The basic formula for savings account interest
The interest your bank pays you depends on three things: how much money you have in the account, what annual percentage yield (APY) the bank offers, and how long the money sits there. The simplest way to calculate it is to multiply your balance by the APY, then divide by the number of days in a year.
Here's the actual math: take your account balance, multiply it by the APY (as a decimal—so 0.5% becomes 0.005), then multiply by the number of days your money was in the account, then divide by 365. That gives you the interest earned for that period.
Example: You have $10,000 in an account earning 4.5% APY. Money sits there for 90 days. The calculation is: $10,000 × 0.045 × (90 ÷ 365) = $110.96 in interest.
Key Takeaways
- Interest earned equals your balance multiplied by the APY rate, multiplied by the fraction of the year your money was deposited.
- Banks compound interest daily, weekly, or monthly, meaning they add earned interest back to your balance and pay interest on that interest too.
- The APY shown on the bank's website already accounts for compounding, so you do not need to calculate compounding separately if you use APY.
- Your actual interest will vary month to month because your balance changes and compounding happens on different schedules at different banks.
- You can verify your interest by checking your monthly statement—banks must show you exactly what they paid.
Why your actual interest differs from the straightforward calculation
Banks do not pay interest once a year. They compound it—meaning they add the interest they owe you back into your account, then pay interest on that new, larger balance. This happens daily at most online banks, weekly at some, or monthly at others. The more often it compounds, the more total interest you earn.
The APY rate you see advertised already includes the effect of compounding. So if a bank shows you 4.5% APY, that 4.5% is the real return you will get after compounding happens all year. You do not have to do the compounding math yourself—the bank has already done it and expressed it as APY.
What changes your actual interest is your balance. If you deposit $10,000 on day one and withdraw $5,000 on day 180, the bank calculates interest on $10,000 for 180 days, then on $5,000 for the remaining 185 days. Your statement will show the exact breakdown.
How to find the APY your bank is actually paying
The APY is not always the same as the interest rate. Banks sometimes advertise a base interest rate and then show you the APY separately—the APY is the number that matters for your calculation because it includes compounding.
You will find the APY on your bank's website, usually on the savings account product page or in the account terms. It should also appear on your monthly or quarterly statement. If you cannot find it, call the bank or log into your online account and look for "Annual Percentage Yield" or "APY"—it will be a percentage.
APY changes. Banks raise and lower rates based on what the Federal Reserve does. Check your statement regularly or set a reminder to look at your bank's website every few months, because the rate you opened the account with may not be the rate you are earning now.
Using a calculator versus doing it by hand
For a single deposit that sits untouched, the math is straightforward enough to do on a phone calculator. But if your balance changes during the month or you want to see what you will earn over a year, a spreadsheet or online calculator saves time and mistakes.
Most banks offer a savings calculator on their website. You enter your starting balance, the APY, and how long you plan to keep the money, and it shows you the projected interest. These are accurate for planning purposes, though your actual interest will vary if your balance changes.
If you use a spreadsheet, the formula is the same: balance × APY × (days ÷ 365). You can also use the compound interest formula if you want to see what happens if you add money regularly, but for a single deposit, the straightforward formula works fine.
What happens when the APY changes mid-year
Banks change rates frequently. When they do, the new rate applies to your balance going forward, not retroactively. If you earned 4.5% for six months and the bank drops the rate to 3.5%, you keep the 4.5% interest you already earned, and the new 3.5% applies to the next six months.
Your statement will show the rate that was in effect during each period. If the rate changed on the 15th of the month, your statement will show interest calculated at the old rate for days 1–14 and the new rate for days 15–30. The bank must disclose this breakdown.
Why the interest you see on your statement might not match your calculation
If you calculate interest and it does not match what the bank paid, the most common reason is that you used a different balance or time period than the bank did. Banks calculate interest on your average daily balance or your ending balance, depending on the account terms. Check your statement to see which method yours uses.
Another reason is rounding. Banks round interest to the nearest cent, so a calculation that shows $110.956 becomes $110.96 on your statement. This is normal and correct.
If the difference is more than a few cents, or if the APY on your statement does not match what the bank advertised, contact the bank. They are required to pay the rate they promised, and if there is an error, they will correct it and pay you the difference.
Frequently Asked Questions
Do I need to do this calculation myself, or does the bank do it for me?
The bank calculates and deposits your interest automatically. You do not have to do anything. The calculation here is so you understand how much you should be earning and can verify that your statement is correct.
What if I add money to my savings account during the month?
The bank tracks your balance daily and calculates interest on each day's balance separately, then adds them together. You do not have to calculate this yourself—your statement shows the total interest earned. If you want to estimate it, calculate interest on your starting balance for the days it was there, then on your new balance for the remaining days.
Is APY the same as the interest rate?
No. The interest rate is the base rate the bank pays. The APY is that rate plus the effect of compounding. Always use APY for your calculation because it is the real return you will get.
How often do banks compound interest?
Most online banks compound daily. Some compound weekly or monthly. The more often it compounds, the slightly more interest you earn, but the difference is usually small. Your account terms will tell you the compounding frequency.
Can I negotiate a higher APY with my bank?
Most banks do not negotiate rates on savings accounts. Your rate is set when you open the account and changes only when the bank changes it for all customers. If you want a higher rate, you may need to move your money to a different bank.