The two formulas that matter: straightforward and compound interest
Interest on a savings account is calculated one of two ways, and which one your bank uses changes how much money you actually earn. straightforward interest pays you a percentage of your starting balance only. Compound interest pays you a percentage of your balance plus all the interest you've already earned — which means you earn interest on your interest.
Most savings accounts use compound interest, usually compounded daily or monthly. That's the better deal for you, but you need to know which method your bank uses and how often they compound, because the difference adds up over time.
Key Takeaways
- straightforward interest is calculated only on your original deposit, while compound interest is calculated on your balance plus accumulated interest.
- The formula for straightforward interest is: (Principal × Rate × Time) ÷ 100, and most savings accounts do not use this method.
- Compound interest uses the formula A = P(1 + r/n)^(nt), where compounding frequency (daily, monthly, quarterly) significantly affects your total earnings.
- Your bank's disclosure documents state the compounding frequency and APY (Annual Percentage Yield), which already factors in compounding for you.
- You can calculate interest yourself using these formulas, or use your bank's online calculator, which applies their exact compounding schedule.
How straightforward interest works (and why your account probably doesn't use it)
straightforward interest is the easier calculation. You multiply your starting balance by the interest rate by the number of years the money sits in the account, then divide by 100. The formula is:
Interest Earned = (Principal × Annual Rate × Time in Years) ÷ 100
If you deposit $5,000 at 4% annual interest for one year, you earn: ($5,000 × 4 × 1) ÷ 100 = $200. After one year, you have $5,200. If you leave it for another year without touching it, you earn another $200 — still only on the original $5,000.
Banks rarely offer straightforward interest on savings accounts because it costs them less. Money market accounts and some promotional savings products occasionally use it, but most standard savings accounts compound instead. Check your account agreement or call your bank to confirm which method they use.
How compound interest works and why the compounding frequency matters
Compound interest calculates interest on your balance plus all interest already earned. The formula is more complex:
A = P(1 + r/n)^(nt)
Where A is your final amount, P is your principal (starting balance), r is the annual interest rate as a decimal, n is the number of times interest compounds per year, and t is the number of years.
Using the same $5,000 at 4% for one year, but compounded monthly (n = 12): A = $5,000(1 + 0.04/12)^(12×1) = $5,000(1.00333)^12 = $5,204.08. You earn $204.08 instead of $200 — an extra $4.08 because interest compounds twelve times.
If the same account compounds daily (n = 365): A = $5,000(1 + 0.04/365)^(365×1) = $5,204.88. Daily compounding earns you $204.88, another $0.80 better than monthly. The more frequently interest compounds, the more you earn, because each compounding adds a small amount to the balance that then earns interest itself.
What APY means and why it's easier than doing the math yourself
Your bank publishes an Annual Percentage Yield (APY), which is the interest rate already adjusted for how often the account compounds. It tells you the actual percentage you'll earn in a year without you having to run the compound interest formula.
If your bank advertises 4% APY, that 4% already includes the effect of daily or monthly compounding — whatever their schedule is. You can multiply your balance by the APY to get a rough annual earnings number. A $5,000 balance at 4% APY earns approximately $200 in a year (though the exact amount depends on when deposits and withdrawals happen).
The APY is always printed in your account disclosure documents, on the bank's website, and in any marketing materials. It's the number to compare when you're shopping between banks, because it shows you the real return, not just the base rate.
Calculating interest month by month or day by day
If you want to track earnings more frequently than annually, you can break the compound interest formula into smaller periods. For monthly calculations, use the same formula but set t to the number of months divided by 12.
For example, to find interest earned in three months on $5,000 at 4% APY compounded monthly: A = $5,000(1 + 0.04/12)^(12×0.25) = $5,000(1.00333)^3 = $5,050.13. You earn $50.13 in three months.
Most banks compound daily, which means interest is calculated and added to your balance every single day. The daily rate is the annual rate divided by 365 (or 360 on some accounts). You don't need to calculate this yourself — your bank does it automatically and shows you the running balance in your account.
Using your bank's tools instead of calculating by hand
Every major bank offers a savings calculator on their website. You enter your starting balance, the APY, how long you plan to keep the money, and whether you'll make regular deposits. The calculator shows you projected earnings using their exact compounding schedule and deposit timing.
These calculators are more accurate than hand calculations for most people because they account for the specific days your bank compounds interest and the exact dates deposits post. They also show you how regular deposits (like $100 per month) change your total earnings over time.
If you're comparing accounts at different banks, use each bank's calculator with the same starting balance and time period. The results will show you the real difference between a 3.5% APY account and a 4.5% APY account, accounting for their different compounding schedules.
What changes your interest earnings besides the rate
The interest rate and compounding frequency are not the only things that affect what you earn. Your balance matters — a higher balance earns more interest. The length of time the money stays in the account matters — longer deposits earn more. And deposits and withdrawals change the balance that interest is calculated on.
If you deposit $5,000 on January 1 and withdraw $2,000 on June 15, interest is calculated on $5,000 for the first 165 days and $3,000 for the remaining 200 days. Banks track this daily, so your actual earnings reflect the exact balance on each day of the year.
Some accounts also have minimum balance requirements or tiered rates — meaning you earn a higher rate if your balance stays above a certain threshold. Check your account agreement to see whether your rate changes based on your balance.
Frequently Asked Questions
Do I need to do these calculations myself, or does my bank do it for me?
Your bank calculates and deposits interest automatically. You don't need to do anything. The calculations shown here are for understanding how much you should earn or comparing accounts — your bank handles the actual math and adds interest to your account on their schedule.
Why is my actual interest earnings different from what I calculated?
The most common reason is timing. If you made deposits or withdrawals during the year, the balance that earned interest changed. Also, banks may compound on a 360-day year instead of 365, and interest posting dates vary. Your bank statement shows the exact amount earned, which is the correct figure.
If I move money between accounts, does that affect interest?
Yes. Interest is calculated on the balance in each account on each day. If you move $1,000 from savings to checking, the savings account balance drops and earns interest on the lower amount going forward. The checking account (if it earns interest) starts earning on the new balance when ready.
Does compound interest mean my money grows exponentially?
Compound interest does accelerate growth, but "exponential" overstates it for savings accounts. At typical savings rates (3% to 5%), the difference between straightforward and compound interest is small in the first few years. Over decades, compound interest becomes more significant, but you're not seeing dramatic growth month to month.
What's the difference between APY and APR?
APY (Annual Percentage Yield) includes compounding and shows your real earnings. APR (Annual Percentage Rate) is the base rate without compounding. Banks must show you the APY for savings accounts so you can compare fairly. APR is used mainly for loans and credit cards.