The basic formula: multiply your balance by the rate and the time

Interest earned on a savings account comes from one calculation: your account balance multiplied by the annual percentage yield (APY), divided by the number of days in a year, then multiplied by the number of days your money actually sat in the account. Most banks do this automatically and deposit the interest monthly or daily, but understanding the math shows you whether the number they credit is correct.

The simplest version assumes your balance stays the same all month. If you have $5,000 in an account with 4.5% APY, and that money sits untouched for 30 days, you earn roughly $18.50. Here is how: $5,000 × 0.045 ÷ 365 × 30 = $18.49. The bank rounds, so you might see $18.50 posted to your account.

That formula works for any time period. A year at 4.5% APY on $5,000 gives you $225. A single day gives you about $0.62. The APY already accounts for compounding (interest earning interest), so you do not multiply it multiple times—the bank handles that behind the scenes.

Key Takeaways

  • Interest is calculated by multiplying your balance by the APY, dividing by 365, and multiplying by the number of days the money was in the account.
  • Most banks calculate interest daily but credit it monthly, so your balance changes slightly each day as interest accrues.
  • If your balance changes during the month, the bank uses the daily balance method: calculating interest on each day's balance separately, then adding them together.
  • APY already includes the effect of compounding, so you do not need to calculate compound interest separately—the posted APY is the true annual return.
  • You can verify your bank's calculation by checking the interest posted against your average daily balance and the stated APY.

How banks handle changing balances throughout the month

Most people do not keep the same balance all month. You deposit a paycheck, withdraw cash, pay a bill. Banks account for this using the daily balance method: they calculate interest on each day's balance separately, then add all those daily interest amounts together at the end of the month.

Here is a concrete example. Say your account starts with $5,000 on January 1st at 4.5% APY. On January 10th, you deposit $2,000, bringing the balance to $7,000. On January 20th, you withdraw $1,500, leaving $5,500. The bank calculates interest like this:

  • Days 1–9 (9 days at $5,000): $5,000 × 0.045 ÷ 365 × 9 = $5.55
  • Days 10–19 (10 days at $7,000): $7,000 × 0.045 ÷ 365 × 10 = $8.63
  • Days 20–31 (12 days at $5,500): $5,500 × 0.045 ÷ 365 × 12 = $8.11
  • Total interest for January: $5.55 + $8.63 + $8.11 = $22.29

This is why your interest amount varies month to month. A month where you deposit early and keep a high balance earns more than a month where you withdraw halfway through. The bank does this calculation automatically, but you can verify it by tracking your daily balance and doing the math yourself.

The difference between straightforward and compound interest

Savings accounts use compound interest, meaning interest earns interest. However, the APY your bank advertises already accounts for this. You do not need to calculate it separately—the number they quote is the true annual return after compounding is factored in.

Here is why this matters: if a bank quoted you a straightforward 4.5% rate and compounded daily, the actual return would be slightly higher—around 4.6%. Instead, banks quote the APY (annual percentage yield), which is the 4.6% number. That is the real amount you earn in a year if you leave your money untouched. When you use the APY in the formula above, you are already accounting for compounding.

Some older savings accounts compound monthly or quarterly instead of daily. The less frequently interest compounds, the slightly lower your true return. A 4.5% APY compounded monthly is not quite as good as 4.5% APY compounded daily, but the bank must quote the APY in both cases, so you can compare fairly.

How to verify your bank's interest calculation

Your monthly statement shows the interest posted, but you can check whether the number is correct. First, find your average daily balance for the month. Add up your balance for each day of the month, then divide by the number of days. If you had $5,000 for 9 days, $7,000 for 10 days, and $5,500 for 12 days, your average daily balance is ($5,000 × 9 + $7,000 × 10 + $5,500 × 12) ÷ 31 = $6,064.52.

Then use the straightforward formula: average daily balance × APY ÷ 365 × number of days in the month. For January with 31 days: $6,064.52 × 0.045 ÷ 365 × 31 = $23.68. This should be very close to what your bank posted (within a few cents, because of rounding).

If the number is significantly different—off by more than a dollar—contact your bank. It is rare, but errors happen. Most banks have online tools that show your daily balance, which makes this calculation easier than it used to be.

Why different banks pay different amounts on the same balance

Two banks offering the same APY will pay you the same amount of interest on the same balance for the same time period. The difference comes from when they credit the interest and how often they compound it.

A bank that compounds and credits interest daily will pay slightly more than one that compounds monthly, even if both quote the same APY. This is because daily compounding means your interest starts earning interest sooner. However, the difference is small—usually a few cents per month on a typical savings account balance.

The bigger difference is the APY itself. A bank offering 4.5% APY will pay roughly double what a bank offering 2.25% APY pays on the same balance. This is why shopping around for the highest APY matters more than worrying about compounding frequency. Over a year, the difference between 2% and 4.5% APY on $10,000 is about $250.

What happens when APY changes mid-month

Banks sometimes raise or lower their APY. If this happens mid-month, the bank calculates interest in two parts: the days before the rate change at the old rate, and the days after at the new rate.

Say your bank raised APY from 4.0% to 4.5% on January 15th. You have $5,000 in the account. The calculation would be: ($5,000 × 0.040 ÷ 365 × 14 days) + ($5,000 × 0.045 ÷ 365 × 17 days) = $7.67 + $10.47 = $18.14 for the month. Your statement should note the rate change, and you can verify the split calculation if you want to check the math.

Rate changes are usually announced in advance, and your bank will show the new rate in your account settings before it takes effect. If you see a rate change on your statement that you did not expect, your bank's customer service can explain what happened.

Using online calculators versus doing the math yourself

Many banks and financial websites offer interest calculators. You enter your balance, the APY, and the time period, and the calculator shows you the interest earned. These are accurate and save time, especially if you want to compare what different balances or rates would earn.

However, knowing the formula yourself is useful for two reasons. First, you can spot-check the calculator's answer to make sure it makes sense. Second, you can do quick mental math to estimate interest without a tool. If you have $10,000 at 4% APY, you earn roughly $400 a year, or about $33 a month. That mental math takes five seconds and helps you decide whether a rate is worth switching banks for.

The formula is straightforward enough that a spreadsheet can do it too. If you track your daily balance in a spreadsheet, you can add a column that calculates daily interest automatically, then sum it at the end of the month. This is useful if you want to see exactly how your deposits and withdrawals affect your earnings.

Frequently Asked Questions

Does interest compound daily or monthly on most savings accounts?

Most banks compound and credit interest daily, meaning they calculate how much you earned each day and add it to your balance. However, the APY they quote already accounts for this, so you do not need to worry about the compounding frequency when comparing accounts—just compare the APY.

If I withdraw money mid-month, do I lose all the interest I earned?

No. Banks calculate interest on your daily balance, so you earn interest on the money for the days it was in the account. If you had $5,000 for 15 days and then withdrew it, you earn interest on $5,000 for those 15 days. You do not earn interest on the days after the withdrawal.

Why does my bank's interest posting not match my calculation?

Small differences (a few cents) are normal and come from rounding. Banks round interest to the nearest cent. If your calculation is off by more than a dollar, check that you used the correct APY and counted the days correctly. If it still does not match, contact your bank—they can show you exactly how they calculated it.

Does the APY change if I keep money in the account longer?

No. APY is an annual rate, meaning it is the return you would get if you left money in the account for a full year. If you leave it in for six months, you earn half the APY. The rate itself does not change based on how long you keep the money, though the bank can change the rate they offer to new deposits at any time.

Can I earn more interest by moving money between accounts?

Moving money between your own accounts at the same bank does not change the interest you earn—you earn interest on whatever balance is in each account. Moving money to a different bank with a higher APY will earn you more, but you pay no interest during the transfer itself (usually one to three business days). The interest calculation resumes once the money settles in the new account.