How savings account interest is calculated

Banks calculate interest on your savings account using one of two methods: simple interest or compound interest. Most savings accounts use compound interest, which means you earn interest not just on your original deposit, but also on the interest that has already been added to your account.

The bank takes your account balance, applies an interest rate (expressed as an annual percentage rate, or APR), and adds the earnings to your account at regular intervals—usually daily, monthly, or quarterly. The more often interest compounds, the more you earn, because each compounding period adds a new layer of interest-on-interest.

Your actual earnings depend on three things: the balance in your account, the interest rate the bank offers, and how often the bank compounds the interest. A bank might advertise a 4.5% APR, but if it compounds daily instead of monthly, you will earn slightly more over the course of a year.

Key Takeaways

  • Compound interest means you earn interest on your interest, not just on your original deposit, and most savings accounts work this way.
  • The interest rate (APR) is annual, but banks usually compound daily or monthly, so your money grows throughout the year even if you do not add to it.
  • A higher APR and more frequent compounding both increase your earnings, so comparing rates between banks matters more than the compounding frequency alone.
  • The bank calculates interest based on your daily or monthly balance, so deposits and withdrawals change how much interest you earn in any given period.

Simple interest versus compound interest

Simple interest is straightforward: the bank calculates interest only on your original deposit. If you put $1,000 in an account earning 5% simple interest per year, you earn $50 that year, and $50 every year after that—the interest never grows. Simple interest is rare on savings accounts today.

Compound interest is what most banks use. The bank calculates interest on your balance, adds it to the account, and then calculates next period's interest on the new, larger balance. This creates a snowball effect: your money grows faster the longer it sits, because each compounding period earns interest on a bigger pile.

The difference becomes visible over time. On a $10,000 deposit at 4% annual interest, simple interest would give you $400 per year forever. Compound interest, compounded daily, would give you about $408 in the first year—and more in the second year, because the second year's calculation includes the $408 you already earned.

How often interest compounds and why it matters

Banks compound interest at different intervals: daily, monthly, quarterly, or annually. The compounding schedule is stated in your account agreement and sometimes on the bank's website under the account details.

Daily compounding is the most common for savings accounts and produces the highest earnings, because interest is calculated 365 times per year instead of 12 or 4. The difference between daily and monthly compounding on a $10,000 balance at 4% APR is roughly $3 to $4 per year—small, but real. The difference between daily and annual compounding is larger, around $40 per year on the same balance.

When comparing savings accounts, the APR matters far more than the compounding frequency. A 4.5% APR compounded monthly will almost always beat a 3.5% APR compounded daily. Focus on the interest rate first, then check the compounding schedule as a tiebreaker.

What the bank uses to calculate your interest

Banks calculate interest based on your account balance during each compounding period. Most use the daily balance method, which means they track your balance every single day, add up all those daily balances for the month (or quarter, or year), divide by the number of days, and apply the interest rate to that average.

This matters if your balance changes during the period. If you deposit $5,000 on the first day of the month and leave it untouched, the daily balance method counts that $5,000 for all 30 days. If you deposit $5,000 on the 15th, it only counts for the last 15 days of the month, so you earn less interest that month.

Some banks use the average daily balance method, which works the same way. A few older accounts use the minimum balance method, where the bank only counts your lowest balance during the period—this is rare and unfavorable to you, so check your account agreement if you have an older account.

Reading the APY versus the APR

Banks advertise two different rates: the APR (annual percentage rate) and the APY (annual percentage yield). The APR is the raw interest rate. The APY is what you actually earn in a year after accounting for compounding.

On a savings account with a 4% APR compounded daily, the APY will be slightly higher—around 4.08%—because of compound interest. The difference is small on savings accounts but real. When comparing accounts, use the APY to see which one will actually put more money in your pocket.

Your bank is required to show you both numbers, usually on the account details page or in the account agreement. If you see only one, ask the bank for the other before opening the account.

How interest rates change and when you earn it

Savings account interest rates are not fixed. Banks raise and lower rates based on what the Federal Reserve does and what competitors are offering. You might open an account at 4.5% and see it drop to 3.8% six months later, or rise to 5.2%. Check your account agreement to see whether the bank can change your rate without notice, or whether it must notify you first.

Interest is usually added to your account monthly, though some banks add it daily or quarterly. When interest is added, it becomes part of your balance and earns interest itself in the next compounding period. You do not have to do anything to receive it—the bank calculates and deposits it automatically.

If you close the account before the end of a compounding period, you typically receive interest only for the days you held the account. Some banks calculate this daily, others round to the nearest month. Check your account agreement or ask before closing.

Why your actual earnings might differ from the advertised rate

The advertised APY assumes your balance stays the same all year. In reality, most people deposit and withdraw money, which changes the daily balance and therefore the interest earned. If you deposit $5,000 and then withdraw $4,000 after three months, you earn less interest than if you had left the full $5,000 in the account.

Some banks also charge monthly maintenance fees, which reduce your earnings. A $10 monthly fee on a $5,000 account earning $20 per month cuts your actual return in half. Look for accounts with no monthly fees, or accounts that waive the fee if you maintain a minimum balance.

Interest rates also change. If you open an account at 4.5% APY and the rate drops to 3.2% after three months, your earnings for the rest of the year will be lower than the original advertised rate suggested. This is normal and legal—banks are not required to lock in a rate for a year on regular savings accounts.

Frequently Asked Questions

Does my savings account interest get taxed?

Yes. Interest earned on a savings account is taxable income. Your bank will send you a 1099-INT form at the end of the year if you earned $10 or more in interest, and you must report it on your tax return. The amount owed depends on your tax bracket.

What is the difference between a savings account and a money market account for interest?

Money market accounts often pay higher interest rates than regular savings accounts, but they usually require a larger minimum balance and limit how many withdrawals you can make per month. Both use compound interest, but the rate is higher on money market accounts because the bank can invest your money more aggressively.

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

You earn interest only on the balance you held during each day of the compounding period. If you withdraw money on the 15th, you stop earning interest on that amount starting the 16th. Interest that was already added to your account stays there and continues to earn interest.

Why do some banks offer much higher interest rates than others?

Online banks and credit unions often offer higher rates than large brick-and-mortar banks because they have lower overhead costs. The money is just as safe—deposits are insured by the FDIC up to $250,000—but the bank can afford to pay you more. Higher rates are not a sign of risk; they are a sign of lower operating costs.

Does the order of deposits and withdrawals change how much interest I earn?

No. The bank uses your daily balance to calculate interest, so only the amount of money in the account on each day matters, not the order of transactions. A $1,000 deposit on the 1st and a $1,000 withdrawal on the 15th produces the same interest as a $1,000 withdrawal on the 1st and a $1,000 deposit on the 15th, as long as the daily balances are the same.