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

Interest earned on a savings account comes from one of two formulas, depending on whether your bank compounds interest daily, monthly, or annually. The simpler version is simple interest: multiply your account balance by the annual percentage yield (APY) and divide by the number of days in a year, then multiply by the number of days your money sat in the account. Most savings accounts use compound interest instead, which means the bank calculates interest on your interest, and the math is handled by your bank's system — but you can still work backward from your statement to verify the number.

The reason this matters: a bank's advertised APY assumes your money stays untouched for a full year. If you deposit money mid-month, withdraw it early, or add to the account, the actual interest you earn will be different. Knowing how to check the math protects you from errors and helps you compare accounts honestly.

Key Takeaways

  • Simple interest is calculated once per year as: (balance × APY ÷ 365) × number of days, and most savings accounts do not use this method.
  • Compound interest recalculates daily or monthly, adding earned interest back to your balance so you earn interest on interest, and your bank does this calculation automatically.
  • The APY shown on your account statement already includes the effect of compounding, so you do not need to calculate it yourself — but you can verify it by checking your monthly statements.
  • Deposits and withdrawals change when interest is calculated, so money added mid-month earns less than money that was there the whole month.
  • You can request an interest calculation breakdown from your bank if the number on your statement does not match your own math.

How simple interest works (and why your account probably does not use it)

Simple interest is the easiest version to calculate by hand. The formula is: Interest = Principal × Rate × Time, where principal is your balance, rate is the APY as a decimal, and time is the fraction of a year.

Example: You deposit $5,000 in an account with a 4.5% APY. You leave it untouched for one full year. Interest earned = $5,000 × 0.045 × 1 = $225.

If you only left the money in for six months: Interest = $5,000 × 0.045 × 0.5 = $112.50.

If you left it in for 90 days: Interest = $5,000 × 0.045 × (90 ÷ 365) = $55.48.

Most savings accounts do not use simple interest because banks want to encourage you to keep money in the account longer. Compound interest earns you more, which makes the account look more attractive.

How compound interest works and why it earns more

Compound interest means the bank adds the interest you earned to your balance, then calculates interest on that new, larger balance. This happens daily, monthly, or quarterly depending on the account. Each time it compounds, you earn interest on the interest from the previous period.

The formula for compound interest is: Final Balance = Principal × (1 + (Rate ÷ Compounds per Year)) ^ (Compounds per Year × Years). For most savings accounts, "compounds per year" is 365 (daily compounding).

Example: You deposit $5,000 at 4.5% APY with daily compounding, left untouched for one year. Final Balance = $5,000 × (1 + (0.045 ÷ 365)) ^ (365 × 1) = $5,230.68. You earned $230.68 instead of $225 — that extra $5.68 is interest on interest.

The longer your money stays in the account and the more often it compounds, the more the difference grows. After five years at the same rate, simple interest would earn you $1,125, but daily compound interest would earn you $1,248.88 — a difference of $123.88.

Why your bank's calculation might differ from yours

Banks use the average daily balance method to handle deposits and withdrawals during the month. They add up your balance for each day of the month, divide by the number of days, then apply the interest formula to that average. This is why a deposit made on the first of the month earns more than one made on the last day.

Example: You start with $5,000 on day one. On day 15, you deposit $2,000. On day 30, you withdraw $1,000. The bank calculates: (($5,000 × 14 days) + ($7,000 × 15 days) + ($6,000 × 1 day)) ÷ 30 = $6,233.33 average daily balance. Interest is then calculated on that average, not on any single balance.

Some banks use the daily balance method instead, which calculates interest on each day's actual balance and adds it up. This usually results in slightly more interest than the average daily balance method, but the difference is small.

Your bank's disclosure documents (usually called the Truth in Savings Act disclosure or account terms) will state which method they use. If you cannot find it, call and ask.

How to verify the interest on your statement

Start with your monthly statement. Look for a line that says "Interest Earned," "Interest Paid," or "Interest Credited." This is the amount the bank calculated for that month.

Next, find your average daily balance for the month. Many banks show this on the statement; if not, you can calculate it yourself by adding your balance for each day and dividing by the number of days in the month. Alternatively, call your bank and ask them to provide it.

Once you have the average daily balance and the monthly interest earned, you can work backward to find the monthly rate: Monthly Interest = Average Daily Balance × Monthly Rate. So Monthly Rate = Monthly Interest ÷ Average Daily Balance.

Example: Your statement shows $8.75 in interest earned and an average daily balance of $5,200. Monthly Rate = $8.75 ÷ $5,200 = 0.001682 (or about 0.168% per month). Multiply by 12 to get the annual rate: 0.168% × 12 = 2.016%. This should be close to your account's stated APY (the small difference accounts for rounding and the compounding effect).

What changes the interest you actually earn

The APY advertised by your bank assumes a full year of deposits sitting untouched. Several things change the actual amount you receive:

Timing of deposits. Money deposited mid-month earns interest only from that day forward, not from the first of the month. Money deposited on the last day of the month earns almost no interest that month.

Withdrawals. When you withdraw money, you stop earning interest on that amount immediately. If you withdraw before the interest is credited, you lose that month's interest on the withdrawn amount.

Rate changes. Banks change their APY frequently. If your rate drops mid-month, the interest calculation uses the old rate for the days before the change and the new rate for the days after. Your statement should show the exact dates the rate was in effect.

Minimum balance requirements. Some savings accounts pay the stated APY only if you maintain a minimum balance. If your balance drops below that threshold, you may earn a lower rate or no interest at all for that period.

Comparing accounts using APY, not just the interest rate

Banks sometimes advertise a "rate" separate from the APY. The rate is the simple interest percentage; the APY includes the effect of compounding. Always compare accounts using APY, not the rate, because APY tells you what you will actually earn.

Example: Account A offers 4.40% APY. Account B offers 4.35% APY. Account A will earn you more money, even though the difference looks small. On $10,000 for one year, Account A earns $450.40 and Account B earns $445.30 — a difference of $5.10. Over five years, that gap widens to $28.50.

The APY also accounts for how often the bank compounds interest. A bank that compounds daily will show a higher APY than one that compounds monthly, even if the underlying rate is the same, because daily compounding generates more interest on interest.

Frequently Asked Questions

Do I need to calculate interest myself, or does the bank do it?

Your bank calculates and credits the interest automatically. You do not need to do the math yourself. The interest appears on your statement each month, and the bank adds it to your balance. You can verify the calculation if you want to check for errors, but the bank handles it.

What is the difference between APY and APR?

APY (annual percentage yield) includes the effect of compounding and is what you actually earn on savings. APR (annual percentage rate) is used for loans and does not include compounding. For savings accounts, always look at the APY.

If I withdraw money before the end of the month, do I lose all the interest?

No. Interest is usually credited on the last day of the month or the day your statement closes. If you withdraw before that date, you lose interest only on the amount you withdrew, not on the money that stays in the account. Some banks have penalties for early withdrawal, but most savings accounts do not.

Why does my interest calculation not match the APY exactly?

The APY assumes your money stays in the account for a full year untouched. If you made deposits or withdrawals, or if the rate changed during the month, your actual interest will be different. Also, banks round interest to the nearest cent, which can create small discrepancies when you calculate by hand.

Can I earn more interest by moving money between accounts?

No. Interest is calculated based on how long your money sits in each account. Moving money between accounts does not change the total interest you earn — it just splits it across multiple accounts. You earn more interest by depositing money and leaving it untouched, or by finding an account with a higher APY.