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

Interest earned on a savings account is calculated by taking your account balance, multiplying it by the annual percentage yield (APY), and multiplying that by the fraction of the year the money sat in the account. The simplest version is: Balance × APY ÷ 365 × Number of Days = Interest Earned.

Most banks use daily compounding, which means they calculate interest on your balance each day, then add that interest back into your account so tomorrow's interest is calculated on a slightly larger balance. This compounds your money over time — you earn interest on your interest. The math gets more complex with compounding, but your bank does this automatically. You do not have to calculate it yourself; your statement shows the total interest posted each month.

What you need to know is what your bank is actually doing with your money and how much you should expect to see. A simple example: if you have $10,000 in an account earning 4.5% APY and leave it untouched for one year, you will earn roughly $450 in interest (before any taxes). If you withdraw $5,000 halfway through the year, the second half earns interest only on $5,000, so your total interest for the year would be closer to $225.

Key Takeaways

  • Daily compounding means your bank calculates interest on your balance each day and adds it back, so you earn interest on interest — but your statement shows the total, so you do not have to track it manually.
  • The basic formula is Balance × APY ÷ 365 × Number of Days, though banks use slightly different methods that produce nearly identical results.
  • Withdrawals reduce the balance on which interest is calculated, so pulling out money mid-month means you earn less interest for that period.
  • Your bank statement lists the total interest posted each month, so you can verify the math by adding up monthly interest to see if it matches your expected annual total.

Why your bank's method matters more than the formula

Banks use one of three methods to calculate interest: daily balance, average daily balance, or end-of-month balance. Most savings accounts use daily balance, which is the most generous to you because it credits interest on your actual balance each day. Some older accounts or money market accounts use average daily balance, which smooths out your balance over the month. A few use end-of-month balance, which only counts what you have on the last day — this is the least generous.

Your account agreement or the disclosures your bank sends you will state which method they use. If you cannot find it, call the bank or check their website. The difference between methods is usually small — a few dollars a year on a typical balance — but it matters if you are moving money in and out frequently.

The APY itself is what moves the needle most. A $10,000 balance at 0.01% APY earns $1 per year. The same balance at 4.5% APY earns $450. Shop around: high-yield savings accounts at online banks often pay 4% to 5%, while traditional brick-and-mortar banks may pay 0.01% to 0.5%. The difference compounds over years.

How to verify your interest calculation on your statement

Your monthly statement shows the interest posted that month. To check if it is roughly correct, take your average balance for the month, multiply by the APY, divide by 12, and compare. If your balance was steady at $10,000 and your APY is 4.5%, you should see about $37.50 in interest posted ($10,000 × 0.045 ÷ 12). If your balance fluctuated, use an average: if you had $10,000 for half the month and $5,000 for half, your average is $7,500, so expect about $28 in interest.

If the posted interest is significantly lower than your calculation, check three things: first, confirm the APY on your statement matches what you think you are earning (rates change, and some accounts have tiered rates). Second, check whether the bank uses daily compounding or a different method — the statement should say. Third, look at whether any fees were charged; some accounts deduct fees from interest. If none of that explains the gap, contact the bank.

Most banks post interest monthly, though some post it daily or quarterly. Check your statement to see when your bank credits it. Daily posting means your balance grows slightly each day; monthly posting means you see a lump sum once a month.

What happens when you withdraw money mid-month

If you withdraw money before the end of the month, you stop earning interest on that amount from the withdrawal date forward. Banks calculate this using the daily balance method: they track your balance each day and credit interest based on that daily amount. So if you have $10,000 on day 1 and withdraw $3,000 on day 15, you earn interest on $10,000 for 14 days and $7,000 for the remaining days of the month.

Some accounts have a minimum balance requirement to earn the stated APY. If your balance drops below the minimum, the bank may pay you a much lower rate (sometimes 0%) on the entire balance, not just the amount below the minimum. Check your account agreement to see if this applies to you. If you regularly dip below the minimum, you are losing money — consider moving to an account with no minimum or a lower one.

Deposits work the same way: money you deposit mid-month starts earning interest from the deposit date forward. If you deposit $5,000 on day 20, you earn interest on that $5,000 for only 11 days that month, not the full month.

How compounding multiplies your interest over time

Compounding is the reason your money grows faster than simple math suggests. With simple interest, you earn the same amount each year: $10,000 at 4.5% earns $450 every year, forever. With compounding, you earn interest on your interest. After one year, you have $10,450. In year two, you earn 4.5% on $10,450, which is $470.25 — not $450. The difference is small in year one but grows over decades.

Most savings accounts compound daily, which is the most frequent compounding available to consumers. Some compound monthly or quarterly, which is slightly less generous. The more often interest compounds, the more you earn. Over 10 years, daily compounding versus monthly compounding on $10,000 at 4.5% APY produces a difference of roughly $20 — not huge, but real.

The APY already includes the effect of compounding, so you do not have to calculate it separately. When a bank advertises 4.5% APY, that is the actual return you will get if you leave the money untouched for a year. The APR (annual percentage rate) is different — it does not include compounding — but savings accounts are quoted in APY, not APR.

Interest and taxes: what you owe the IRS

Interest earned on a savings account is taxable income. At the end of each year, your bank sends you a 1099-INT form listing the total interest you earned. You must report this on your tax return, even if it is only a few dollars. The IRS taxes it as ordinary income at your regular tax rate.

If you earned more than $10 in interest during the year, your bank is required to send you a 1099-INT. If you earned less, the bank may not send one, but you still owe tax on it. Keep your own records of interest posted each month so you can report the correct total if the bank does not send a form.

Some accounts, like Roth IRAs and traditional IRAs, have tax-advantaged treatment of interest — you do not owe tax on the interest until you withdraw the money (or in some cases, never). If you are saving for retirement, ask your bank whether they offer tax-advantaged savings accounts.

Comparing interest across different account types

Savings accounts, money market accounts, and certificates of deposit (CDs) all earn interest, but at different rates and with different rules. A regular savings account typically earns the lowest rate but lets you withdraw money anytime. A money market account pays slightly more but may require a higher minimum balance and limit withdrawals. A CD pays the most but locks your money away for a set term — if you withdraw early, you pay a penalty.

To compare, look at the APY, not just the interest rate. APY includes compounding, so it is the true return. A CD advertising 5% APY for one year will earn you $500 on $10,000. A savings account at 4.5% APY earns $450. The difference is $50 per year, but you cannot touch the CD money without penalty. Decide whether the extra $50 is worth locking up your cash.

Online banks almost always pay higher rates than brick-and-mortar banks because they have lower overhead. If you are not using an online bank, you are likely earning significantly less on your savings. Moving $10,000 from a 0.01% account to a 4.5% account means an extra $449 per year in your pocket.

Frequently Asked Questions

How often does interest post to my account?

Most banks post interest monthly, though some post daily or quarterly. Check your account agreement or call your bank to confirm. Daily posting means your balance grows slightly each day; monthly posting means you see a lump sum once a month. Either way, the total interest you earn over a year is the same.

What if my bank changes the APY mid-year?

Banks can change APY at any time, and they often do when the Federal Reserve changes rates. Your new rate applies to interest earned going forward, not retroactively. If your rate drops from 4.5% to 3.5%, interest posted next month will be calculated at 3.5%. You do not owe anything back.

Can I calculate interest if my balance changes every day?

Yes, but it is tedious. Add up your balance for each day of the month, divide by the number of days, multiply by the APY, and divide by 12. Your bank does this automatically using daily compounding, so your statement shows the correct total. You do not need to calculate it yourself — just verify it is in the ballpark using the average balance method.

Does a higher APY mean I should move my money?

Not automatically. If you are earning 0.01% and can move to 4.5%, yes — the difference is huge. But if you are moving from 4.4% to 4.5%, the extra $1 per year on $10,000 may not be worth the hassle of opening a new account. Consider switching if the rate difference is at least 0.5% and you have a substantial balance.

What happens to interest if I close my account mid-month?

You keep the interest earned up to the day you close. If you close on day 15 of a 30-day month, you earn interest on your balance for 15 days, and the bank posts that interest before closing the account. You will receive it as a final deposit or check, depending on how you close the account.