How banks calculate the interest you earn

Banks calculate savings account interest by multiplying three things: your account balance, the annual interest rate, and the time period the money sits in the account. The formula is straightforward: Interest = Balance × Annual Rate × Time. If you keep $1,000 in an account earning 4.5% annually for one full year, you earn $45. The catch is that most banks don't wait a full year to pay you—they calculate interest daily and add it to your account monthly or quarterly.

The timing matters because of how the calculation compounds. When a bank says it pays interest "daily," it means it calculates what you've earned each day based on your balance that day, then adds all those daily amounts together at the end of the month or quarter. This is called daily compounding, and it means money you earn early in the month starts earning interest on itself for the rest of the month.

Different banks compound at different intervals. Some compound daily, some weekly, some monthly. The more frequently interest compounds, the more total interest you earn over time, because you're earning interest on your interest. The difference is usually small—a few dollars per year on a typical balance—but it compounds faster the longer your money stays in the account.

Key Takeaways

  • Banks calculate daily interest by dividing the annual rate by 365, multiplying by your balance, then adding those daily amounts together when they credit your account.
  • Compounding frequency (daily, weekly, or monthly) affects how much total interest you earn, because interest earned early in the period starts earning interest on itself.
  • The annual percentage yield (APY) shown on a bank's website already includes the effect of compounding, so it's the number to compare between accounts.
  • Your balance matters more than the rate—doubling your balance doubles your interest, but moving from 4.5% APY to 5% APY increases interest by only about 11%.
  • Interest posts to your account on a schedule (usually monthly or quarterly), not continuously, so you won't see it appear every day even though it's calculated daily.

The difference between APR and APY

Banks advertise two different rates, and they mean different things. APR (annual percentage rate) is the straightforward interest rate before compounding—the raw percentage. APY (annual percentage yield) is the rate you actually earn after compounding is factored in. APY is always higher than APR on a savings account, because it includes the effect of earning interest on your interest.

When you're comparing savings accounts, always look at the APY, not the APR. A bank might advertise 4.5% APR, but if it compounds daily, the actual APY might be 4.60%. That 0.10% difference sounds small until you do the math: on $10,000, that's about $10 per year in extra earnings. On $100,000, it's $100 per year. Banks are required to show you the APY prominently, usually near the interest rate on their website or in account disclosures.

How to calculate interest yourself

You can calculate your own interest to verify what a bank is paying you or to estimate what you'll earn in a new account. The simplest method uses the APY and assumes your balance stays the same all year: Annual Interest = Balance × APY. If you have $5,000 at 4.5% APY, you'll earn about $225 per year, or roughly $18.75 per month.

If your balance changes during the year—because you deposit or withdraw money—the calculation gets more complex, but most banks do this work for you. Some banks show a running interest total in your online account, updated daily or weekly. If yours doesn't, you can ask customer service to tell you how much interest you've earned year-to-date, and they can pull that from their system in seconds.

For a rough estimate of interest on a changing balance, add up your balance on the first day of each month, divide by 12, then multiply by the APY. This gives you an average-balance method that's close enough for planning purposes. If you had $5,000 on January 1, $6,000 on February 1, and $5,500 on March 1, your average for those three months is about $5,500, so you'd earn roughly $206 per year at 4.5% APY.

Why your interest rate can change

Savings account interest rates are not locked in. Banks change them based on what the Federal Reserve does with its benchmark interest rate, which it adjusts several times per year. When the Fed raises rates, banks typically raise savings rates within days or weeks. When the Fed cuts rates, banks cut savings rates more slowly, but they do cut them.

You might open an account at 4.5% APY and see it drop to 4.0% six months later if the Fed cuts rates. This is normal and happens to every customer at that bank—it's not a penalty or a change in your account terms. The rate you see when you open the account is not a may provide for the life of the account. If you want to lock in a higher rate, some banks offer certificates of deposit (CDs), which may provide a fixed rate for a set period (usually three months to five years), but your money is locked away and you pay a penalty if you withdraw early.

How interest posts to your account

Interest doesn't appear in your account the moment it's calculated. Banks calculate it daily but post it on a schedule—usually monthly or quarterly. On the posting date, the interest amount appears as a deposit in your account, and your balance goes up. You can then withdraw that interest or leave it in the account to earn interest on itself next month.

The posting date varies by bank. Some post on the last day of the month, some on the first day of the next month, some on a specific date like the 15th. Check your account statements or ask your bank when interest posts. This matters if you're trying to time a large withdrawal—if interest posts on the 30th and you withdraw on the 29th, you'll miss that month's interest.

How to find the best interest rate

Interest rates vary widely between banks. A traditional bank branch might offer 0.01% APY on savings, while an online bank might offer 4.5% or higher on the same type of account. The difference comes down to overhead: online banks have lower costs, so they pass higher rates to customers. Brick-and-mortar banks have physical locations and staff, which costs more.

To compare rates, visit several banks' websites and note the APY for their basic savings account. Look for accounts with no monthly fees, no minimum balance requirements, and no restrictions on how often you can withdraw. Some banks offer higher rates if you maintain a large balance or set up automatic deposits, so read the fine print. Websites like Bankrate and DepositAccounts list rates from many banks in one place, though you should verify the current rate on the bank's own website before opening an account, because rates change frequently.

What happens to interest if you close your account

If you close your account before interest posts, you lose the interest earned up to that point. If interest posts on the 30th and you close on the 25th, the bank keeps the interest. However, if you close on the 30th or later, after interest has posted, you keep it. This is another reason to know when your bank posts interest—if you're planning to close an account, wait until after the posting date to do it.

Some banks will mail you a final statement showing interest earned through the closing date, even if it hasn't posted yet. Ask your bank about this before you close. If you're moving money to another bank, the new bank won't credit you for interest you earned at the old bank—only the old bank pays that.

Frequently Asked Questions

Does interest compound if I don't touch my account?

Yes. Interest compounds automatically whether you withdraw it or leave it in the account. If you leave interest in the account, it becomes part of your balance and earns interest on itself the next compounding period. This is called earning interest on interest, and it's why leaving money untouched for years builds faster than you might expect.

Why is my interest so low compared to the advertised rate?

The advertised rate is annual—it's what you'd earn if your balance stayed the same for a full 12 months. If you've only had the account for a few months, or if your balance is lower than when you opened it, your actual interest will be proportionally lower. Also check that you're looking at APY, not APR. If you opened the account recently, interest might not have posted yet.

Can I lose money if interest rates drop?

No. Interest rates dropping means you'll earn less interest going forward, but you won't lose the principal (the money you deposited) or the interest you've already earned. Your balance can only go down if you withdraw money or if the bank charges fees. Interest rates dropping is different from investment losses, which can reduce your principal.

What's the difference between a savings account and a money market account?

Money market accounts often pay slightly higher interest than savings accounts, but they usually require a larger minimum balance and limit how many withdrawals you can make per month. Both are FDIC-insured up to $250,000. If you need frequent access to your money, a savings account is usually better. If you have a large balance and rarely withdraw, a money market account might pay more.

Does the bank owe me interest if it makes a mistake in my favor?

If the bank credits you with more interest than you actually earned, they will eventually catch it and reverse the overpayment. Banks reconcile accounts regularly and audit interest calculations. It's not worth spending the overpayment—you'll have to return it, and the bank may charge you fees for the negative balance.