Banks multiply your balance by an interest rate, then divide by the number of days in a year

Most savings accounts use daily compounding, which means the bank calculates interest on your balance every single day, then adds that interest back into your account. The formula is straightforward: your current balance multiplied by the annual interest rate, divided by 365 (or 360, depending on the bank), then multiplied by the number of days the money sat in the account.

The result is that interest compounds — meaning you earn interest on the interest you already earned. If you deposit $1,000 at a 4.50% annual rate and never touch it, after one day you earn roughly $0.12. The next day, the bank calculates interest on $1,000.12, not just $1,000. Over a year, this compounding adds up to noticeably more than simple interest would.

The actual rate you see advertised is called the Annual Percentage Yield (APY), and it already accounts for compounding. So if a bank shows you 4.50% APY, that is the real return you will get over a year if you leave the money untouched — you do not need to do any math yourself.

Key Takeaways

  • Banks calculate interest daily by multiplying your balance by the annual rate and dividing by 365, then add that amount to your account.
  • Daily compounding means you earn interest on interest, which is why your balance grows faster than simple math would suggest.
  • The APY shown by the bank already includes the effect of compounding, so you can compare rates directly between banks without adjusting them.
  • Interest rates vary by bank and change over time, so the rate you see today may be different next month.
  • Some banks use 360 days instead of 365 in their calculation, which slightly reduces the interest you earn.

Why the bank uses 365 days, not the actual days in your account

The daily interest calculation uses a fixed 365-day year, not the actual number of days between deposits or withdrawals. This is called the 365/365 method or actual/365 method, and it is the standard for most consumer savings accounts in the United States.

A smaller number of banks use the 360/360 method, which divides by 360 instead of 365. This sounds like a minor difference, but it reduces your interest by roughly 1.4% over a year. If you earn $45 in interest at a 365-day bank, you would earn about $44.37 at a 360-day bank, all else equal. Always check which method your bank uses — it is usually in the account disclosure document.

How compounding frequency changes your earnings

Most savings accounts compound daily, but some older or specialty accounts compound monthly, quarterly, or annually. The more often interest compounds, the more you earn, because each compounding event adds interest to the balance that will earn interest in the next period.

The difference is small at lower rates but becomes visible at higher rates. At 0.01% APY, daily versus monthly compounding makes almost no difference. At 4.50% APY, daily compounding earns you roughly $0.15 more per year on a $1,000 balance than monthly compounding would. The APY already reflects the compounding frequency, so you do not need to recalculate — just compare APYs directly.

When you see a bank advertising a rate, always check whether it is APY or just the annual percentage rate (APR). APY includes compounding; APR does not. Savings accounts should always show APY.

When interest posts to your account and when you can use it

Interest is calculated daily, but it does not post (appear in your balance) every day. Most banks post interest monthly, on the last day of the month or the first day of the next month. Some post quarterly or even annually, though this is rare for consumer savings accounts.

Once interest posts, it becomes part of your balance and you can withdraw it immediately. Until it posts, it is calculated but not yet in your account. This matters if you are tracking your balance closely or planning a withdrawal — the interest you earned this month may not show up until the first of next month.

If you close the account before interest posts, you typically lose the unposted interest. Check your account agreement to confirm the posting schedule and what happens to accrued interest if you close early.

How rate changes affect your interest earnings

Banks change savings rates frequently, sometimes weekly. When a rate changes, it applies to your balance going forward, not retroactively. If your rate drops from 4.50% to 4.25% on the 15th of the month, you earn 4.50% on the balance from the 1st through the 14th, then 4.25% from the 15th onward.

The bank must notify you before lowering your rate, usually by email or through your online account. The notification period varies — some banks give 30 days' notice, others give less. Read the notification carefully to see when the new rate takes effect.

If rates rise, your bank may or may not raise your rate automatically. High-yield savings accounts at online banks tend to raise rates quickly when the Federal Reserve raises rates. Traditional banks and credit unions may lag behind. If your rate has not moved in months while others have risen, it may be worth comparing to other banks.

What happens to interest if you withdraw money mid-month

Interest is calculated on your balance each day, so if you withdraw money, you stop earning interest on that amount immediately. If you have $5,000 on the 1st and withdraw $2,000 on the 15th, you earn interest on $5,000 for 14 days and $3,000 for the remaining days of the month.

Some accounts have a minimum balance requirement to earn interest. If your balance drops below the minimum, you may earn no interest for that month, or you may earn interest only on the amount above the minimum. Check your account agreement to see if this applies to you.

Frequent withdrawals do not hurt your interest rate itself, but they do reduce the balance earning interest, so your total interest earnings go down. If you need to access money regularly, a savings account is still the right choice, but you will earn less interest than someone who leaves the money untouched.

How to compare interest rates between banks

Always compare APY, not just the advertised rate. APY accounts for compounding and the bank's calculation method, so it is the true number to use. A bank showing 4.50% APY will earn you more than a bank showing 4.48% APY, assuming you leave the money in for a full year.

Check whether the rate is promotional or permanent. Some banks offer a higher rate for the first few months, then drop it. The fine print usually says "introductory rate" or "promotional rate" and tells you when it expires. If you plan to keep the money in the account for years, a slightly lower permanent rate may be better than a high introductory rate.

Look at the minimum balance required to earn the advertised rate. Some banks only pay the full APY if you maintain a certain balance — $25,000, for example. If your balance is lower, you may earn a much lower rate. Confirm that the rate shown applies to your expected balance.

Frequently Asked Questions

Does interest compound on interest I have already earned?

Yes. Once interest posts to your account, it becomes part of your balance and earns interest in the next compounding period. This is called compounding, and it is why your balance grows faster over time than simple interest would suggest.

What is the difference between APY and APR on a savings account?

APY (Annual Percentage Yield) includes the effect of compounding and shows your true annual return. APR (Annual Percentage Rate) does not include compounding. Savings accounts should always show APY. If you see only APR, the actual return will be slightly higher.

If my bank uses 360 days instead of 365, how much less do I earn?

You earn roughly 1.4% less per year. On $1,000 earning 4.50% APY, a 360-day bank would pay you about $44.37 instead of $45. The difference grows with larger balances and higher rates.

Can I lose interest if I withdraw money before the end of the month?

No, you earn interest on your balance for each day the money is in the account. If you withdraw on the 15th, you keep the interest earned from the 1st through the 14th. You simply stop earning interest on the withdrawn amount after that date.

Why do some banks pay higher interest than others?

Online banks typically pay more because they have lower overhead costs than traditional banks with physical branches. Banks also adjust rates based on their funding needs and the Federal Reserve's interest rate decisions. Shop around — rates vary significantly even for the same type of account.