The difference between simple and compound interest sounds academic until you see how much it actually changes the growth of savings over time. Nearly all modern savings accounts use compound interest, but understanding the mechanism helps you interpret an advertised APY correctly and compare offers accurately.
How simple interest works
Simple interest calculates a return only on the original principal amount, regardless of how much interest has already accumulated. A $10,000 deposit earning 4% simple interest annually earns exactly $400 every year, with no growth in that annual amount even after several years of accumulated interest sitting in the account.
Virtually all savings accounts today use compound interest, calculated and credited daily or monthly rather than once a year — the more frequent the compounding, the slightly higher the effective annual yield compared to the same nominal rate compounded less often.
How compound interest changes the math
Compound interest calculates returns on both the original principal and any interest already earned, meaning your balance grows the base on which future interest is calculated. The same $10,000 at 4% compounded daily earns slightly more than $400 in the first year, and that gap widens every subsequent year as the growing balance itself earns increasing amounts of interest.
Why APY already accounts for compounding
The annual percentage yield (APY) figure banks advertise already factors in the compounding frequency, converting it into a single effective annual rate for easy comparison. This is different from the nominal interest rate, which doesn't account for compounding frequency on its own. Comparing APY directly between accounts is the correct method, since it normalizes for any differences in how often interest compounds.
- Always compare APY, not the nominal interest rate, when evaluating savings account offers
- Understand that more frequent compounding (daily versus monthly) produces a marginally higher effective yield at the same nominal rate
- Recognize that compounding's effect becomes more significant the longer money remains untouched in the account
- Don't assume a slightly lower APY with more frequent compounding underperforms a higher APY with less frequent compounding — APY already accounts for this
Frequently asked questions
Does withdrawing money reset compound growth?
Withdrawing reduces your principal going forward, which reduces future interest earned on that lower balance, but it doesn't erase interest already earned and credited to your account before the withdrawal.
Is compound interest only relevant to savings accounts?
No, the same principle applies to many other interest-bearing products, including CDs, certain bonds, and notably, the interest charged on credit card debt, where compounding works against the borrower rather than for them.