The basic formula: multiply your balance by the rate, then by time

Interest on a savings account is calculated by taking your account balance, multiplying it by the annual interest rate (shown as a decimal), and multiplying again by how long your money sits in the account. The simplest version is: Balance × Annual Rate × Time = Interest Earned. If you have $1,000 in an account earning 4.5% APY for one full year, you earn $45. For six months, you earn $22.50.

Banks use two methods to calculate this: simple interest and compound interest. Simple interest pays you once on your original balance. Compound interest pays you on your balance plus any interest already earned, which means you earn "interest on interest." Most savings accounts use compound interest, and the compounding frequency matters—daily compounding earns more than monthly compounding on the same rate.

The APY (Annual Percentage Yield) shown on your account already includes the effect of compounding, so you can use it directly in calculations. The APR (Annual Percentage Rate) does not include compounding, so it will give you a lower number if you calculate with it instead.

Key Takeaways

  • Multiply your balance by the APY rate (as a decimal) and the fraction of the year to find interest earned: $1,000 × 0.045 × 1 year = $45.
  • APY already includes the effect of compounding, so use APY for your calculations, not APR.
  • Compound interest means you earn interest on interest, which is why daily compounding earns slightly more than monthly compounding at the same rate.
  • Your actual interest will vary if your balance changes during the month, because banks calculate interest on your average daily balance or your ending balance depending on the account terms.

Simple interest: the straightforward calculation

Simple interest is the easiest to calculate by hand. The formula is: Principal × Rate × Time = Interest. Your principal is the amount you deposit. The rate is the APY written as a decimal (so 4.5% becomes 0.045). Time is the portion of a year (one month is 1/12, six months is 6/12 or 0.5).

Example: You deposit $5,000 at 4.5% APY for three months. The calculation is $5,000 × 0.045 × (3/12) = $5,000 × 0.045 × 0.25 = $56.25. You earn $56.25 in interest over those three months. This assumes your balance stays at $5,000 the entire time and you make no withdrawals or deposits.

Simple interest is rarely used for savings accounts anymore, but it is useful for understanding the baseline. Most banks compound interest daily or monthly, which means the real interest you earn will be slightly higher than a simple interest calculation shows.

Compound interest: how you actually earn money in savings

Compound interest recalculates your interest on a schedule—daily, monthly, or quarterly—and adds it to your balance. Then the next calculation includes that interest as part of your principal. This creates a snowball effect where your money grows faster than simple interest alone.

The formula for compound interest is: Final Amount = Principal × (1 + Rate/Compounds per year)^(Compounds per year × Years). For a $5,000 deposit at 4.5% APY compounded daily for one year, the calculation is $5,000 × (1 + 0.045/365)^(365 × 1) = $5,230.11. You earn $230.11 instead of $225 (which simple interest would give you).

The difference grows larger with bigger balances and longer time periods. At $50,000 for five years at 4.5% APY, compound interest earns you about $2,500 more than simple interest would. Your bank's website or app usually shows you the projected interest without requiring you to do this math—but knowing how it works helps you compare accounts fairly.

Why your actual interest might differ from the calculation

Banks calculate interest on your average daily balance or your ending balance, depending on the account. If you deposit $10,000 on the first of the month and withdraw $5,000 on the 15th, the average daily balance method counts roughly $7,500 for the month. The ending balance method counts only $5,000. The same account and rate will pay you different interest depending on which method the bank uses.

Your balance also changes when you make deposits or withdrawals. If you deposit $1,000 on the 10th and withdraw $500 on the 20th, the interest calculation includes both changes. Banks typically show you the interest earned at the end of each month in your statement, which reflects these actual balance changes.

Interest rates also change. If your account earns 4.5% in January and the bank lowers it to 4.0% in February, your February interest will be lower even if your balance stays the same. Banks notify you of rate changes, usually by email or in your account dashboard, but the change takes effect on the date they specify.

How to find the APY on your account

Your bank displays the APY in several places. Check your account statement (usually at the top or in a summary section), your online banking dashboard, or the account details page. If you are shopping for a new account, the APY appears on the bank's website next to the account name, often labeled "APY" or "Annual Percentage Yield."

The APY may also include a note like "APY accurate as of [date]" because rates change. If you opened an account three months ago at 4.5% APY and the current rate is now 3.8%, your account earns the rate that was in effect when you opened it—unless your bank has a variable rate account, which adjusts automatically. Check your account agreement or call your bank to confirm whether your rate is fixed or variable.

For high-yield savings accounts, APY rates change frequently and vary by bank. Comparing APY across banks is the only fair way to choose, because a $10,000 deposit at 4.5% APY earns $450 per year, while the same deposit at 3.5% APY earns $350—a $100 difference annually.

Using online calculators versus doing the math yourself

Most banks provide a savings calculator on their website. You enter your starting balance, the APY, and how long you plan to keep the money, and the calculator shows you the projected interest. This is faster and more accurate than hand calculations, especially for compound interest over multiple years.

Doing the math yourself is useful if you want to understand what is happening or compare two accounts quickly without opening a calculator. For rough estimates, remember that at 4% APY, you earn about $40 per year on every $1,000 in the account. At 5% APY, you earn about $50 per year per $1,000. These are approximations, but they work for quick mental math.

If your balance changes frequently or you are comparing accounts with different compounding schedules, use the bank's calculator or a spreadsheet. The small differences add up over time, and a calculator removes the chance of arithmetic errors.

Frequently Asked Questions

Does the interest rate on my savings account ever go down?

Yes, if you have a variable rate account. Banks can lower the rate at any time, though they usually notify you first. Fixed-rate accounts keep the same rate for a set period (like one year), but when that period ends, the bank may offer a new rate. Check your account agreement to see whether your rate is fixed or variable.

How often does the bank add interest to my account?

Banks compound interest daily, monthly, or quarterly depending on the account. Daily compounding is most common for high-yield savings accounts. The interest is usually posted to your account once per month, even if it is compounded daily. You will see the total interest earned listed on your monthly statement.

If I withdraw money mid-month, do I lose all the interest for that month?

No. Banks calculate interest based on your average daily balance or ending balance, so a mid-month withdrawal reduces the interest you earn that month but does not eliminate it. If you withdraw on the 20th, you still earn interest on the money you held for the first 19 days.

What is the difference between APY and APR for savings accounts?

APY includes the effect of compound interest, while APR does not. For savings accounts, always use APY to calculate what you will actually earn. APR is more commonly used for loans. A 4.5% APY account earns more than a 4.5% APR account because APY already accounts for compounding.

Can I calculate interest if my balance changes multiple times a month?

You can estimate it, but the bank's calculation will be exact. Each deposit or withdrawal changes your balance, and the interest is calculated on that changing balance. Your monthly statement shows the actual interest earned, which reflects all the balance changes that occurred. For precise calculations across multiple transactions, use your bank's online calculator or ask them directly.