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

Interest on a savings account works like this: the bank takes the money you deposit, uses it to lend to other customers or invest, and pays you a small percentage of what you left with them. That percentage is called the interest rate. The bank calculates how much you earn by multiplying your account balance by that rate, then breaking it into smaller daily or monthly payments so you earn a little bit constantly rather than all at once.

The actual math is straightforward. If your account holds $1,000 and the bank offers 4% annual interest, you would earn $40 per year if the balance never changed. But most banks don't wait a year to pay you. Instead, they calculate what you've earned each day and add it to your account monthly or daily, depending on the account. This means you start earning interest on your interest — a process called compounding — which makes your money grow slightly faster than the straightforward math suggests.

Key Takeaways

  • Banks calculate daily interest by dividing your account balance by 365 (or 366 in a leap year), multiplying by the annual interest rate, then adding that amount to your account each day.
  • Compounding means you earn interest on the interest the bank already paid you, so your balance grows faster than a straightforward percentage would suggest.
  • The interest rate your account earns changes based on what the Federal Reserve does with its own rates, so the rate you see today may be different in three months.
  • Different banks offer different rates for the same type of account, so comparing rates before you open an account can mean earning significantly more over a year.

How the daily calculation actually works

Most banks calculate interest daily, even if they only deposit it into your account once a month. Here's the step-by-step process: the bank takes your balance at the end of each day, divides it by 365 days, multiplies that by the annual interest rate, and that's what you earn that single day. They repeat this for every day of the month, then add all those daily earnings to your account at once, usually on the first day of the next month.

This matters because your balance changes throughout the month. If you deposit $5,000 on the 15th, you only earn interest on that $5,000 for the remaining days of the month, not the whole month. If you withdraw $2,000 on the 20th, the bank stops earning interest on that $2,000 from that day forward. Some banks use your lowest balance during the month instead of calculating daily, which means you earn less — so it's worth asking your bank which method they use.

What compounding means for your money

Compounding is the reason your savings grow faster than you might expect. When the bank deposits your monthly interest into your account, that interest becomes part of your balance. The next month, you earn interest not just on your original deposit, but on the interest from the previous month too. Over time, this creates a snowball effect where your money grows on itself.

The difference is small in the first few months, but it adds up. On a $10,000 balance earning 4% annual interest, you'd earn about $40 in the first month. In the second month, you earn interest on $10,040, not just $10,000 — so you earn slightly more than $40. By the end of a year, you'll have earned more than the straightforward $400 calculation would suggest. The longer your money sits in the account, the more noticeable this effect becomes.

Why interest rates change and what affects yours

The interest rate your bank offers is not fixed forever. Banks set their rates based partly on what the Federal Reserve — the central banking system of the United States — does with its own rates. When the Federal Reserve raises its rates, banks usually raise the rates they offer on savings accounts. When the Federal Reserve lowers its rates, banks typically lower yours too. This can happen several times a year, so the rate you locked in three months ago may be different today.

Your personal rate also depends on the type of account. A regular savings account usually earns less interest than a money market account or a certificate of deposit (CD). Banks also compete with each other, so online banks often offer higher rates than brick-and-mortar banks because they have lower costs. Shopping around before you open an account — or moving your money to a higher-rate account if your current bank's rate drops — can make a real difference in how much you earn.

The difference between APY and APR

Banks use two different numbers to describe interest rates: APR (annual percentage rate) and APY (annual percentage yield). APR is the straightforward yearly rate without compounding — it's the percentage the bank quotes. APY includes the effect of compounding, so it's always slightly higher than the APR. When you're comparing accounts, APY is the number that matters because it shows what you'll actually earn.

For example, a bank might advertise 4% APR on a savings account. But because interest compounds monthly, the actual APY might be 4.07%. That extra 0.07% comes entirely from compounding — earning interest on your interest. On a large balance or over many years, that difference adds up. Always look for the APY when comparing accounts, not the APR.

How to find out what your account is earning

Your bank is required to tell you the APY on your account, and they must disclose it before you open the account and in your account statements. You can find it in several places: the account agreement you signed when you opened it, your monthly or quarterly statement, or the bank's website in the account details section. If you can't find it, call your bank's customer service line or visit a branch and ask — they're required to provide it.

You can also calculate roughly how much interest you should earn in a month by taking your average balance, multiplying by the APY, and dividing by 12. If your bank is paying you significantly less than that calculation suggests, ask them why. Sometimes there are fees that reduce your interest, or the rate may have changed since you last checked.

Frequently Asked Questions

Does my interest get taxed?

Yes. Interest earned in a savings account is considered income by the IRS, and you owe federal income tax on it. Your bank will send you a form called a 1099-INT at the end of the year listing all the interest you earned. You report this on your tax return. Some states also tax savings account interest, depending on where you live.

Why do some banks offer 0% interest?

Banks that offer no interest are usually older, traditional banks with physical locations in many cities. They have higher costs to run those branches, so they don't need to offer high rates to attract deposits. Online banks and credit unions often offer higher rates because they have fewer expenses. If your bank offers 0%, moving your money to an online bank could earn you real money with no extra work.

Can I lose money if interest rates drop?

No. The interest rate dropping means you'll earn less going forward, but you won't lose what you've already earned or your original deposit. Your balance only goes up, never down, because of interest. The only way to lose money is if you withdraw it yourself or if fees exceed your interest earnings.

What happens to my interest if I withdraw money mid-month?

You keep all the interest you've already earned. If you withdraw money before the month ends, you straightforward stop earning interest on that withdrawn amount from that day forward. The interest already added to your account stays there. Some accounts have penalties for early withdrawal, but those are separate from interest calculations.

Is there a maximum amount of interest I can earn?

No. The more money you keep in the account, the more interest you earn. There's no cap on how much interest a savings account can generate. However, the FDIC insures only up to $250,000 per account at each bank, so if you have more than that, you'd need to split it across multiple banks to keep it all insured.