How banks calculate the interest you earn
Banks calculate savings account interest using your daily balance, the annual percentage yield (APY) the bank offers, and the number of days in the period. The formula is straightforward: multiply your balance by the APY, divide by 365 (or 366 in a leap year), then multiply by the number of days your money sat in the account. That gives you the interest earned for that period.
Most banks compound interest daily, meaning they calculate what you owe you each day, add it to your balance, and then calculate the next day's interest on that larger amount. This compounding effect is why the interest you actually receive is slightly higher than the straightforward APY number suggests — you earn interest on your interest.
The timing matters. Interest accrues from the moment a deposit clears, not from the moment you make it. If you deposit money on a Friday and it clears on Monday, you start earning interest on Monday. Similarly, if you withdraw money, interest stops accruing on the day the withdrawal processes, not the day you request it.
Key Takeaways
- Banks calculate daily interest by multiplying your balance by the APY, dividing by 365, and multiplying by the number of days the money was in the account.
- Daily compounding means interest is added to your balance each day, so the next day's interest calculation includes yesterday's interest earnings.
- Interest accrues only from the date a deposit clears and stops on the date a withdrawal processes, not on the dates you initiate the transactions.
- The actual interest you receive will be slightly higher than the stated APY because of compounding, though the difference is small on typical balances.
- Different banks may use different compounding schedules (daily, monthly, quarterly), so comparing APY alone does not tell you which account will earn more.
The difference between APY and APR
Banks advertise APY (annual percentage yield), not APR. APY already includes the effect of compounding, so it shows what you will actually earn over a year. APR (annual percentage rate) does not account for compounding and is rarely used for savings accounts — it is more common for loans and credit cards.
If a bank offers 4.5% APY on a savings account, that 4.5% already reflects daily compounding. You do not need to do any additional math to account for compounding — the bank has already done it. The APY is the number that matters for comparing one account to another.
What happens when you deposit or withdraw mid-month
Interest accrues based on your actual balance each day. If you deposit $5,000 on the 15th of the month and the account earns 4.5% APY, you earn interest only on that $5,000 from the 15th onward, not on the full month's balance. The bank calculates this by taking your daily balance, multiplying by the APY, dividing by 365, and multiplying by the number of days.
Withdrawals work the same way. If you withdraw $2,000 on the 20th, your balance drops on that day, and interest for the 20th onward is calculated on the lower amount. Some banks post interest monthly, so you might not see the reduced interest until the next statement, but the calculation reflects the actual days your money was in the account.
This is why timing matters for large deposits or withdrawals. A $10,000 deposit on the 1st of the month earns interest for 30 days; the same deposit on the 28th earns interest for only 3 days. Over a year, that difference compounds.
How often interest is credited to your account
Banks calculate interest daily but credit it (actually add it to your balance) on different schedules. Most common savings accounts credit interest monthly, meaning the bank adds up all the daily interest earned during the month and deposits it once, usually on the last day of the month or the first day of the next month.
Some accounts credit interest quarterly or even annually, though this is less common for savings accounts. The more frequently interest is credited, the sooner it starts earning interest itself — which is why monthly crediting is better than quarterly. However, the difference is small unless your balance is very large.
Your account statement will show when interest was credited. Look for a line item labeled "Interest Paid" or "Interest Earned" with a date. That is when the bank actually added the money to your account.
Why your interest earnings might be lower than expected
The most common reason is that the APY changed during the month. Banks adjust rates frequently, and if your rate dropped mid-month, you earned the higher rate only on the days before the change. Your statement should show the rate that applied each day, though not all banks display this clearly.
Another reason is that you are comparing the APY to a straightforward calculation. If you see 4.5% APY and think "that is $45 per year on $1,000," you are correct — but only if the money sits untouched for the full year. Any deposits or withdrawals change the calculation. A $1,000 balance for only 6 months earns roughly $22.50, not $45.
Finally, some accounts have minimum balance requirements. If your balance drops below the minimum, the account may earn a lower rate or no interest at all. Check your account agreement for these terms.
Calculating interest on your own
To calculate what you should have earned, gather three pieces of information: your daily balance for each day of the month, the APY that applied on each day, and the number of days in the month. Then use this formula for each day: (Daily Balance × APY ÷ 365) = Daily Interest.
Add up all the daily interest amounts for the month, and that is what you should see credited. In practice, most people do not do this — it is tedious and the amounts are small. But if you suspect an error, this is how you verify it.
A simpler approach: divide the APY by 12 to get a rough monthly rate, multiply your average balance by that rate, and divide by 100. This gives you an approximation. For example, 4.5% APY ÷ 12 = 0.375% per month. If your average balance was $5,000, multiply $5,000 × 0.375 ÷ 100 = $18.75 for the month. This is not exact (because it ignores daily compounding), but it is close enough to catch major errors.
How to find the best interest rate for your situation
Compare APY across banks, not APR or stated rates. APY is the only number that reflects what you will actually earn. Banks are required to display APY prominently, usually near the account name or in a table of rates.
Pay attention to minimum balance requirements and whether the rate applies to your balance size. Some banks offer high APY only on balances above $25,000, or they tier the rate so that only part of your balance earns the advertised rate. Read the fine print, or call and ask directly.
Also check how often the bank changes rates. Some banks adjust rates weekly based on market conditions; others change them monthly or quarterly. If rates are rising, a bank that adjusts frequently will get you higher interest faster. If rates are falling, the opposite is true.
Frequently Asked Questions
Does interest start accruing the day I deposit money?
No. Interest accrues from the day your deposit clears, not the day you make it. A deposit made on Friday may not clear until Monday, so interest starts on Monday. Check your bank's clearing timeline — it varies by deposit method (direct deposit, mobile check deposit, in-person deposit).
If I withdraw money mid-month, do I lose all the interest for that month?
No. You earn interest only on the balance you actually held. If you had $5,000 for 15 days and $3,000 for 15 days, you earn interest on both amounts for their respective periods. The bank calculates this daily, so you do not lose anything.
Why is my interest lower than the APY shown on the website?
The APY is an annual rate. If your money was in the account for less than a year, or if your balance was lower than the example used, your actual interest will be lower. Also, if the rate changed during the month, you earned the old rate on some days and the new rate on others.
Can I calculate interest myself without a calculator?
You can estimate it. Divide the APY by 12 for a monthly rate, multiply your average balance by that percentage, and divide by 100. This gives you a rough figure. For exact calculations, you need to account for daily balances and daily compounding, which is tedious without a spreadsheet.
Does it matter which bank I choose if they all offer the same APY?
Check the fine print. Some banks offer high APY only on large balances or require frequent deposits. Others have monthly fees that reduce your earnings. Two accounts with the same APY can deliver different results depending on minimum balances, fee structures, and how often the rate adjusts.