The basic formula: multiply your balance by the APY, then divide by 365

To find out how much interest you'll earn in a year, take your account balance, multiply it by the APY (annual percentage yield) as a decimal, and that's your yearly interest. For example, if you have $10,000 in an account with a 4.50% APY, you multiply $10,000 × 0.045 = $450 per year.

To find daily interest, divide that yearly amount by 365. In the example above, $450 ÷ 365 = about $1.23 per day. Banks calculate and add interest daily or monthly depending on the account, but the yearly total stays the same.

The reason this works is that APY already includes the effect of compounding — the way interest earns interest. You don't need to do anything extra for that math. The APY number does the work for you.

Key Takeaways

  • Multiply your balance by the APY as a decimal to find your yearly interest: $10,000 × 0.045 = $450.
  • Divide the yearly interest by 365 to see what you earn per day, though banks may credit interest monthly or daily depending on their schedule.
  • APY already includes compounding, so you don't need to calculate that separately.
  • Your actual interest will be lower if your balance changes during the month, because interest is calculated on the balance that exists each day.
  • The APY can change at any time, so the interest you earn next month may differ from this month.

Why APY matters more than the interest rate

You might see two different numbers on a savings account: the interest rate and the APY. The interest rate is the raw percentage the bank pays. The APY is that rate plus the effect of compounding — earning interest on your interest.

For savings accounts, the difference is usually small, but it's real. A 4.50% interest rate might become 4.60% APY once compounding is included. Always use the APY for your calculation, because that's the actual return you'll receive.

How daily balance changes affect your earnings

The formula above assumes your balance stays the same all month. In real life, you deposit and withdraw money, so your balance changes. Banks handle this by calculating interest on the balance that exists each day, then adding those daily amounts together.

You don't have to do this math yourself — the bank does. But it means that if you deposit $5,000 on the 15th of the month, that $5,000 only earns interest for the remaining 16 days, not the full month. Similarly, if you withdraw $3,000, the interest on that $3,000 stops accruing the day after the withdrawal.

This is why the interest you actually receive might be slightly different from what you calculated using the straightforward formula. The formula gives you a reasonable estimate if your balance is fairly stable, but the actual amount depends on the exact timing of your deposits and withdrawals.

Monthly versus daily compounding

Some accounts credit interest monthly; others do it daily. This affects when you see the money in your account, but not how much you earn in a year. Whether the bank adds $1.23 every day or waits and adds $37 at the end of the month, you end up with the same $450 yearly.

Daily compounding does have one small advantage: if you leave the interest in the account, it starts earning interest itself the next day. With monthly compounding, you wait until the end of the month. Over many years, this tiny difference adds up, but for most people it's negligible.

What happens when the APY changes

Banks can raise or lower the APY at any time. When they do, your interest earnings change when ready. If your account is paying 4.50% and the bank drops it to 4.00%, next month's interest will be calculated on the lower rate.

This is why it's worth checking your account's APY every few months. If it drops significantly and other banks are offering more, you might consider moving your money. High yield savings accounts compete on rate, so banks that lose customers to higher rates often raise their own rates back up within weeks.

Using a calculator versus doing it by hand

The math is straightforward enough to do on any calculator or even on paper. Multiply your balance by the APY as a decimal. That's it. You don't need special tools or software.

Some banks and financial websites offer interest calculators where you enter your balance and APY, and they show you the result. These are convenient but not necessary — they do the same multiplication you can do yourself. The value is mainly in seeing the number spelled out clearly, which can help you decide whether an account's rate is worth your attention.

Frequently Asked Questions

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

The bank calculates and credits all interest automatically. You don't have to do anything. Calculating it yourself is just a way to understand how much you're earning or to compare accounts before you open one.

What's the difference between APY and APR?

APY includes compounding; APR does not. For savings accounts, always use APY. APR is mainly used for loans and credit cards. A savings account's APY will always be slightly higher than its APR because of compounding.

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

No. Interest is calculated daily on whatever balance exists each day. If you withdraw money on the 15th, you keep the interest earned from the 1st through the 14th. You just stop earning interest on the withdrawn amount starting the 15th.

Can I predict exactly how much interest I'll earn next month?

Only if your balance doesn't change. If you make deposits or withdrawals, the actual interest will differ slightly from the straightforward calculation because interest is earned only on the balance that exists each day. The difference is usually small.

Why do different banks offer different APYs for savings accounts?

Banks set their own rates based on how much they need deposits and what they can earn by lending that money out. Online banks often offer higher rates than brick-and-mortar banks because they have lower operating costs. Rates also move up and down with the Federal Reserve's interest rate decisions.