The math is simpler than you think, and your bank already does it
Your savings account earns interest based on the balance you hold and the rate your bank pays. To find out how much you'll earn in a month, you multiply your balance by the annual interest rate, then divide by 12. That's the whole calculation. Your bank runs this math daily or monthly depending on how they compound interest, but you can do it yourself in under a minute with a calculator or a spreadsheet.
The reason to calculate it yourself is not to check your bank's math—they're required by law to get it right—but to see whether the rate you're earning is worth keeping your money there. A savings account paying 0.01% annually will earn you almost nothing. One paying 4.5% will earn you something real. Knowing the actual dollar amount makes the difference obvious.
Key Takeaways
- Monthly interest = (Your balance × Annual interest rate) ÷ 12, and this gives you the amount your bank will add to your account that month.
- The annual percentage yield (APY) printed on your account disclosure is the rate to use; it already accounts for how often interest compounds.
- Interest compounds, meaning you earn interest on interest, so your actual earnings grow slightly faster than a straightforward monthly calculation suggests over time.
- Your bank calculates and deposits interest on a schedule—usually monthly or daily—so the timing of deposits and withdrawals within a month affects what you actually earn.
- Comparing rates across banks matters: the difference between 0.01% and 4.5% APY on $10,000 is roughly $450 per year.
The basic formula and what each number means
The calculation uses three pieces of information: your account balance, the annual interest rate, and the number of months in a year.
Monthly interest = (Balance × Annual rate) ÷ 12
If you have $5,000 in a savings account earning 4.5% APY, the math looks like this: ($5,000 × 0.045) ÷ 12 = $18.75 per month. Your bank will add $18.75 to your account that month (assuming your balance stays at $5,000 and the rate doesn't change).
The rate you use must be the annual percentage yield (APY), not the annual percentage rate (APR). APY includes the effect of compounding—the way interest earns interest—so it's the true number to use. Your bank is required to show you the APY on your account disclosure statement and on their website. It's usually listed next to the account type or in a table comparing different savings products.
Why your actual earnings may differ from the straightforward calculation
The formula above assumes your balance stays the same all month. In reality, most people deposit and withdraw money, so the balance changes. Banks handle this in different ways.
Some banks calculate interest on the average daily balance—they add up what you had each day of the month and divide by the number of days. Others use the ending balance on the last day of the month. A few use the minimum balance you held during the month. Your account disclosure tells you which method your bank uses. If you want to know exactly what you'll earn, you need to know this.
If your bank uses average daily balance and you deposit $5,000 on the first day of a 30-day month, you earn interest on $5,000 for all 30 days. If you deposit it on the 15th, you earn interest on $5,000 for only 15 days, so your interest that month is cut roughly in half. The formula still works—you just plug in the average balance instead of the starting balance.
Interest also compounds, which means you earn interest on the interest your account already earned. If your bank compounds monthly, the interest from month one gets added to your balance before month two's interest is calculated. This makes your money grow slightly faster than straightforward math suggests, but the effect is small in the first few months and only becomes noticeable over years.
How to calculate interest on a changing balance
If you know your balance will change during the month, calculate the average daily balance first, then use that in the formula.
Here's an example: You start the month with $5,000. On day 15, you deposit $2,000. The account earns 4.5% APY. For days 1–14, you had $5,000 (14 days). For days 15–30, you had $7,000 (16 days). Average daily balance = [($5,000 × 14) + ($7,000 × 16)] ÷ 30 = $6,067. Then: ($6,067 × 0.045) ÷ 12 = $22.75 for the month.
If you make multiple deposits or withdrawals, the math gets tedious to do by hand. A spreadsheet is faster: create a column for each day, enter the balance for that day, then use the AVERAGE function to find the average daily balance. Then multiply by the annual rate and divide by 12.
Many online banks show you the interest earned in real time on your account dashboard, so you don't have to calculate it yourself. If yours does, you can verify the number using the formula above to make sure it's correct.
Comparing what different rates actually earn you
The difference between a low rate and a high rate becomes clear when you see the dollar amounts. Here's what $10,000 earns per month at different rates:
| Annual Rate (APY) | Monthly Earnings on $10,000 | Annual Earnings on $10,000 |
|---|---|---|
| 0.01% | $0.08 | $1.00 |
| 0.5% | $4.17 | $50.00 |
| 2.0% | $16.67 | $200.00 |
| 4.5% | $37.50 | $450.00 |
| 5.0% | $41.67 | $500.00 |
The rates shown here reflect what was available at different times. Rates change based on what the Federal Reserve does with its benchmark rate. When you're shopping for a savings account, use the formula to calculate what you'd actually earn at each bank's current rate, then compare. A bank paying 4.5% on $10,000 earns you $450 a year more than a bank paying 0.01%. That's real money.
When interest posts and how it affects your balance
Banks don't add interest to your account every single day, even though they calculate it daily. Most post interest monthly, on a set date. Some post it quarterly or even annually. Your account disclosure tells you the schedule.
When interest posts, it becomes part of your balance. If you earned $18.75 in interest during the month and your balance was $5,000, your new balance is $5,018.75. Next month, if the rate stays the same and your balance stays the same, you earn interest on $5,018.75, not $5,000. That's compounding at work.
The posting date matters if you're about to withdraw money. If you withdraw your balance the day before interest posts, you miss that month's interest. If you withdraw the day after, you keep it. Check your account statement to see when interest typically posts, and plan large withdrawals accordingly if the amount is significant.
Using a spreadsheet to track earnings over time
If you want to see how your balance grows month by month with compounding, a spreadsheet makes it straightforward. Create columns for the month, starting balance, interest earned, and ending balance. In the interest column, use the formula =(starting balance × annual rate) ÷ 12. In the ending balance column, add the starting balance and interest earned. The ending balance of one month becomes the starting balance of the next.
This shows you the real power of compounding over time. On $10,000 at 4.5% APY, you earn $37.50 in month one. In month two, you earn interest on $10,037.50, so you earn $37.64. The difference is tiny at first, but after a year, compounding adds about $23 to what you'd earn with straightforward interest. After five years, the difference is over $120. The longer your money sits, the more compounding matters.
Frequently Asked Questions
Do I need to calculate interest myself, or does my bank do it for me?
Your bank calculates and deposits interest automatically. You don't need to do anything. Calculating it yourself is useful only if you want to verify the amount, compare rates across banks, or project what you'll earn over time. Your account statement shows the interest you earned each month.
What's the difference between APY and APR for savings accounts?
APY (annual percentage yield) includes the effect of compounding and is the true rate you earn. APR (annual percentage rate) does not include compounding. For savings accounts, always use APY. Banks are required to show you the APY on disclosures and websites.
If I withdraw money mid-month, do I lose the interest I earned?
It depends on your bank's method. If they use average daily balance, you earn interest only on the days you held the money. If they use ending balance, you earn nothing that month if your balance is zero on the last day. Check your disclosure to know which applies to your account.
How often should I recalculate my interest to check if my bank is paying correctly?
You don't need to check regularly. Banks are required by law to calculate interest correctly, and errors are rare. If you want to verify once, calculate the interest for one month using the formula and compare it to what your statement shows. If they match, your bank is doing it right.
Will my interest earnings change if the bank changes its rate?
Yes. If your bank lowers or raises the APY, the interest you earn in future months changes when ready. Interest already posted to your account stays there. Your next statement will show the new rate and the new monthly earnings based on it.