The basic formula: your balance, the rate, and the time period

Banks calculate savings account interest by multiplying three things: the money you have in the account, the interest rate the bank is paying, and how long that money sits there. The formula is Interest = Principal × Rate × Time. If you keep $1,000 in an account paying 4% annual interest for one full year, you earn $40. If you keep it for six months, you earn $20.

The catch is that most banks do not wait a full year to pay you. They calculate interest daily or monthly, then add it to your account on a schedule — usually monthly or quarterly. This matters because once interest is added to your account, the next calculation includes that interest too. That is called compounding, and it means you earn money on the money you already earned.

Key Takeaways

  • Banks calculate interest by multiplying your account balance by the annual interest rate, then dividing by the number of days in a year to find the daily amount.
  • Interest compounds, meaning the bank adds earned interest back to your balance, and the next calculation includes that added amount.
  • How often interest compounds — daily, monthly, or quarterly — changes how much total interest you earn over time.
  • The annual percentage yield (APY) on your account statement already includes the effect of compounding, so you can compare accounts directly.
  • Your actual interest earned depends on your lowest balance during the period, not your average or ending balance, at most banks.

How daily interest calculation works

Most savings accounts calculate interest on a daily basis. The bank takes your account balance at the end of each day, multiplies it by the annual interest rate, then divides by 365 (or 366 in a leap year) to find that day's interest. If your balance is $5,000 and the rate is 4.5% annual, the daily interest is roughly $0.62 per day.

The bank does this calculation every single day, even though you do not see the money appear in your account every day. Instead, the daily interest amounts add up during the month or quarter, and then the bank deposits the total into your account all at once. That deposit date is when compounding kicks in — your next day's calculation includes the interest you just earned.

Compounding: earning interest on your interest

Compounding is the reason a savings account grows faster than simple math suggests. After the bank deposits your first month's interest, your balance is now higher. The next month's interest calculation uses that higher balance, so you earn slightly more. The month after that, you earn even more, because your balance has grown again.

Over time, this effect becomes noticeable. A $10,000 deposit at 4% annual interest compounded monthly will grow to about $10,408 after one year. The same $10,000 at 4% simple interest (no compounding) would grow to only $10,400. The difference is small in year one, but it grows larger the longer the money sits there. After five years, compounding adds hundreds of dollars.

The frequency of compounding matters. Interest compounded daily grows faster than interest compounded monthly, which grows faster than interest compounded quarterly. Most online savings accounts compound daily, which is why they often pay higher total returns than brick-and-mortar banks that compound monthly or quarterly.

Annual Percentage Yield (APY) versus Annual Percentage Rate (APR)

Banks list two different rates on savings accounts: the Annual Percentage Rate (APR) and the Annual Percentage Yield (APY). The APR is the simple interest rate before compounding. The APY is the rate you actually earn after compounding is included. If a bank advertises 4% APY, that is the real return you will see over one year, assuming you do not add or withdraw money.

The difference between APR and APY is usually small — often less than 0.1% — but it matters when you are comparing accounts. Always compare APY to APY, not APR to APY. A bank advertising 4.5% APR with daily compounding might deliver 4.6% APY, while another bank offering 4.4% APY with monthly compounding will pay less total interest, even though the APR looks close.

The lowest balance method and how it affects your earnings

Most banks use the lowest balance method to calculate interest. This means the bank looks at the lowest amount of money in your account during the interest period — usually one month — and calculates interest based on that balance, not your average balance or your ending balance.

This rule can cost you money if you withdraw funds mid-month. If you keep $10,000 in the account for 29 days, then withdraw $5,000 on day 30, the bank calculates the entire month's interest based on $5,000, not the $10,000 you had for most of the month. Some banks use the average daily balance method instead, which is more generous — they add up your balance each day and divide by the number of days. Before opening an account, check which method the bank uses.

How interest rates change and what that means for your earnings

The interest rate on your savings account is not fixed. Banks raise and lower rates based on what the Federal Reserve does with its benchmark interest rate. When the Fed raises rates, banks usually raise savings rates within days or weeks. When the Fed cuts rates, banks cut savings rates too, though sometimes more slowly.

This means the APY you see today might not be the rate you earn all year. If you open an account at 4.5% APY in January and the Fed cuts rates in March, your bank might lower your rate to 4.0% APY starting in April. Your existing balance keeps earning at the new rate from that point forward. Check your account statement or log into your online account to see your current rate — it should be listed clearly.

What happens to interest if you close the account or withdraw money early

Interest accrues up to the day you close the account or make a withdrawal. If you close your account on the 15th of the month, you earn interest through the 15th, and the bank deposits that interest into your account (or sends it to you) when you close. You do not lose accrued interest by closing early — you just stop earning interest after that date.

Some savings accounts have withdrawal limits or penalties if you withdraw too often, but these are separate from interest calculations. The interest you have already earned is yours to keep. The only way to lose interest is if your bank charges a monthly fee that exceeds the interest you earned that month, which is rare at online banks but can happen at some brick-and-mortar institutions.

Frequently Asked Questions

Why does my bank say I earned $12 in interest but my balance only went up by $10?

Your bank likely charged a monthly maintenance fee. Interest is calculated and added separately from fees. If you earned $12 in interest but the bank charged a $2 monthly fee, your net gain is $10. Check your account statement for a line item labeled "service charge" or "monthly fee."

If I deposit money mid-month, does it earn interest right away?

Yes, but only from the day it is deposited. If you deposit $1,000 on the 15th of the month, that $1,000 earns interest from the 15th through the end of the month. The first full month of interest comes the following month. Most banks calculate daily, so the interest starts accruing the same day the deposit clears.

Does a higher APY always mean I will earn more money?

Yes, if all other factors are equal. A 4.5% APY will always pay more than a 4.0% APY on the same balance over the same time period. However, make sure you are comparing APY to APY, not APR to APY. Also check whether the higher rate is temporary — some banks offer promotional rates that drop after three or six months.

What is the difference between compounding daily and compounding monthly?

Daily compounding calculates and adds interest 365 times per year; monthly compounding does it 12 times. Over one year, daily compounding pays slightly more because interest starts earning interest sooner. The difference is usually less than 0.1% annually, but it grows larger over many years.

Can I predict exactly how much interest I will earn?

You can estimate it, but not predict it exactly, because interest rates change. Use your bank's interest calculator (most online banks have one on their website) with your current rate and balance. Remember that the rate may change, so the actual interest you earn could be higher or lower than your estimate.