate) ÷ 12 = Monthly Interest. This ignores compounding, so it slightly underestimates, but it is close enough for planning.

For a more accurate number that includes compounding, use the compound interest formula: Final Balance = Principal × (1 + Rate ÷ Compounding Periods) ^ Number of Periods. If you have $10,000 at 4.5% compounded daily for one year, that is: $10,000 × (1 + 0.045 ÷ 365) ^ 365 = $10,460.45. Your interest earned is $460.45.

Most banks provide an interest calculator on their website where you enter your balance and the rate, and it does this math for you. Your monthly statement also shows the interest posted that month, so you can track what you actually earned.

Why your actual interest might differ from the advertised rate

Banks can change their interest rate at any time. If you open an account at 4.5% and the bank drops it to 3.5% three months later, you earn 4.5% for those three months and 3.5% going forward. The advertised rate is what new customers get on the day they open the account, not a may provide for the life of the account.

Some accounts have tiered rates, meaning the interest rate changes based on your balance. A bank might offer 4.5% on balances up to $25,000 and 3.8% on anything above that. If you have $30,000, the first $25,000 earns 4.5% and the remaining $5,000 earns 3.8%. The bank calculates interest on each tier separately.

Promotional rates are temporary. A bank might advertise 5% APY for the first three months, then drop to 2% after that. Read the fine print to see when the promotional period ends and what the standard rate will be.

The difference between APR and APY, and why it matters

APR (Annual Percentage Rate) is the interest rate before compounding. APY (Annual Percentage Yield) is the rate after compounding is included. On a savings account, APY is always equal to or higher than APR.

Here is why the difference exists. A bank might advertise an APR of 4.39% compounded daily. When you factor in daily compounding over a full year, the actual return is 4.5% APY. The 0.11% difference is small, but it adds up. On a $100,000 balance, that 0.11% difference is $110 per year.

When you compare savings accounts, always compare APY to APY, not APR to APY. Comparing APR to APY makes one account look worse than it actually is. If one bank advertises 4.5% APY and another advertises 4.39% APR, the first bank is paying more, even though the numbers look close.

How interest posting works and when you see the money

Banks calculate interest daily, but they do not add it to your account every day. Most banks post interest monthly, on the last day of the month or the first day of the next month. Some post quarterly or even annually, though this is rare for savings accounts.

When interest is posted, it becomes part of your balance and starts earning interest itself. If your bank posts interest on the last day of each month, you see the deposit hit your account on that day. Your statement will show the amount posted and the date.

If you withdraw money before interest is posted, you do not lose the interest you have already earned — it is already calculated and waiting. If you withdraw on the 28th of the month and interest posts on the 31st, you still receive the interest earned through the 28th. The bank does not penalize you for withdrawing before the posting date.

Frequently Asked Questions

Do I earn interest on interest?

Yes, that is compounding. The interest you earn each day becomes part of your balance and earns interest the next day. This is why APY (which includes compounding) is higher than APR (which does not).

What happens to my interest if I close the account before the month ends?

You receive the interest earned up to the day you close the account. If you close on the 15th and interest posts on the 30th, the bank calculates interest through the 15th and either deposits it before closing or includes it in your final withdrawal.

Can a bank change the interest rate on my account?

Yes, banks can change rates at any time. The rate you see when you open the account is not locked in for life. Read your account agreement to see if there are any rate guarantees or promotional periods.

Why do different banks offer different interest rates?

Banks set their own rates based on what they pay for deposits and what they earn from loans. Online banks typically offer higher rates because they have lower overhead costs than brick-and-mortar branches. Rates also move with the Federal Reserve's decisions about short-term interest rates.

Is the interest I earn on a savings account taxable?

Yes. Interest earned on a savings account is ordinary income and must be reported on your tax return. Banks send you a 1099-INT form in January if you earned $10 or more in interest during the year. Your tax liability depends on your overall income and tax bracket.