See exactly how your money grows with compound interest. Choose your compounding frequency — daily, monthly, quarterly, or annually — and instantly see your final balance and total interest earned over any time period.
Compound interest is calculated using the formula A = P(1 + r/n)^(nt), where A is the final amount, P is the principal (starting amount), r is the annual interest rate as a decimal, n is the number of times interest compounds per year, and t is the time in years. Unlike simple interest — which only earns returns on the original principal — compound interest earns returns on both the principal and the previously earned interest. This creates an exponential growth curve rather than a linear one.
The key insight of compound interest is that time is the most powerful variable. Doubling your principal doubles your final amount. But doubling your time period more than doubles it because of the compounding effect. This is why financial advisors consistently emphasize starting to invest as early as possible.
$10,000 at 5% compounded monthly for 10 years: A = $10,000 × (1 + 0.05/12)^(12×10) = $16,470.09. Interest earned: $6,470.09 — a 64.7% return without adding another dollar.
Compare $10,000 at 7% compounded monthly: after 10 years = $20,097. After 20 years = $40,388. After 30 years = $81,136. The money roughly doubles every 10 years — but the absolute dollar gains accelerate dramatically. The last 10 years (20→30) add $40,748, more than the first 20 years combined.
$10,000 at 5% for 10 years compounded: annually = $16,288.95; monthly = $16,470.09; daily = $16,486.65. Daily compounding earns only $197.70 more than annual compounding over a decade — frequency matters far less than rate and time.
A quick mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 4%, money doubles in 18 years. At 6%, 12 years. At 9%, 8 years. At 12%, 6 years. This works in reverse for debt: a credit card at 24% APR doubles what you owe in just 3 years if unpaid.
Simple interest is calculated only on the principal: Interest = P × r × t. On $10,000 at 5% for 10 years, simple interest earns exactly $5,000. Compound interest (monthly) earns $6,470 — $1,470 more from the same investment. Over 30 years, the gap widens dramatically: simple interest earns $15,000 while monthly compounding earns $71,136 on the same initial $10,000.
APR (Annual Percentage Rate) is the stated interest rate without compounding. APY (Annual Percentage Yield) reflects the actual return after compounding within the year. A savings account with 5% APR compounded monthly has an APY of 5.116%. Always compare APYs when evaluating savings accounts.
Banks calculate interest on your daily balance and credit it to your account periodically (monthly for most savings accounts). Each time interest is credited, it becomes part of your balance and earns interest in subsequent periods. This is why leaving money untouched in a high-yield savings account grows faster than withdrawing interest as it's earned.
Yes — compound interest works just as powerfully on debt as on savings. Credit card balances, unpaid student loans, and other high-interest debts compound against you. A $5,000 credit card balance at 20% APR compounds monthly; if you only pay the minimum, you could pay more than $5,000 in interest alone before paying off the original balance.
Continuous compounding is a theoretical concept where interest compounds at every instant. The formula is A = Pe^(rt), where e is Euler's number (approximately 2.71828). In practice, daily compounding is extremely close to continuous compounding. The difference between daily and continuous compounding is negligible for most personal finance purposes.
Start as early as possible, reinvest all returns rather than withdrawing them, choose accounts with higher interest rates, and contribute regularly. Even small monthly additions dramatically accelerate growth. Avoiding high-interest debt is equally important — paying off a 20% credit card balance is equivalent to earning a guaranteed 20% return.